The harness for the o2 sensor should be a branch around the fuel injector harness. But since yours was modified, it’s hard to tell how much of the harness is original.

Exhaust leak is the doughnut gasket. Check the joints in the exhaust system.

Vacuum hand pump on the brake booster should hold 5 inches of mercury and stall with vacuum applied. If not, it’ll stay running and you have a leak.

Rich condition is probably due to the sensor being missing. Unless you found a way to trick the ecu.

 

Thanks for answering,

As for the o2 sensor, I found one connector with 4 wires (I assume the o2 sensor is like other Toyotas so has 4 wires) and it looks like this. Think this is it?

How do you expect the computer to control the air-fuel mixture without an O2 sensor? How else would it possibly know how much fuel to put in? It would just be guessing. The front O2 sensor is used to control the air-fuel mixture. Any O2 sensor behind the cat (or where it should be) is just to monitor the cat efficiency and isn't used to control the air-fuel ratio.

I see, I frequently see people talking about cat delete, so I thought they'd remove the o2 sensors too. I did find a connector, just not sure what it is for thank you

Right on, DrZ. The fifth Gen 4A-GE black-top is quite a nice engine. Has 4 individual throttle bodies, variable valve timing, 11:1 compression ratio, uses a MAP, all pretty nice. And that engine depended upon an ECU that required a lot of inputs for it to control the engine. And that engine wasn't available in USDM Toyota's, which makes getting parts a challenge.

All that said, helping with someone else's swap is beyond challenging. Your best bet is to talk to the person who did the swap. If that person really did not install an oxygen sensor in the exhaust system somewhere, find out what he/she did to make it work without one. The ECU depends upon the oxygen sensor, as DrZ stated earlier. If it is not there, you have to have a check engine light on somewhere. But that is another issue, as you don't know how the wiring for the 4A-GE black-top interfaces with the vehicle that wasn't set up for the 4A-GE engine.

Have you done a thorough inspection of the exhaust system to verify there is no oxygen sensor anywhere? There are many de-cat exhaust systems out there, but they all still have oxygen sensors.

Also, you said you started your car after sitting a month and discovered it was running rich - did it run rich before you parked it for a month (i.e.: did you buy it knowing it was running rich then)?

Finding parts is indeed hard and always expensive 😅 I bought someone's car about a year ago as my first car having close to 0 experience but I've learned a ton to this day. The car has always been running rich since the day I bought her and I've been working on it since then. On top of that, the wiring was halfway done so only essential components have been wired (Instrument cluster isn't wired = no engine light / rpm / speed).
I know for sure there are no o2 sensors connected, since I've taken the exhaust apart a few times already. However, I found a 4 wire connector that could be the o2 sensor connector and it would help if you can give your input on it Thanks for the input

Just something to keep in mind...an engine running (idling) on today's fuels will turn the spark plugs black (BTDT before). This is because there's no heat from combustion being generated, along with no advance of the ignition timing. Add in that any fuel that may have been sitting for a month or longer will more than likely have water mixed in (absorbed by the ethanol mixed in the fuel by weather changes) and will not want to burn. I've run into it multiple times, and a change of plugs will get the engine to run correctly.
But like DrZ and others have mentioned above, without all of the sensors functioning correctly, or the wrong ECU, this engine may not run correctly.

I have changed the plugs less than 100 miles ago. Double checking if the ECU is the right one would be a good idea (I think it already is). As for the o2 sensor, I am actively looking to rewire it back!

 

How do you expect the computer to control the air-fuel mixture without an O2 sensor? How else would it possibly know how much fuel to put in? It would just be guessing. The front O2 sensor is used to control the air-fuel mixture. Any O2 sensor behind the cat (or where it should be) is just to monitor the cat efficiency and isn't used to control the air-fuel ratio.

 

Right on, DrZ. The fifth Gen 4A-GE black-top is quite a nice engine. Has 4 individual throttle bodies, variable valve timing, 11:1 compression ratio, uses a MAP, all pretty nice. And that engine depended upon an ECU that required a lot of inputs for it to control the engine. And that engine wasn't available in USDM Toyota's, which makes getting parts a challenge.

All that said, helping with someone else's swap is beyond challenging. Your best bet is to talk to the person who did the swap. If that person really did not install an oxygen sensor in the exhaust system somewhere, find out what he/she did to make it work without one. The ECU depends upon the oxygen sensor, as DrZ stated earlier. If it is not there, you have to have a check engine light on somewhere. But that is another issue, as you don't know how the wiring for the 4A-GE black-top interfaces with the vehicle that wasn't set up for the 4A-GE engine.

Have you done a thorough inspection of the exhaust system to verify there is no oxygen sensor anywhere? There are many de-cat exhaust systems out there, but they all still have oxygen sensors.

Also, you said you started your car after sitting a month and discovered it was running rich - did it run rich before you parked it for a month (i.e.: did you buy it knowing it was running rich then)?

 

Just something to keep in mind...an engine running (idling) on today's fuels will turn the spark plugs black (BTDT before). This is because there's no heat from combustion being generated, along with no advance of the ignition timing. Add in that any fuel that may have been sitting for a month or longer will more than likely have water mixed in (absorbed by the ethanol mixed in the fuel by weather changes) and will not want to burn. I've run into it multiple times, and a change of plugs will get the engine to run correctly.
But like DrZ and others have mentioned above, without all of the sensors functioning correctly, or the wrong ECU, this engine may not run correctly.

 

For the 4AG Gurus, do you recognize any of those connectors? 😶

1) 3 pin connector close to the OBD1 reader

2) Weird 2 wire connector with ...something

3) 4 wire connector (some colors match those from a Toyota's o2 sensor), but I'm not sure

4) 3 wire connector also coming from the injector harness

5) I might have a IAC? (but it does not ressemble the one I've seen on YT from a 4AG Blacktop since it wasn't under the back of the engine).

 

The o2 needs to be retained, otherwise the ecu isn’t able to calculate the fuel injection pulse to balance with the air that’s coming in.

 

I've worked on wiring for a while on this car.. since I cant find the OEM o2 sensor, here's what I might do as last resort:

Coming from another post on another forum:
This is the harness connector off a blacktop AE111 motor. The wires are connected thus:

Pink - goes to HT on the ECU. This is the grounded side of the heater element
Black - goes to B+ on the ECU. This is the switched, +12V supply rail and provides power to the heater
Brown - goes to chassis earth. This is the ground reference for the sensor
White - goes to OX on the ECU. This is the sensor output signal

So basically Pink and Black are the Heater side of the O2 sensor, while brown and white are the O2 signal to the ECU

If I get all of this right, I should be able to splice into the wires and make my own o2 sensor harness.

 

ae101 4agze in ae92 where is O2? -

 

#2 is the Intake Air Temperature (IAT) sensor. It's supposed to be mounted inside the air filter box. The ECU uses the air temperature to calculate the density of the air to know how many oxygen molecules are going into the cylinders.

 

Oh I see, I dont have an air filter box so I guess I'll just leave it there close to the TB.

This might help you regarding the O2 sensor (the parts diagram for a 1998 Trueno AE111 w/4A-GE - Oxygen Sensor).

Note the part number (89465-19685). That part number has an eBay listing: Oxygen Sensor 89465-19685 For Toyota Corolla AE101 AE111 4AGE Levin Trueno 97-00 | eBay

That eBay listing provides a clear view of the wiring harness connector. It does not match the receiving connector you posted on your wiring harness that you thought might be the one for the O2 sensor (post #6). But it does match the receiving connector of the wiring harness in your post #9.

You may find other options for this O2 sensor, if you opt to purchase this part number, as the eBay I found with the clear picture of the wiring connector is out of Australia.

The thing is, if your current wiring harness has the connector you found belonging to an oxygen sensor, but it doesn't match the correct oxygen sensor for a 4A-GE black-top engine, is it possible the wiring harness isn't original to that engine?

To precise, the 4 pin connector is one I've found, but I do not know what it belongs to. I can wire from the ECU and make my own o2 harness if I cannot find the OEM one.
DrZ had said the connector should come from the injector harness part, so I'll look futher into it.
I'm pretty sure the entire harness is the right one, because the 1989 Corolla sr-5 is carb injected system while the Blacktop is EFI. I have the injectors wires obviously because the engine runs.

That’s the original thought I had. In the link I provided, the o2 sensor for the ae92 is a single wire connection. In the op’s image, it’s a 4 pin connector. This could be a modified harness and it makes it tricky.

Unless the op can find the layout of the engine harness for his chassis, it’s hard to determine if the entire harness was swapped in or it was modified at certain junctions.

I was thinking it’s a big port 16v 4age. Guess it’s the big brother 20v.

Yup! It's a blacktop 20v and I have the right harness so it's a 4pin connector. After looking at the link, a 1 wire o2 sensor didn't make much sense 😕

 

This might help you regarding the O2 sensor (the parts diagram for a 1998 Trueno AE111 w/4A-GE - Oxygen Sensor).

Note the part number (89465-19685). That part number has an eBay listing: Oxygen Sensor 89465-19685 For Toyota Corolla AE101 AE111 4AGE Levin Trueno 97-00 | eBay

That eBay listing provides a clear view of the wiring harness connector. It does not match the receiving connector you posted on your wiring harness that you thought might be the one for the O2 sensor (post #6). But it does match the receiving connector of the wiring harness in your post #9.

You may find other options for this O2 sensor, if you opt to purchase this part number, as the eBay I found with the clear picture of the wiring connector is out of Australia.

The thing is, if your current wiring harness has the connector you found belonging to an oxygen sensor, but it doesn't match the correct oxygen sensor for a 4A-GE black-top engine, is it possible the wiring harness isn't original to that engine?

 

That’s the original thought I had. In the link I provided, the o2 sensor for the ae92 is a single wire connection. In the op’s image, it’s a 4 pin connector. This could be a modified harness and it makes it tricky.

Unless the op can find the layout of the engine harness for his chassis, it’s hard to determine if the entire harness was swapped in or it was modified at certain junctions.

I was thinking it’s a big port 16v 4age. Guess it’s the big brother 20v.

 

Also, this may help you a lot with sensors, connectors, testing, etc. It is the AE111 4A-GE Black Top EFI Manual (attached).

 

Hi, so having 4age Blacktop 20v engine running rich

The OP says this (not a 16v 4age).

 

I think there are many Toyota's from the late 90's that would have the same O2 sensor wiring harness connector on the harness side. On a salvage car, that part of the connector could be cut off with a sufficiently long pigtail that you could bring your custom wires over to your O2 sensor with. You'll need to have a bung on your exhaust system somewhere to attached your O2 sensor onto. An exhaust shop should be able to provide the correct O2 sensor bung (once you get the O2 sensor) to attach to your existing exhaust system.

You don't have an air inlet filter to your throttle bodies? Don't ruin that poor engine without running an air filter. And that inlet air temperature sensor needs to be in the air flow going to the throttle bodies. Setting that sensor in the engine bay, not in the air flow going to the throttle bodies, will cause it to read temps too high than what is really going to the throttle bodies, which will cause the ECU to compensate incorrectly.

I don't understand someone who swaps in such a good engine but fails to provide the sensors it needed to function properly.

 

I agree, unfortunately the swap was poorly done so I'm trying to fix it. From what I've seen, most Toyota's from the 90s use the same o2 sensor so that'll help.

I am not running any air filter box, but have installed some motorcycle filters in the throttle bodies instead. As for the inlet air temperature sensor, I'll install it close to the TB. (I'll add a picture later if you're curious about the setup)

 

I’m the one that suggested the o2 sensor connector would be around the area of the injector harness. That’s not correct. Look near the exhaust manifold. Not too familiar with obdI.

I’d assume you could just buy a o2 sensor, but do you know what chassis the engine and harness came from?

 

I do not know exactly what chassis the engine and harness came from but there arent many (AE101, AE111 are just a few). Every parts for the Blacktop are very expensive since it's only manufactured in Japan and like 2 other countries. I've seen people used Benso's o2 4wire sensor with a little wiring. As for the fit on the exhaust, I learned to weld a few months back so finding a nut shouldn't be an issue.

The bigger issue would be to wire the connector to the engine harness/ECU.

 

You know for sure that the terminal pins for the o2 sensor have been removed when the swap was done by the previous owner?

 

No, I haven't had the time to carefully inspect the harness and every connector, but I do know I do not have an o2 sensor connected as of now. Might have the harness/connector hanging loose somewhere so that'll be my first option, last option would be to splice/cut(wires) from the ECU and make my own harness.

 

Continuity check the connector to the ecu. If it’s in the ecu, then you’ll just need to buy the sensor with a bung. Then weld into the exhaust.

Edit : this is one of the hardest projects to work on. A swap that was not done properly. I hope you fix it properly and have a fun time when it’s finished.

 

That'll be the next step thanks
So many of my friends are wondering how I have not given up yet 😅 I think it really is me imagining myself driving the car on public roads

 

Wow..ah I'll try the take it easy on me part.

Try reading what the O2 sensors do and don't do. Your assumptions are way out of line big time son.

Hard to believe you don't have a idle control mechanism. Try reading about them in 1989 Toyota literature, consider it continuing education..(Item #2)

Your leaky injector(s) could be the whole problem. The jerk PO that cut the o2 sensors probably muffed the inj install or used wildly oversized ones? Again you need to find out

My 89 Mr2 has a pressure regulator on the fuel rail so any pressure is confined to that parameter. (Your item #3)

My total advice is get rid of that bad attitude that you can't fix everything. Dude, you can.
Education is the key not piddling to others for answers. Quit cheating yourself. Make a commitment to better your car knowledge. Your 50 year old self will thank you.

Ride on,
tim

 

I'm assuming your 4A-GE blacktop is OBD-II - I think these first made their appearance in 1995, so I think Toyota was no longer making OBD-1 units anymore by 1995.

Either way, do you have an OBD-II port to plug a scanner into on your engine? Or is your engine OBD-1, and if so, do you have an OBD-1 diagnostic port in your wiring harness?

 

dumb answer, but no I haven't tried to plug an OBD-1 (I think is what I have). I haven't bothered trying since even the dash and all wasn't wired..

 

dumb answer, but I have not tried checking the OBD1 port (I think it’s what I have). Since the instrument cluster wasn’t even wired properly, I have not bothered doing it.

 

Can you post a picture of your OBD-1 diagnostics port?

 

If that is a picture of the car that is the topic of this thread, then yes, it looks like it is OBD-I. When you get home, or if your friend is willing, open the cap and take a picture that includes the terminals in the body of the port and the label on the underside of the cap (which will be visible when the cap is opened). Post that picture. And, lastly, do you have a pin-out diagram showing all the pin labels of your ECU?

 

I’ll post them pics ASAP, and yes I have the ECU pinout

 

Great. On your way home, if you don't already have something like this, pick this up at an automotive parts store. It is a 6-12 VDC troubleshooting / check light. It already has a fuse built into the housing. This can become your temporary MIL. We can test this out after you are back home and ready to try it. Not sure what your schedule or even mine will be, but I'm pretty sure this will work. If you aren't comfortable with the process - don't do it. But I think you will find it useful. If you try it and it works, you could rig up something a little better than this until you get all things worked out and get your instrument cluster tied in properly.

 

Will do! I’m willing to try because that’s how I’ve learned everything.

 

Position the intake air sensor just behind the grill, it's job is to read ambient air temperature. PS just a thought, some CAT5 wire is a cheap way to make a neat looking harness.

 

Will look into those to if I end up making my harness! Thanks

 

Those pictures are great. Thank-you. I'm glad to see terminals E1, TE1 & W on that diagnostic port.

Let's explain what I am going to ask you to do. Below is a diagram on how the MIL works on a 1990-1993 Toyota Celica 4A-FE system. The data link connector on this example is not identical to yours, but the key terminals exist, and the operation of the MIL is the same on yours as it is on the example below. The MIL is the check engine light in your instrument cluster.

If you look at how the MIL is wired, it is connected to 12VDC power through the 15A Gauge Fuse. It gets power when the ignition key is turned to ON. It will only illuminate if it is grounded. That grounding is done in the ECU through terminal W. When you first turn your ignition key to start the engine, the ECU goes through a check process, part of that process involves turning on the MIL for a few seconds and then, if there are no DTC's, turns the MIL off (if there are DTC's present, the MIL stays on). The ECU accomplishes this internally by grounding and opening circuit W. Also, if terminals E1 & TE1 in the diagnostic port are shorted together with a jumper, the ECU will go into diagnostic mode when the ignition key is in the ON position. Many things are different in the diagnostic mode, but regarding the MIL, if there are no DTC's present, the MIL will flashing rapidly and constantly (telling you that you are in diagnostic mode and that there are no DTC's present). If the MIL flashes slowly, it will flash the DTC or DTC's it has stored. I'll assume you know how to read those kinds of flashed to get the DTC numbers. If not, just say so, I'll provide you a brief description of how to decode what the MIL tells you.

In your situation, you don't have the wiring harness connected to your instrument cluster, so the MIL in the instrument cluster is not currently functional. So we will make a temporary MIL.Because you have terminal W in your diagnostic port, your diagnostic port is connected with your ECU terminal W, so we can use terminal W on your diagnostic port with a 12VDC test lamp.

If you bought a 12VDC test lamp similar to the one I showed you in the picture in post #43, then you have all you need (except you may need some short small conductors, like extra wires, to help make contact in the diagnostic port - but you most likely will not need them).

In the diagram below, note that E1 is simply a ground terminal (same as any metal chassis object). +B is battery voltage which is present when the ignition key is turned to ON. To test, put your 12VDC test light alligator clip end on the +B terminal and the pointed end on terminal E1 with the ignition in the OFF position - should not have a light present. Then turn the key to the ON position. The 12VDC test light should light up. Does it? NOTE: If the alligator clip doesn't work with the diagnostic port terminals, you may have to put a short conductor in the diagnostic port terminal and clip the alligator clip onto that.

if it does, turn your ignition key back to OFF. Then take the 12VDC test light and put one end on the diagnostic terminal +B and the other end on the diagnostic terminal W. Test light should be out. Then turn the ignition switch to ON. Test light should turn on. Does it? If it does turn on, go to the next paragraph. If it doesn't, stop here and respond back.

If the test light turned on in the above paragraph, and because I don't know if you've disconnected your battery frequently and recently, let's start the engine with the test lamp still connected as described in the paragraph above. If everything checks out fine, the test light (now the temporary MIL) will turn off within a few seconds of the engine starting, or it may stay on indicating the presence of DTC's. What did the temporary MIL do?

If the test light stayed on, turn the engine off and ignition switch in the OFF position. Test light should turn off. With a separate jumper, connect diagnostic terminals E1 & TE1. Test light should still be off. Now turn the ignition switch to ON (don't start the engine). The MIL should be flashing - most likely slowly and indicating DTC numbers. Record the number or numbers it is flashing to you. These are telling you what problems the ECU has recognized.

I'll stop now and let you absorb this. Ask any questions if you are not certain. Otherwise, try this out. I think it will provide help to you. I'm pretty sure you will get an O2 sensor code and an intake air temperature sensor code at the very least (if neither are plugged in).

 

Those pictures are great. Thank-you. I'm glad to see terminals E1, TE1 & W on that diagnostic port.

Let's explain what I am going to ask you to do. Below is a diagram on how the MIL works on a 1990-1993 Toyota Celica 4A-FE system. The data link connector on this example is not identical to yours, but the key terminals exist, and the operation of the MIL is the same on yours as it is on the example below. The MIL is the check engine light in your instrument cluster.

If you look at how the MIL is wired, it is connected to 12VDC power through the 15A Gauge Fuse. It gets power when the ignition key is turned to ON. It will only illuminate if it is grounded. That grounding is done in the ECU through terminal W. When you first turn your ignition key to start the engine, the ECU goes through a check process, part of that process involves turning on the MIL for a few seconds and then, if there are no DTC's, turns the MIL off (if there are DTC's present, the MIL stays on). The ECU accomplishes this internally by grounding and opening circuit W. Also, if terminals E1 & TE1 in the diagnostic port are shorted together with a jumper, the ECU will go into diagnostic mode when the ignition key is in the ON position. Many things are different in the diagnostic mode, but regarding the MIL, if there are no DTC's present, the MIL will flashing rapidly and constantly (telling you that you are in diagnostic mode and that there are no DTC's present). If the MIL flashes slowly, it will flash the DTC or DTC's it has stored. I'll assume you know how to read those kinds of flashed to get the DTC numbers. If not, just say so, I'll provide you a brief description of how to decode what the MIL tells you.

In your situation, you don't have the wiring harness connected to your instrument cluster, so the MIL in the instrument cluster is not currently functional. So we will make a temporary MIL.Because you have terminal W in your diagnostic port, your diagnostic port is connected with your ECU terminal W, so we can use terminal W on your diagnostic port with a 12VDC test lamp.

If you bought a 12VDC test lamp similar to the one I showed you in the picture in post #43, then you have all you need (except you may need some short small conductors, like extra wires, to help make contact in the diagnostic port - but you most likely will not need them).

In the diagram below, note that E1 is simply a ground terminal (same as any metal chassis object). +B is battery voltage which is present when the ignition key is turned to ON. To test, put your 12VDC test light alligator clip end on the +B terminal and the pointed end on terminal E1 with the ignition in the OFF position - should not have a light present. Then turn the key to the ON position. The 12VDC test light should light up. Does it? NOTE: If the alligator clip doesn't work with the diagnostic port terminals, you may have to put a short conductor in the diagnostic port terminal and clip the alligator clip onto that.

if it does, turn your ignition key back to OFF. Then take the 12VDC test light and put one end on the diagnostic terminal +B and the other end on the diagnostic terminal W. Test light should be out. Then turn the ignition switch to ON. Test light should turn on. Does it? If it does turn on, go to the next paragraph. If it doesn't, stop here and respond back.

If the test light turned on in the above paragraph, and because I don't know if you've disconnected your battery frequently and recently, let's start the engine with the test lamp still connected as described in the paragraph above. If everything checks out fine, the test light (now the temporary MIL) will turn off within a few seconds of the engine starting, or it may stay on indicating the presence of DTC's. What did the temporary MIL do?

If the test light stayed on, turn the engine off and ignition switch in the OFF position. Test light should turn off. With a separate jumper, connect diagnostic terminals E1 & TE1. Test light should still be off. Now turn the ignition switch to ON (don't start the engine). The MIL should be flashing - most likely slowly and indicating DTC numbers. Record the number or numbers it is flashing to you. These are telling you what problems the ECU has recognized.

I'll stop now and let you absorb this. Ask any questions if you are not certain. Otherwise, try this out. I think it will provide help to you. I'm pretty sure you will get an O2 sensor code and an intake air temperature sensor code at the very least (if neither are plugged in).


View attachment 412974

First of all, I'd like to thank you a lot for taking the time to help me and guide me through my learning journey. Whether you think it or not, you help me understand more about wiring. I did went through some trouble and stopped, so I'll tell you step by step what I've done. Do NOTE that I had the battery disconnected just prior to the testing and haven't started the engine after connecting the battery.

Step 1: E1 terminal is grounded properly. +B does not have power, instead seems like its a ground too. When trying to connect E1 to +B (I tried at the OBD-1 port and directly at the ECU connector), I had no power.
E1 wired directly to a fuse with ignition ON (let's call it FON) turns the light on and the same happenns with +B to FON. Since +B should already be IGNITION +B, connecting it to FON shouldn't turn it on (no ground) therefore I have an issue there.
In conclusion, E1 and +B are ground.

Step 2: I wired W to +B and light was off no matter what. I also tried W to FON and still nothing.
While I do not understand completely ECUs and the function of every terminal, would the culprit here be +B, which isn't Ignition Key ON, but rather a ground in my case? Is +B responsible to ''power on'' the W terminal? If so, having a defective +B would explain the problem.

I am open to wire +B (from ECU) to ignition Key ON myself if it's safe and okay to do so. I do want to avoid cutting the harness more, but will do if necessary.

 

I believe you have to ground the W terminal to ground E1 to check if the MIL bulb works. The ECU normally grounds the light to turn it on. This will manually ground it to check it.

 

Just did with Ignition Key turned ON and had no light on.

 

Either the bulb isn't wired correctly to the W terminal and the ECU or the bulb is burned out.

 

I'm really thinking that the wiring harness, even if it was meant for that engine and that ECU, has been so cut up and changed that the original wiring is way compromised. When I see Willhu's picture of his ECU & wiring harness, I see a mess. It looks like one of the harness connectors is unplugged from the ECU. I see wires cut away from the connectors that are opened and not capped. We could take the temporary 12VDC lamp unit over to the ECU itself and abandon the diagnostic port. But that area looks very fragile to me - as in if it is touched, something bad could happen.

+B is switched battery voltage feeding the ECU along with other items. I'm thinking the +B wire going to the diagnostic port was cut. If you think it might be a ground, you can get your DVOM (set it to ohms or continuity) and check the +B terminal of the diagnostic port to a chassis ground. Is there continuity or very low (if any) resistance value? If no continuity (or infinite resistance), then this wire was just cut through and perhaps the +B wire ahead of the cut is being used for another purpose.

Willhu, why don't you check the +B terminal on the wiring harness connector at the ECU with your 12VDC trouble light, with the alligator clip attached to a chassis ground and the tip pushed in at the +B terminal. Light should be out until you turning your ignition switch to ON. Does this work at your ECU?

 

Note that the engine starts and runs (I've driven the car on short distances like the way it is now). The connector in the picture doesn't belong to the ECU, most likely from the radio since all of that has been removed.

+B has no power with key switched ON and shows continuity when grounded with chassis.