Did you check the coil for heat cracks?

 

That primary side coil resistance target seems incredibly low. Just think how much current would flow through that at such low resistance with a 12VDC supply. You might want to double check that spec. For my 1993 5S-FE, the primary side coil resistance spec is 1.1-1.7 ohms at a temperature between 14 deg F to 104 deg F. Your 3S-FE may be different, as you may have a resistor in the 12V feed to the coil. Not sure.

 

These are the specs from the manual.
I haven't checked for cracks specifically but I didn't see any obvious ones while checking the resistance.

You might want to recheck your '93 resistance unless '94 is different. My manual from the '94 I used to have specs are similar to my '90.

 

You might want to recheck your '93 resistance unless '94 is different. My manual from the '94 I used to have specs are similar to my '90.

My bad. Those 93 specs were for the 4A-FE engine, not the 5S-FE engine. Here are the specs for my 5S-FE engine (93 version). Much closer to yours:

 

Well that's good 'cause usually there are cracks.

 

I can easily get a new OEM coil for around $120 but could a slightly higher coil resistance cause any running issues? I don't know if my coil is original or not. The cap, wires and plugs look like they have been replaced not that long ago.

 

For that price you can get a brand new "World" brand distributor from RockAuto. I purchased one years ago with no regrets. Of course you're also getting new bushing/bearings, and 0-rings and coil.

 

Yeah all the aftermarket parts are usually cheaper but I prefer OEM for most parts but especially electrical parts.

 

Yep, that's what I used to think until I realized these cars are not Rolls Royces and you have to draw the line somewhere. I prefer OEM but...

 

Coil primary resistance specs call for 0.38 to 0.46 ohms cold. I checked mine and I get 0.90 to 1.0 ohms.

Touch your test leads directly together and subtract that from your original measurement. Testing resistances below 2 or so ohms is not easy - especially with standard multimeters. Your coil may very well be in spec.

-Charlie

 

I have a Fluke multi-meter that I've used for many many years and it has been really accurate for me.
I scored an NOS OEM coil for $20.

 

A quality Fluke with quality leads is helpful, but still isn't the whole answer below 1ohm... (I'm an electrical engineer with >20 years of experience)

-Charlie

 

I would get OEM cap and wires too.

 

Cap and rotor is on the list. I have to double check what kind of wires I have currently as well as plugs.

 

Only a high end test meter is accurate below 1 ohm, and getting reliable readings requires a controlled environment. In other words outside of finding an open circuit testing the primary coil is pointless.

 

So I have another idea. What about adding a known resistor in line while measuring that is over an Ohm. That way if I subtract it from the reading I may get a value that’s a little more accurate.

 

I would first check your manual for your Fluke meter and look for the measurement accuracy for the resistance readings. Let's us know what the manual specifications are for resistance readings on your Fluke meter. Then, when reading resistance with your Fluke meter, cross both test contacts/clips together and check out close to zero your resistance reading is. If it is zero or virtually zero, I would tend to trust the accuracy of your Fluke meter. You might want to get a known 1 ohm resistor and check it with your Fluke meter also to see how close to 1 ohm it reads.

 

I have a Fluke 23 which I've had and used since about 1986.
I found it's specs online from Fluke.

 

Yeah, as others said, your Fluke meter is not very accurate in the very low resistance range.

In the lowest resistance range (320 ohms) with a resolution of 0.1 ohms, the accuracy is +/-(0.5 + 2). The first number (0.5) is the percentage of the actual reading (0.5%). The second number (2) is the amount of potential error is the lowest resolution value. So, in other words, a 0.9 ohm reading on your Fluke meter could have an actual resistance between 0.69955 to 1.10045 ohms (+/- 0.2045 ohms). You need a much more accurate meter to read the actual resistance to know if you are between 0.38 - 0.48 ohms. However, even though your accuracy is not good enough for the resistance test it is good enough to indicate you are outside the range you should be in (0.38-0.48 ohms) as even the lowest possible resistance of 0.69955 ohms is above the tolerance range.

I'm not totally clear on the coils that have 2 difference resistance ranges on the primary side, but I think the coil you have with the lower primary resistance range uses a distributor wire from the wiring harness connector that contains a resistor in it. The coil you have in it now likely is made for a different distributor type that does not have this external additional resistor, so the primary resistance is designed to be a bit higher. I have a feeling your coil in your distributor now is not the correct coil. Not sure, but I'm leaning that way. I believe the Toyota Part No. for the coil for your car is 90919-02135 (not sure, would need your VIN to verify). That was used in a lot of Toyota models early 1980's to about 1991. You might want to go to a salvage yard and get a used coil that has a lower primary resistance and swap it into yours to see if it makes a difference. If you want to consider that, give me your VIN so I can verify your coil part number. Then I can tell you what Toyota models with what engines in what model years used this same ignition coil.

 

Even if you could test with 100% accuracy that doesn't help much the coil can fail when it gets hot via running engine.

 

This makes me wonder if I should buy a coil from Toyota to keep on hand

 

If this is available $20 shipped
Toyota 90919-02135 (9091902135) COIL ASSY, IGNITION

 

If this is available $20 shipped
Toyota 90919-02135 (9091902135) COIL ASSY, IGNITION

$20 from Dubai. $113+ from US dealers. Just wow.

 

Sounds too good to be true doesn't it. I'd trying buying one but already have spares.

 

I've done many transactions the site is legit.

 

Agreed - they are not great with communication and sometimes report availability/stock of things they can't get... but you always get your $$$ back if that's the case. Partsouq is also legit for OEM parts.

-Charlie

 

I bought it

 

Did you ever get the coil?

 

Did you replace the coil? Did it solve your problem?
I'd wager it did not.
I think you were barking up the wrong tree. Resistance reading are "in the ball park" and close enough.
Check the resistance on the new coil. It's going to be close. It is possible that temperature could affect coil performance but my experience is that the change would occur at high temps or wet conditions and hold for a while and (possibly) return to normal after cooling or drying for a while.
When you cleaned your throttle body, did you clean the coolant passages? Several components can affect the idle, including the AC gizmo and the aforementioned throttle body temp sensor.
Finally, the engine computer needs to re-learn operating parameters after the battery has been disconnected for a while. I believe it takes a few start, drive, & stop intervals to figure out what "normal" is.

 

I replaced the coil today and it did make a difference. I no longer have the slight surging while moderately accelerating. It feels like it has a little more power going uphill.
I don't recall the exact resistance readings I had on the new coil but they were in the low hundredths (0.10-0.19). The old coil was original and had some cracks on the back side.
I cleaned every orifice the throttle body and IAC had. I did that a week ago or two and even there was some improvement but I still had the light surge.
As you said it may take a few drives to fully see the results.

 

The old coil was original and had some cracks on the back side.

This is the issue. The cracks allow internal arcing and low/no spark voltage to the plugs.

-Charlie

 

See my post number two after OP's.

 

I finally got around to taking a photo of my old coil. The crack is pretty bad. I’m surprised it even worked. I wonder what happened to make that crack.

 

Oil + plastic = bad. Heat cycles too.

-Charlie