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I just went to check my son's Avalon (we have 5 in the family). I checked his relay voltages which are as they should be term 5 12.07v, 1, 2, &3 are 0v.
The above is correct for a battery connected, no key in ignition situation.
Terminal1 is 10.9v,
2 is 0v,
5 is 11.94v,
3 is 2.28v
The above is not correct for a properly operating system.

With the EFI Relay removed, battery connected, key out of the ignition (key in OFF position), you should get these values:
  • Terminal 5: 12.6 VDC
  • Terminal 3: 0 VDC
  • Terminal 1: 0 VDC
  • Terminal 2: 0 VDC
So your son's Avalon is good. Your own Avalon is not (plus your battery is weak/undercharged). The 10.9 VDC on terminal 1 is very low, likely causing very low magnetic output from the coil side of the relay, and causing the contacts to open/close rapidly. It is not necessarily the fault of the ECU though (but it certainly could be). There could be a problem where a live power wire "B-W" (or black insulated wire with white stripe) is contacting the "L-O" wire (or blue insulated wire with orange stripe) within the wiring harness somewhere. A good way to figure this out would be to disconnect the "E" wiring harness connector (a 22-pin wiring harness connector) at the ECU and then check the voltage on all 4 EFI Relay socket terminals again. If the problem with voltage on terminal 1 remains, the ECU is not at fault - there has to be a wiring harness defect causing a live wire to be in contact with the "L-O" wire feeding EFI Relay socket terminal 1. If the voltage on terminal 1 goes to zero with the "E" wiring harness connector removed from the ECU, then the issue is within the ECU.

So a little more diagnostic testing and you should be able to narrow down the source of the power problem.
 

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Discussion Starter · #22 ·
I'm starting to resign myself to it being the ECM as well. I know that the battery was disconnected in the parts donor car, but I'm not certain it was on the "new" car.... I usually remember to disconnect the ground cable, but I was in a rush. I didn't change the switch. Just the key cylinder and the ECM.
When I pull the 15A fuse, terminal 1 still shows 10.9v.... which it should NOT. (I mistakenly said earlier that it should. It should be 0v)
馃 I still have the old ECM ( but no matching chip key). I could switch it back in to see if Terminal 1 returns to 0v rather than the 10.9v.....
 

Long-haired Southern-Squidbillie
2004 Camry 2AZ engine; 2018 Camry LE
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Good luck JV, hope you get it sorted out.

There are some Always Hot power feeds to the ECM and other modules. If their connectors are removed/installed while power is live (Hot Swapped), then there is a chance to blow some internal components. Live circuits with current flowing also don't like having their ground path interrupted either, so safest thing is remove both battery terminals whenever messing with ECU connections.

i don't know or see how the ECM could be getting a power feed when the 15A fuse was removed, so it could be a wiring issue; but 93CC has some good suggestions so go down that path.

EWD page 170
  1. (8) Battery signal circuit
    Voltage is always supplies to TERMINAL BATT of the engine control module.
    If you turn on the ignition SW, the current goes from TERMINAL MREL of the engine control module to the EFI relay and put on the relay, and the voltage related to the engine control module operation is supplied to TERMINAL +B of the engine control module through the EFI relay.
Found a wire color diagram, that EWD manual is great, gotta find one for my cars.

Rectangle Schematic Font Slope Parallel
 

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Discussion Starter · #25 ·
Back at it again for an hour or so. With E connector unplugged at ECU, Terminal 1at the EFI Relay goes to 0v. So, unfortunately, it does look like I've got an ECU issue.

So, a thought occurred to me. If I install a manual switch inline on the L-O (blue-orange) wire between the ECU (MREL -8) and the EFI relay it would stop the power flow to the relay when the car is turned off. Seeing as the car runs well in all other functions I may be lucky enough not to have other internal ECU damage. What do you think?
 

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I also still have the original ECU which was fine other than I lost all the chip keys that were stored in it. Is it somehow reprogramable for a decent price?
Found this out on the internet:
Product Font Rectangle Screenshot Circle

I think this bulletin has been revised significantly since July 10, 2001, and is no longer being followed by Toyota. There were later years where it was possible with a special scan tool to obtain the original code and to modify it to a new code pertaining to a replacement set of chipped keys. But since the control on your 2000 Avalon was built into the ECU rather than an immobilizer integration module, you would need a new ECM along with the keys programmed for it.
 

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I also still have the original ECU which was fine orher than I lost all the chip keys that were stored in it. Is it somehow reprogramable for a decent price?
Given you said your old ECU was fine, just to make absolutely sure your replacement ECU is faulty, why don't you plug your old ECM back in again (with connector E plugged in) and check if you still have 0 VDC at terminal 1 in the EFI Relay socket. If you have 0 VDC with your original relay, then without doubt your replacement relay is faulty. If your old relay is faulty on this terminal, I would be worried that it has problems that you may not have experienced yet, or your may experience soon, in that there may be a bad capacitor on the board that is leaking and the corrosive material may spread to other circuits. If it was me, I wouldn't trust that ECU. Maybe you can get your purchase price returned to you if the seller is a decent one.

What is the part number on your ECU cover? Or better yet, provide a picture of the label on the top of your ECU cover.
 

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There is also this video of a replaced ECU on a 2003 Avalon, which the person later uses the OBD2 port to short 2 terminals for at least 30 minutes (I've seem the same practice used in other internet sites that run a 50-60 minute period to clear the ECU, then start the car with their original keys. Don't know if this 2003 Avalon example applies to your 2000 Avalon. If it does, I would expect the reverse to work then, using your replacement keys with your original ECU. I'm not suggesting you do this, as I can't find a 2000 Avalon example on a you-tube where the same procedure works for them. Just wanted to let you know about this.
 

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Discussion Starter · #30 ·
Good video. His ECU part number is different from mine. I'll do a little more digging before trying to clear my original ECU. I noticed my brother's 2003 Avalon doesn't have the underhood diagnostic connector (on the passenger side of the intake), so I guess there was a change between 2000 and 2003.
And of course, just to make life interesting, my car is now behaving totally normal. Lol
 

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If normal, I wouldn't make any attempts to change anything until it starts doing the issue with the EFI relay clicking again. It probably will, but you never really know for sure. If everything is normal now, as that voltage on terminal 1 now at 0 VDC when the ignition is OFF?
 
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