SC400 no start issue

[ST165ar]

This is a copy and paste from the Club Lexus forum. Hasn't got much traffic over there. However, this seems to be a more technically inclined forum.

Today my 93 SC400 started acting up on me. I went for a short drive and returned. Went back out a few hours later and would not start. The starter spins fine. During the no start issue, the check engine light does NOT come on with the key in the "ON" position. The fact that the CEL is not illuminated when the key is in the "ON" position gives a good indication that the EFI MAIN relay is a good place to start looking.

I replaced the relay with one from my Land Cruiser. Still no start. Put original EFI MAIN relay back in, still no start. Land Cruiser starts fine with both relays. 2nd try, I turned the key to "ON" (still no CEL), pulled the EFI MAIN relay and put it back in. This time there was some servo sounds, a few relay clicks, CEL illuminated and car started.I was able to replicate the results easily 3 times.

Did a little more reading on how the system works. I think I will start by checking for voltage at the IGSW and MREL terminals during the fail condition. My theory right now is that the ECU has internal issues between the IGSW and MREL terminals. If correct, I will have input voltage at IGSW and no output voltage at MREL. The only questionable issue is: what is happening with the key "on" and the removal and installation of the EFI MAIN relay that causes the system to work?

Ryan
93 SC400
92 Land Cruiser

 

[gloverman]

Check for power at the +B terminal. Test while turning the key on and off. I had an ecu doing the same which had a poor connection at the M-REL pin of the ECU. I knew the ECU was good as we had rebuilt it and tested it fine. You can test M-rel output at the relay itself.

 

[ST165ar]

Thanks for the tips. My computer hasn't been rebuilt as far as I can tell. Is the purpose of testing +B with key on and off to check for open circuit while off? I haven't considered +B part of the picture because it is reactionary to the EFI MAIN relay and MREL. I'll do some poking around soon and figure it out.

 

[George_R]

CEL power connects to ignotion switch rather than master relay output. ECU drives the other CEL's pin low. I would start tracing from ignition switch. I would use a 10Wt bulb to see if some wire has enough power (not only voltage)

 

[ST165ar]

Had a chance to finally dig into this a little more. Attempt to start car - CEL not illuminated when key is "on". Starter spins fine - car does not start.

Tested for voltage between IGSW and E1 on ECU - 10 VDC
Tested for voltage between MREL and E1 on ECU - 3 VDC
Tested for voltage at EFI Main Relay Socket and E1 on diagnostic connector - 10 VDC on pin 1 and 3
Tested for voltage between +B and E1 on diagnostic connector - 0

Reinserted EFI Main relay, CEL illuminated, MREL at ECU output is 9 VDC, +B gets 10 VDC at diagnostic connector, and car starts.

 

[Zuffen]

All those reading are at least 30% below what you would expect to see.

I'd suggest another fully charged battery and give it a go.

 

[ST165ar]

I tried the fresh battery, still same results. I've tested and chased my tail a few times.

When in the fail condition:
key in on position, no CEL.
IGSW to E1 at the ECU has always has voltage within specs.
MREL to E1 at ECU has voltage below specs, also confirmed at Pin 1 on EFI Main relay and E1 test connector.

This is what I originally though was happening, just had to confirm it a few times in my own mind

Also confirmed that MREL and E1 at ECU does turn off with the key, although thought this was supposed to stay on for a few seconds when turned off.
+B does not get power until EFI MAIN relay/ECU get power, so during the fail condition +B does not have power.
checked for loose/disconnected grounds and tested resistance. EFI Main pin 3 (the real pin 3, not the first time I thought pin 4 was pin 3) to ground, E1 at ECU to ground, and E1 at test connector to ground are all less than 1 ohm.

The 2 unexplained-

1. I put a small piece of test wire to test EFI Main pins without removing the relay. During the fail condition Pin 1 will only have 3 VDC, remove the relay and pin 1 now has 10 VDC.
2. remove and reinstall relay with key in on position, and most times this will stir something up and make things work

I'm just going to call it a bad ECU. I feel better about either tracking another one down or having it repaired now instead of just blindly throwing parts at it.
I did take the covers off of the ecu and some of the capacitors were leaking but, it was very minor compared to the pictures that I've seen about this.

 

[gloverman]

Another test would be to check the M-REL pin on the inside of the ECU. Running a wire externally for this circuit can also be helpful.

Half the ECU's we repair have no visable issues yet test with faults.

 

[ST165ar]

I do agree that would be the next step to further narrow faulty items. I feel that I do not have the proper equipment/knowledge to start testing the IGSW and MREL circuits within the ECU. I would probably do more harm than good. I'm confident that I've properly identified the fault, so further testing would not benefit me very much as I will not be performing the repair on the ECU. I will send a note with annotated trouble areas though.

 

[ST165ar]

Just a quick update.

Sent the computer off over the holidays to be repaired. Got it back and everything is working as it should. It has been 4 days of daily use without any trouble. I'm going to call this one fixed. Thanks for the help and ideas.