Intermittant Brake Lights (Bad idea)

[Rockabilly Rebel]

That is weird. I see power to the tranny coming off the main relay, one of the 87 contacts, but that shouldn't affect the coil input, 86. My guess would be someone has been in there and has connected the tranny power supply to 86 instead of 87. Because what happens on 87 should not affect 86. Unplug the TCU if it's easy and see what happens, start wise.

Thanks for the help Mike.
Do you mean unplug the TCU, leave the rest plugged in and just start the car?
Thanks

[Mike W.]

Yep. Obviously the tranny won't be happy, but it will still have fluid circulating in it so I can't imagine any damage. Don't drive it, it would probable go into limp home mode, but that should tell you if it's the problem it appears to be. Or tear into the wiring right now for the main relay focusing on the red/blue going to the tranny.

[Rockabilly Rebel]

Yep. Obviously the tranny won't be happy, but it will still have fluid circulating in it so I can't imagine any damage. Don't drive it, it would probable go into limp home mode, but that should tell you if it's the problem it appears to be. Or tear into the wiring right now for the main relay focusing on the red/blue going to the tranny.

The red/blue is fed through the lower 87 pin. I unplugged the TCU and started it. It ran for about 3 seconds, then stalled because power was cut off to fuel pump (I had the tset light on green/red from OBC Relay). Then there was no power to the fuel pump for about 5 seconds, then it came back. So, I cranked again, it started, then stalled after a second, leaving no power to the fuel pump for about 5 seconds. It seems that whenever we change something, the car starts and idles for a few extra seconds, building up my hope that we found it, only to stall out again.

Any ideas?

[Rockabilly Rebel]

What is the process when the ECU goes from taking info from the refernce sensor to getting info from the speed sensor? And if you stop the car and restart it quickly, will the computer drop the speed info sensor and go back to the reference sensor for the start process? What is involved in that switching process? And could my issue be there?

[Machtig]

This is a shot in the dark but it sounds like this car has an auto tranny, yes? Double check it is in park or otherwise ensure the park interlock switch is working (if it has one?).

[Rockabilly Rebel]

It is automatic transmission. It is in park, I have tried starting it in Neutral, same result. I'll check that switch. Thanks.

[Mike W.]

If it was the neutral safety switch it wouldn't crank, so that's not it.

There should be two red/blue, one goes to the tranny, the other to the engine ECU.



You're fixed on fuel, so lets try this. Jumper out the fuel pump relay and see what happens. Typically if you pull the fuse to the fuel pump it will run for a few seconds. OK at first, then less ok as fuel pressure goes down until it stalls. But more than a second. But kind of gradually die as it goes lean and leaner, not a shutoff like a switch.

[Rockabilly Rebel]

Alright, so when I jumper #30 to #87 on the fuel pump relay, the exact same thing happens. Starts, runs a second, stalls. I have jumpered #30 fuel pump directly to Fuse #1 and it is the only time when I hear the fuel pumps actually spinning but it does the same thing. Only way to keep it running is to jumper #30 to both #87s on main or jumper green to green/red at OBC relay.
Thanks

[Rockabilly Rebel]

Maybe worth mention: The tach isn't working.

[Mike W.]

Alright, so when I jumper #30 to #87 on the fuel pump relay, the exact same thing happens. Starts, runs a second, stalls. I have jumpered #30 fuel pump directly to Fuse #1 and it is the only time when I hear the fuel pumps actually spinning but it does the same thing. Only way to keep it running is to jumper #30 to both #87s on main. or jumper green to green/red at OBC relay.

Ok, I think we're getting there. I hadn't seen that before. The OBC relay box is a known problem, connect Green to Gn/Rd and take it out of the equation. You should be back in business. You will lose the anti theft portion of the OBC, but who uses that anyway?

Maybe worth mention: The tach isn't working.

Now you should already be in business, but the tach is noteworthy. While I haven't heard of it affecting the OBC relay, the SI board and failing batteries can cause multiple instruments to fail and do weird things. And looking at the snippet of the schematic I posted the feed to the coil of relay box relay goes right thru the SI board, although named "instrument cluster printed circuit" But this generally applies to earlier SI boards with NiCad batteries and an '88 would have lithiums.

So... for now at least, connect Gn and Gn/Rd to bypass the relay box. That should get you running and I would not consider it a Mickey Mouse job, it's bypassing a known problem child. But to restore full instrumentation, and possibly reenable the relay box, it sound like you need to either replace the SI board or the batteries, although the late ones with the lithium batteries are generally not a problem. But it's still a 35 year old battery so anything goes, although it all comes off fuse 5 so tracing that circuit back to the fuse may be in order. But for now bypass the OBC relay box.

[Rockabilly Rebel]

Hi guys,
indeed Green to Green/Red gets the car started and working fine. So far, so good. When I get a minute, I will pull the cluster and have a look at the SI board. Also, I may check out the insides of the OBC. Why not have a bit of fun, right? Will keep you posted.
Thanks for all the help and input.