Hello, I am new to the forums, basically since my 1988 740 is a relentless tank, I have been able to figure out most things that have gone bad.

I have hit that limit.
The electrical system has been the weak link, and it is not my strong suit.
The car will now, seemingly randomly, cut off, by cut off I mean all power to the vehicle is shut down. No dash lights, no radio, no "ping ping ping" sounds, nothing dead. This can happen while driving, or when I get into the car and try to start it. Then after some time the car gets power again and starts right up.

I am wondering what the culprits could be other than the battery connections being bad.
I recently cut the positive terminal wire cleaned it and reattached with some dielectric grease, So I think that is good now.

I am thinking that there may be some relay going bad or the ignition switch.

I hope that someone on this forum can help me trouble shoot this, before I get stuck for good.

 

How old is the battery? You can get it checked at the big box auto part stores if you aren't sure about its condition.

You could try checking ground connections, start with the battery first then work your way forward.

You didn't mention the negative cable or cleaning the battery posts as well, that would be quick and easy.

I hope it is resolved soon. Post back the answer, you will find it.

Good Luck

 

I checked the negative pole of the battery and any ground wire I could find. They all seem tight and free from corrosion. However I do have major crumbling of the insulation on my wires under the hood. I believe this is a common problem with the 1988 740 wiring harness that they fixed in 1989-90. But I have already had to unplug a few sensors and 'paint' on some insulation due to total bare wires. I cannot find a replacement wiring harness and if I could I am afraid that it would be too much $$$.

I picked up the car for $600 about 12 years ago and have been doing what I can to keep her running.
Though car is a rust bucket, holes everywhere, the engine and transmission are still strong. I would like to do what I can to keep it on the road and squeeze every last mile out of her.

Back to the problem at hand:

It seemed like it would happen more frequently if I engaged the right turn signal. By that I mean a few times just as I turned on the right turn signal the car lost all power.

If I wait a few then I get full power back and the car continues on its way. If I have to restart the car it cranks strong and starts after turning over just once.

This made me believe it may have been a relay.
[About 6 years ago I had to fix the Bulb (Red Cylinder) Relay by soldering a connection (lights were not working). ]

So... I unplugged the turn signal wires from the steering column and sure enough for two days the problem did not pop up, almost long enough to make me think I was on to something. Well this morning I go out and the car turns over not even once before all power is lost dead again. So though maybe still a relay possibly not directly related to the turn signal.


Is there a relay that could cause this...and does anyone have a relay diagram that they could share?
What is the radio suppression relay?

Thanks,
-R

 

Could the Fuel pump/ F.I. relay cause the car to lose power? I pulled mine and noticed some charring around one of the poles.

 

i can't see any one of those relays causing *everything* to cut out the way you say. sure, teh fuel pump relay would kill the engine, but the lights would still woork.

I'd measure voltage differential between the battery negative post and the engine block when the engine is running. any more than a .1V is a likely sign of a ground problem.

 

What about a bad ignition switch?

 

complete engine wiring harness for a 88/89 740T, last one. RBR is closing their store, selling everything they have in stock off cheap.

Engine wiring harness new 740 turbo 87-89

 

It could be the ignition switch, though I think that something would stay one, or flash...

Have you tried jiggling the key when this happens? Bad ground is likely, but it's weird that sometimes it happens when you initially try to start it. Did you jump on that wiring harness?

 

I will call about the harness... still $250 is not exactly cheap.

Yes I have jiggled the key, I have also inspected the ground wires and the positive wires.
I guess I would like to be sure it is not the IGN.Switch b4 I fork out $250 for a wiring harness.

However, now that you mention it, the clock stops and the door lights that are on the inside frame also shut down when this happens. So I guess that tells me that it is more than the IGN switch.

-R

 

Wiring harness sold...I will keep looking...
It seems that the wiring harnesses are rare, I have emailed David Barton, and checked IPD Classifieds, ebay etc, and I have so far come up empty.

 

I sympathize..my 84 760 had the worst wiring of all time. The Titanic's wiring is more serviceable. The symptoms you desrcribe do point to your ignition barrel...my 1990 940 sometimes does the trick where it starts, but nothing else works...the wife is baffled by this. YOu have to release the transmission manually and you can drive about, but nothing, no lights, noises, indicators or anything will work. Switch off and switch on again, its all good. Replace your ignition barrel and I suspect it will fix it.