You definitely have either a dry joint or cracked PCB track. My first guess with bending the PCB would be a cracked track. Easiest way to find these is with a light behind the PCB.

This sounds like the connector as Greg has alluded to.

First Rule of fault finding is start at one end and systematically work your way through. Repairing systems with multiple faults is difficult even for the experienced professional technician.

I would address the connector first as you have reasonably diagnosed this as faulty. Clean the contacts on the ECU with 99% IPA (NOT rubbing alcohol)and a cotton bud. Then look at the plug (I have never looked at one of these so I don't know what type of contacts they have) clean the contacts and re-tension them. Put it back together and test. DO NOT move on to the ECU tracks until you 100% confident this is fixed.

 

I agree with that multi pin socket having issues.

I have used Lemon Juice here on numerous units, including mine. Rinsed off with Metho, and of course care used when doing the clean.

Being a 1985 car, I suspect a 6CU spec ECU. These are well noted as having fuel pump control circuit issues, and other erratic events.

Most of these have been replaced with the 16CU unit, that is way more reliable and better in all areas. It is a plug and play swap.

 

Greg,
Bingo, you were exactly right, that glitch was due to a poor connection at the computer plug. (wasn't snapped in) Silly me. I'll do what you say about cleaning the connection because it's surely something back there that's causing the flood problem.
thanks, Bob

 

I had a bit of time today to poke and prod and flex the pc board while it was plugged in with key on. It's definitely the pc board with the rectangular hole in it that's giving trouble. Flexing the board one way or the other will make my injector test light go on and off. Also even pushing quite gently on some components will make the light come on. I'm going to try Warren's trick with the lamp behind the pc board now that it's dark, and I'll get those connections clean tomorrow. Oh, it is a 6cu
Thanks for all the info. We're getting close now.
Bob

 

In that case get a 16 CU from ebay or somewhere and spend no more time on the 6 CU. Much better item.
Greg

 

With the computer pins and plug cleaned I tried again to see if my test lamps would stay off with the ignition on and not cranking. My hopes weren't too high thankfully, as the computer still failed. When one of the pc boards was held and flexed one direction the test light would light, and would go out when the board was flexed the other direction. This result was consistent and could repeated every time. Myself and others could not find a visible flaw in the board (with a lot of looking) so Greg, I'm looking for a computer. I see that a 16CU was used in a 6 cyl. as well as the V12. Would I have to get one that was specifically out of a V12? That seems like a stupid question but with the magic of electronics, maybe the computer somehow knows what engine it's managing. ???
thanks, Bob

 

Get a specifically V12 one. Loads on UK ebay. But you probably have cats so a USA spec one would be safer, though I doubt there is any difference. Note that ONLY the 16CU will work, later ones (26CU I think or anyway a higher number) will not.
Greg

 

I don't think the UK market cars had O2 sensors in that era, US/Canadian cars did. I would get the proper spec. There shroud be a sticker on it with the part number, the 6CU, 16CU etc is just a generic family of ECU's, not one for a specific application.

 

There is a crack in that PCB for sure. Shine a light on the the Front of the PCB and inspect it with a magnifying glass. Basically shine the light through the board.

The ECU does not know what engine it's connected to but it will be programed for certain parameters based on the input information from sensors.

 

I had pretty much given up on the computer when a friend with a lot of analytical know how and a bit of printed circuit experience volunteered to have a look. At the end of about 2 hours of flexing, inspecting, and poking the electronic parts of the board, finally the culprit solder joint did manifest. With me holding the remote injector test lamp (which reminded me of my childhood where my father worked on the car, and I held the trouble light) and Jim poking various components, the problem was discovered to be a cracked solder joint on one of 14 connections of a chip. The chip measured only 16mm long so that's why it was so tough to spot. After soldering, no amount of flexing or tapping or anything could make the test lamp light, so everything got put back together. It fired up pretty much instantly and is purring like a Jaguar kitten.
Warren, you nailed it. Thank you to all of you that guided me with your suggestions. It was quite the ordeal, and quite the learning experience.

 

Very good news, thanks for letting us know (an important coutesy that I regret to say is far from universal) and very well done you. Also we bow before Warrjon's superb diagnosis.


Greg

 

Well done from me also.

I would still have a 16CU on the shopping list.

 

+1 on the 16cu. I had a 6cu in my '85 with intermittant failures, and corrected it by resoldering suspect connections, although I never found an obvious bad joint. However, the 6cu's are notorious for cracked joints, while the 16cu's are not, so I opted to buy a 16cu for peace of mind.

 

I'm glad you fixed it, those pesky dry joints can be a bit tricky to find even for experienced peddlers.