e12 ecu issues

centralcoaster · Post by **centralcoaster** » Thu May 26, 2016 5:10 pm

I have a 78 530 which I am trying to get running again. It was my main car until the gas prices started to climb around 10 years ago. Since then I have tinkered with it a little but not too much driving. for the last couple of months I have been trying to get it to run without much success. I have verified the fuel delivery system is working properly and the cold start injector is functioning. I just found a problem in the ignition system.
sometime in the past while messing with things under the dash, I miswired the tach. this effectively shorted out the pulse signal from the coil/distrib. the result was no spark. With the black wire from the distributor now disconnected from the tach, there is pulse signal to the ecu and spark to the plugs. but there is no pulse to the injectors. I got a noid light set and plugged the lignts into injector plugs for #1 and # 6. no light comes on when the starter is engaged. I disconnected the ecu main wiring harness and found a good pulse signal on pin #1. I alsp tested the ground connections at pin # 5, 16, and 17. All checked out fine. I installed a second ecu with identical part # as my original. Same problem.
am I missing something here, or does this indicate that both ecu's are bad?
could my miswiring of the tach have caused my original ecu to fail?
if I can't find an identical ecu , which others can be substituted.
I did a lot of reading in the forum and couldn't find answers. Hopefully some of the gurus here can help

RonDwyer · Post by **RonDwyer** » Thu May 26, 2016 8:56 pm

We spoke last week. I have an ECU from a 77 530i if you are interested. It worked when I pulled the engine.

franky2fingaz · Post by **franky2fingaz** » Thu May 26, 2016 9:11 pm

have you looked at the harness itself? did you test with harness connection right after ECU...?

ECU does need RPM signal from ignition to operate.. if that helps

L-jet pdf:

http://www.firstfives.org/faq/ljet/jetronic.pdf

centralcoaster · Post by **centralcoaster** » Fri May 27, 2016 1:01 am

ey thanks for responding.
franky, I have visually inspected the harness and all looks fine. I have soldered new terminals on all of the injector plugs , temp plugs and all of the other connections, and I was careful to observe the correct polarity on all of them. So I don't think that is an issue.

Ron thank you for your time and input. since our conversation i have verified that the injectors are not getting the correct signal to fire. After speaking with you I went back over the wiring diagram, found that I got no spark signal to the plugs with the wire which runs to the tach and the ecu, but when I disconnected that wire, placed a jumper ftom the coil to the distributor then I got spark. that made me look more closely at the wiring . when I disconnected the wirnig to the tach I got spark with the original wiring.
so now I have gas, spark and air, but no injector pulses
should I be looking at the dual relay, does that do more than just work the fuel pump?
Pls. I have "bench tested" all of the injectors, connectimg them individually to gas under pressure and putting 12 volts to the injectors and all work.
as for your ecu, ron, you may hear from me soon
thanks again

RandyM · Post by **RandyM** » Fri May 27, 2016 11:00 am

I have a pinout list from an unknown Ecu and diagrams from an 80 & 81 that list injectors as ECU 14,15, 30-33. The injectors 14,30,31 (injectors # 4,5,6 share a lead to dbl. relay (I can't read the number 88b maybe, that it also is connected to Ecu 29 brown ground(?). The other injectors, 15,32,33 (inj. #1,3,2) share a lead to dbl relay 88e if I read correctly. Double check your Dbl. Relay connector numbers.

Pm me your email if u want the files.

RonDwyer · Post by **RonDwyer** » Fri May 27, 2016 9:20 pm

Ahh yes, vindication. Process of elimination to verify everything attached actually works. Bad grounds actually cause some of the weirdest faults in the lighting system.

centralcoaster · Post by **centralcoaster** » Fri May 27, 2016 11:25 pm

gave it another look today. from a tech manual for vw l jetronic, it said :with key on pins 88b and 88z of the dual relay should be hot and I read 12 volts on each. then back to the ecm harness I got 12v on pin 10 which is for the main relay and pins 14,15,30,31,32, and 33 which are the injectors. ground pins 5, 16,17,35 all checked out
still have spark, still have fuel pressure but no pulse to the injectors. pulse to the ecm from the coil/distributor is good.
what am i missing?
I tried a second identical model # ecm with the same results.
From what I have read these units are pretty durable, one bad unit is not out of the realm of possiblilities, but 2 bad ones?

bestow knowledge on me O Wise Ones

RandyM · Post by **RandyM** » Sat May 28, 2016 11:22 am

There's a pretty comprehensive combo relay troubleshooter here:

http://www.njfiats.org/joomla/images/st ... BoschL.pdf

Also a clear electrical diagram here:
http://www.bowlsby.net/914/Classic/zTN_Man08.pdf

I'm guessing bad ground. Is there pulse at each injector pin on ECM? DOne a test light on injector plugs while cranking?

PS After a quick trip to the hardware store yesterday,I couldnt start, no crank, nothin. Half hour later I discover I was in Drive. Still have dent in forehead from palm slap.

centralcoaster · Post by **centralcoaster** » Sat May 28, 2016 10:03 pm

with the ecm harness disconnected, there is a pulse at pin 1 from the coil. I did connect noid ligts on injector plugs for cylinder 1 and 6, but no pulse there. I have tested all of the ground wires for continuity, and cleaned the ends where they bolt to the upper intake manifold. I haven't done a voltage drop test at any of the grounds, thinking that the output current draw to each of the injectors was so small that just checking the continuity would be ok.
I guess I will check that out tomorrow.
thanks for all of the input.
I hate electrical problems, especially when I don't really understand the circuitry

centralcoaster · Post by **centralcoaster** » Sun May 29, 2016 3:11 pm

i just ran a voltage drop test on all of the ground wires in the ecm wiring harness and the main ground on the top of the intake manifold. I connected my $5 harbor freight voltmeter directly to the battery terminals, then with the positive still connected to the battery connected my negative vom lead to the grounds at the ecm harness and the harness ground on the manifold. there are no voltage drops of any significance.

RandyM · Post by **RandyM** » Sun May 29, 2016 4:14 pm

Is there continuity from hot thru the dbl relay(88z) to the ecm end of the harness on injector circuits i.e. 14,15,30-33, and 29? Key on. note that 88a connects to 29, see http://www.firstfives.org/faq/ljet/528i ... squirt.PDF ignore the meagsquirt parts

Also since you said you messed with the under dash wiring, I'd be double check for faults to anything you may have changed/fixed.

PS does the starter work? per my, ahem, mishap yesterday, could it be a shift indicator issue assuming you have auto trans.

centralcoaster · Post by **centralcoaster** » Mon May 30, 2016 8:35 pm

went back to the drawing board and may have found the problem in the dual relay. more testing after getting a new relay.
I did a google search for the bosch part # which was 0 332 514 103 and didn't have any success. I found a relay # 0 332 514 120 on fleabay which claims to be compatible. anyone know if this is true?

centralcoaster · Post by **centralcoaster** » Wed Jun 01, 2016 2:13 pm

looks like another misdiagnosis. after further testing, the dual relay seems good. my dvom was low battery and couldn't test the diodes. with new battery in the dvom all is well.

so rechecking the wiring harness from the ecu. All wiring is good.
Now I am back to my original question. there is spark to the plugs, coil pulse to the ecu, fuel pressure is good, but no signal pulse to the injectors.
Is this a bad ecu or is there something that I am missing.
I am 99% sure that it is not bad grounds as I get continuity to ground and there does not seem to ba any significant voltage drop between the ecu wiring harness and chassis ground.
As previously mentioned, I did get a second ecu online. the results are the same with both ecu's

centralcoaster · Post by **centralcoaster** » Wed Jun 01, 2016 9:48 pm

i just put in a new dual relay , but the problem remains. no injector pulse. with everything that i have done to date, and checked and rechecked, can the problem be anything other than a bad ecm?

RonDwyer · Post by **RonDwyer** » Sun Jun 05, 2016 12:06 pm

That is challenging. I'd open up one of the ECU's and look at the circuitry very closely. The little current tracks on the circuit board are called a 'Trace' and sometimes these can get broken. Use a magnifying glass. Look closely at all the traces and connections to the plug, when in doubt touch up the solder joint with an iron, not a gun. Broken traces will tend to arc when they sever. A black smudge mark on the varnish is a telltale sign.

Rust inside the box is a bad sign.
Lots of dirt and dust is bad as well.

Fixing a trace depends on your skill and the location. Usually just a piece of wire can be soldered to either side.

Let us know what you find. These ECU's are about as complex as a transistor radio.

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