Need help finding electrical problem

[Dandy dog]

My 88 325ic is no stranger to these pages, and I've gotten a lot of help several years ago from several of the more experienced regulars, especially Geez Tech. Are any of you guys still patrolling this forum?
I've got an electrical problem that I need help diagnosing: The engine cut out in the middle of a trip back up the hill two nights ago. (I'm pretty sure that the problem is electrical because the idle air solenoid isn't buzzing when the ignition is on.)
I'm gonna go out and see what else isn't working, and I'll post some details later on. If you're still out there, GT, can you give me a shout?
Neil

[tanya_charbury]

Hi Neil,

I'm not a guru. However, I have access to one but he's forum-shy so I might need to be the go-between. Did it cut out and stay dead, or is this intermittent?

~T

[Dandy dog]

Okay: All the auxilary stuff is working (lights, signals, wipers, radio, etc.); what isn't is the idle air solenoid, the power windows and the radar detector (which is hooked into a switched hot lead (green wire) under the dash.) The fuel pump runs when jumped directly to the battery, but not when the relay contacts 30 and 87 are jumped.
My best guess at this point: either the main circuit breaker went south, or I've a short somewhere in the under-dash wiring. (The guy that owned the car before me had a stereo system installed, so the wiring is a rats-nest of unfastened connectors and loose wires.)
I am, of course, praying for the former, and I'll pick up a replacement from a yard later on today.
Anybody got any ideas or suggestions.
Neil

[Dandy dog]

Hi Neil,

I'm not a guru. However, I have access to one but he's forum-shy so I might need to be the go-between. Did it cut out and stay dead, or is this intermittent?

~T

I'd appreciate your help - so far I haven't eared from my rabbi, Geez Tech.
The problem is SO not intermittent: She's, like, stone dead.
Neil

[tanya_charbury]

Wow. So, "stone dead" is still open to my misinterpretation. With everything that you can turn of being off, then when you turn the ignition key on, to barely-not the crank-the-starter position, does the car seem normal as to what you see and hear? Then, when you turn the key to the crank-the-starter position, does it turn over but not fire the engine, or not even turn over? (Sorry if I should already know this without asking.)

~T

[Dandy dog]

Wow. So, "stone dead" is still open to my misinterpretation. With everything that you can turn of being off, then when you turn the ignition key on, to barely-not the crank-the-starter position, does the car seem normal as to what you see and hear? Then, when you turn the key to the crank-the-starter position, does it turn over but not fire the engine, or not even turn over? (Sorry if I should already know this without asking.)

~T

With the ignition turned On, all the auxilary devices (listed above) can be operated; the car looks and feels normal. When the key is turned further, the starter motor is actuated and the engine turns over but will not start.
From my previous experiences with the Motronic ECU, I've run some tests that show that the fuel pump is functioning properly, but it's not being activated by the ECU in Start mode as it should be. In addition (and possibly most significantly), the idle air solenoid doesn't hum and vibrate when the ignition is turned On: that's what makes me think that the main relay could be the problem.
I've gotten a replacement relay, and I'll sub that in in the morning; however, it'd be useful to know what the readings should be at the various pins of the main relay, in case I've got a short somewhere.
I've also got a backup ECU, but with the possibility of a short I'm kind of uneasy about trying that quite yet.
Thanks again for your interest: Please let me know if you need any more information, or if you've got any suggestions about test procedures that'd give us some more to go on.
Neil

[tanya_charbury]

Neil,

You evidently know a lot about what you're doing. I buy your point that the symptoms you're experiencing are relevant to the issue. Sadly, you're also beyond the point of sophistication where my guru can help you long-distance. So, it's up to you and I.

Let's think this through from a non-BMW electrical specialist perspective.

The folks at BMW and Bosch are probably concerned about how easily an ECU can be fried. If a short or bad voltage input from the outside world can make the thing self-destruct, that's not a very robust design. I make professional-grade software for a living, and I plan things to survive bad input, and if I can think of that, I'm pretty sure the folks at Bosch did, too. I don't have an 1988 325i ECU in stock or I'd offer it at a price that might entice you to try your backup unit.

Here's another option:

The problem is presumably either ignition or fuel, or both. A sad ECU could cause either or both, and also the symptoms you describe seem to fit well with a central cause, whether the ECU or something very central to the electrical system. Swapping out central components until the problem goes away -- this might be a good approach.

If you're trying to back into the problem, there might be value in isolating if it's fuel or spark, so the old trick of checking for spark might be good: take a spare spark plug, tape it to a clean ground and then attach the high tension lead to that instead of the plug that's screwed into the head. This is best done in dim light. Then, have a friend turn the ignition key so that the motor cranks, and see if you observe a spark. If yes, the problem is probably fuel. If not, then at least part of the problem is spark.

Going by heuristics, I've read that a weak spot on these cars is the component that helps the ECU figure the position of the flywheel, so purely from a betting perspective, that's not a bad place to look.

~T

[Dandy dog]

T -
My mentor Geez Tech told me repeatedly to diagnose the problem before throwing parts at the machine, so here goes: I'm almost certain this is an electrical problem, mainly because the idle control stabilizer doesn't purr with the ignition On (a really big red flag.) The fact that all the power windows no longer function ought to be factored into the diagnostic effort, also.

So tomorrow I'm gonna 1) check for voltage going into the ICS (which I'll bet is 0), then trace the wiring back to the ECU. Also, I'll 2) check for voltage at the window fuse (which I'll also bet is absent), then take a look at relay K5 (which is where the fuse gets current.) (Does anybody know if K5 gets voltage directly from the ECU? That certainly seems like where this thing is heading.)

[dandydog]

Guys -
Results from this morning's testing:
1) There IS current at fuse 17, so the windows may be a separate problem. (I've had trouble with them previously.)
2) However, there is NO voltage at the center contact of the ICS connector. I'll check continuity to the DME.

With the DME connected:
I've got battery voltage at terminal 30 of the fuel pump relay.
I've got battery voltage at terminals 30 and 86 of the main relay.
I've got NO voltage at the center terminal of the idle air stabilizer.
With the DME disconnected,
The main relay clicks when terminal 36 of the DME connector is grounded.
With 36 and 2 jumped, there's battery voltage at 37 and 18.
With 36 and 2 jumped, there's battery voltage at the center terminal of the IAS.
With 36 and 2 jumped, the fuel pump runs when 3 is grounded.
With 36 and 2 jumped, the injectors click open when 16 or 17 is grounded.

All of which looks totally normal to me (except, of course, for the IAS reading.) anybody see anything I'm missing?

I'm gonna send in the back-up DME.


Rats. Nothing.

For the time being, I'm out of ideas. If anybody's got any suggestions, I'll be glad to follow up on them tomorrow.
Neil

[dandydog]

There's a short in the wiring controlled by fuse #10 that sends 2.25 volts to terminal 87 of the main relay when the ignition is On.
Does anyone have a suggestion about how to proceed from here?
Neil
PS: Does the 2.25 voltage (a constant reading) remind anyone of anything? Does the step-down relay (K5) do what it says it does? What does it cut the voltage down to?