Reading Electrical Diagram

Lawrence Coppari · 09-04-2002, 02:58 AM

I have a 1986 951. It has an intermittent problem with the cruise control although for the past month it has been almost totally disfunctional except for a few minutes while in Atlanta two weekends ago. Here are the symptoms:

If it comes on, it works perfectly.
It does not come on now.
If I turn the ignition on and off it has come on before but not now.
It has never quit once on.
I have cleaned all grounds and tail light wires. I soldered the grounds to the plate in the tail lights.
Another list member sent me his spare cruise control. It did not work either. It is unknown whether the spare cruise module was in working condition, however.

I'm thinking it is not being powered up. I have the electrical diagrams but cannot discern where 12V should be. There are about 15 connections on the back and all I know is brown is ground. Anyone know which wires should be hot?

Lawrence Coppari · 09-04-2002, 05:26 AM

Just went over diagrams with an EE. Looks like #4 is hot.

AFJuvat · 09-04-2002, 11:32 AM

Don't know if you have done this or not, but check your relay. It is the one with the fuse tester on it (#16 I believe).

I'm sure you have already checked it, but a bad brake light bulb will keep the cruise from working too.

Lastly, what condition are your steering column switches in?

AFJuvat

Lawrence Coppari · 09-04-2002, 12:39 PM

Brake lights all ok. Talked with EE today and now can read wiring diagrams. Bloody things are like road maps . My childhood "Fun With Electricity" set did not prepare me for this.

drew1 · 09-04-2002, 09:45 PM

Anybody that can learn Road Atlanta can learn those diagrams.

drew1

Lawrence Coppari · 09-05-2002, 02:39 AM

Don't know about steering column switches but I plan to find out by checking voltages on module input when I move the switch on the column through its positions. Also plan to check relays. I'll bypass them with a wire/spade.

Plan to get to it this weekend. Will report on what I find. I have been after this problem for over a year. But now it is much worse. It used to work 80% of the time. Now it rarely works.

Anyone going to the BMW event in two weeks at Road Atlanta?

Lawrence Coppari · 09-06-2002, 02:57 AM

Switched some of the identical relays around last night but did not try car out. Drove to work this morning. Dash instruments now do not work. This says I have at least one bad relay. Plan to work on it this weekend.

drew1 · 09-06-2002, 04:54 PM

Sounds like you're narrowing it down.

drew1

Lawrence Coppari · 09-07-2002, 08:13 AM

Spent the morning on the cruise control. The module is getting power as it should. One part that should be a ground according to the diagram has a couple of ohms resitance to ground. I really don't know why there are two separate terminals on the plug to ground. Anyway, here is what I found.

With my module in it I can get the cruise to work if I disconnect and reconnect the leads at the servo under the hood. There is a big connector there with 7 prongs in it. When I pull it apart and put it back together it makes a big click. I disassembled the servo. One side has an electic motor. I cleaned the junk off the brushes and where the brushes contact the armature. On the other side of the unit there is a big electromagnet that engages the pinion from the armature shaft to a nylon gear. This is what makes the click sound when you pull the plug apart. The rest is all gears except for one gear that has two sweeper contacts. I cleaned the contacts and the path they ride on. I can see the resistance change nicely when the sweepers go along their path. They are attached to a gear that pulls on the throttle.

So, I really don't know what is wrong. Is something in the control module not working properly that necessitates disconnecting and reconnecting the servo? The other control module I have is totally dead. It put no voltage out to the servo.

Anyone have any ideas other than getting another servo to try?

Actually, I only use it when I go to track events. I can easily disconnect and reconnect the servo before I leave on the trip. You have to do it each time you start the engine.

I might put a hard ground on the pin that is showing some resistance. The diagram shows it should be grounded just like a nearby pin that is.

Brandon85-944 · 09-07-2002, 08:47 AM

Here some helpful links:

These guys fix CC's:

http://www.fix.net/~gdl/begin1.html

Look at Kaos's reply in this post:
cruise control that sticks

Hope this helps...

Brandon85-944

Lawrence Coppari · 09-09-2002, 04:01 AM

Spent a bunch of time on the cruise this weekend. Here is what I found.

The switches on the steering column work properly. From the diagram, I can determine where and when voltage should appear at the input to the circuit board when I move the switch. It does.

The servo seems ok. I removed it from the car and determined which leads make the motor work. Motor always starts no matter where the armature is stopped. The electromagnet always works when I touch it off with 12V. It causes a gear to engage the armature pinion.

The resistance of the slider brushes varies nicely as I move them.

I have noticed that if I start the engine and let it idle, if I pull the plug at the servo apart and reconnect, the electromagnet makes a click sound. Then the cruise will almost always work. Before I pull the plug apart, I move the cruise stalk to the set position. I had the rear of the car raised and cruise disconnected from throttle linkage to do this. Cruise will not work properly without a big load. It is unstable with no load - rear wheels raised.

Cruise will also work by doing the unplug/plug thing without rear wheels raised.

It's beginning to sound like it's my module. I only use the cruise to get to track events. I can do the unplug/plug thing before I go. Only other thing I can think of is there is no signal from speedo circuit, but speedo works fine and has always worked fine. Once the cruise begins working, it continues until you shut engine off.