Cruise Control Gremlin

[Por_sha911]

Symptoms: cruise sets and holds but then slowly loses speed (5 mph over a matter of a 2-5 minutes). When I hit the brakes and then reset, it goes back to what ever the last held speed was.
It holds better at slower speeds (35-50) but won't hold on the highway (60-80).

Checked:
-hoses for vacuum: when the motor is shut off when I pull off the vacuum hose that runs from the engine to the cruise acutator in the engine compartment, I get the sound of air hissing from the release of vacuum),
-ran diagnostics via Bentleys.
-all switches work just as they should.
-all solder joints on board look good

Latest attempt: replaced two capacitors (see pic below).


Any suggestions? Is there something else I can change on the board?
Help!

[RickM]

Joe, have someone ride with you and have them spray component cooler when the problem appears. Not sure if possible to have the circuit exposed but thought I'd suggest it.

Did you go over each solder joint or just visually inspect them?

[Por_sha911]

Visual but even with a magnifier they look solid (car only has 39K). Are you saying spray component cooler directly onto the board? Where do I find the stuff?

[BURN-BROS]

When you checked the hose for vac, did you isolate the line, apply vac (typically done with hand pump that has a gauge)and measure to see if you have a slow leak? It is usually the diaphragm or old/damaged hose that causes that condition you have described.

Aaron

[targa80]

In all the time I have been repairing electronic's it is rare that a cap would be the problem. Unless you have a schematic of that cruise control circuit card I would say you are troubleshooting in the dark. You should see if you could borrow someone elses controller that way you will atleast know your on the right track.

[RickM]

Originally posted by Por_sha911
Visual but even with a magnifier they look solid (car only has 39K). Are you saying spray component cooler directly onto the board? Where do I find the stuff?

Component cooler is sold at electronic supply houses or at Radio Shack. When the circuit is failing you spray the cooler on one component at a time. If the failure is due to heat then the component may be identified by this method.

I have repaired two items this way, both leaking caps. But that's two times out of many...

I'd try the other suggestions as well. Swapping out a known working unit should be simple enough.

[Por_sha911]

Originally posted by BURN-BROS
When you checked the hose for vac, did you isolate the line, apply vac (typically done with hand pump that has a gauge)and measure to see if you have a slow leak? It is usually the diaphragm or old/damaged hose that causes that condition you have described. Aaron

There is a vac line running from a "t" to the acuator. I pulled that hose off with the car having been off for 2 days and got a hiss of air. There is definately no leak problem there. Am I reading this condition wrong?

it is rare that a cap would be the problem

I've had several folks tell me that the capacitors are the first thing to go and since they are dirt cheap it made sense to try them first.

When the circuit is failing you spray the cooler on one component at a time.

When you say circuit, do you mean resistor, component, or just printed wiring on the board?

borrow someone elses controller

Without further testing, I can't say that the problem happens 100% of the time. It seemed to work sometimes but I may not have noticed that it was above or below 50 MPH (see original post)
What would be the symptoms of a bad acutator? (I'm assuming that this is the correct name for the cannister thing in the left side of the engine compartment where the vacuum runs in.

Thanks for the help.

[BURN-BROS]

Originally posted by Por_sha911
There is a vac line running from a "t" to the acuator. I pulled that hose off with the car having been off for 2 days and got a hiss of air. There is definately no leak problem there. Am I reading this condition wrong?

Thats strange, I don't think that the system would have any vac until you required cruise control. Where are the valves located to introduce and vent the system?The valves may be part of the actuator.I would guess that you pulled the line before the valves. I would test the actuator itself and know for sure it isn't bad.

Aaron

[Por_sha911]

The arrow points to the hose that holds vacuum. Is this correct or is there a problem with the accuator? Remember, it does work, it just won't hold speed for very long at higher speeds.
I'm assuming that if it's hissing when I pull that hose, it MUST be holding vacuum with no problem. True?

[BURN-BROS]

Joe, the actuator should only need vac when in use. If vac was on at all times the actuator would engage at all times.

The system should work like this.

A valve would open to let vac in to the actuator to set it to the desired position(speed). Note that the system should not have vac until system is utilized.

Another valve would vent the preasure to atmosphere to disengage actuator.

The control box should modulate these valves when going up or down a grade or when you step on the brakes.If the actuator was actually storing vac it would be at WOT.


Are there electrical wires that go into the actuator? If so the valves are internal and your test was not accurate. One caveat is that the systems that I studied were domestic cars. I believe that the systems are fundamentally the same. If the chance that I am about to eat crow I wanted to throw that out there.


One more thing to check are the switches at the brake pedal for proper alignment. Earlier systems had a vac valve down by the brake to vent the system.

I hope that this helps you find out the problem.

Aaron

[RickM]

Originally posted by Por_sha911

When you say circuit, do you mean resistor, component, or just printed wiring on the board?

You spray each component (resistor, capacitor, transistor, diode) on the circuit board to see if the unit reverts back to proper operation. Do one component at a time.
Many times capacitors will experience "leakage" when the heat from operation sets in. The coolant often reverses the failure temporarily.

[ChrisL]

Don't know if this'll help, but I've got an old one from my '86 sitting in the garage gathering dust. It had the same symptoms as your's does -- slowing slowing down from the set speed. I replaced it and the cruise control started working perfectly. PM me if you want it.

Chris.

[Por_sha911]

Thanks to everyone for speaking up.

Originally posted by BURN-BROS
Joe, the actuator should only need vac when in use. If vac was on at all times the actuator would engage at all times.
One more thing to check are the switches at the brake pedal for proper alignment. Earlier systems had a vac valve down by the brake to vent the system. Aaron

I'm refering to the vacuum line running from the engine (constant vacuum) to the acuator (uses vac as needed) is holding vac. THis tells me the line and the actuator are not leaking. Yes?
The cruise disengages from the brake light switches located on the MC-see original post: both were replaced.

RickM-I replaced both caps already.

ChrisL-I appreciate the offer but if your brain box is doing the same thing then I already have one of those. Thanks for speaking up because it tells me that the problem may still be lurking somewhere on the board. I hate to start blindly replacing parts (especially if they can be fixed). FWIW, where did you get the replacement and what was the cost?

[Lorenfb]

"I hate to start blindly replacing parts (especially if they can be fixed). FWIW, where did you get the replacement and what was the cost?"

Best Troubleshooting Approaches:

1. Find a good module to test with.
2. Test your unit in a "good" car.

Any other way given the type of problem described is VERY difficult.
Also, I'd advise against R/R of any components, as this usually results
in other problems or a bad cruise control for sure. It would be like
attempting to fix your video board in your PC by starting at the component
level for troubleshooting. You always start with a functional test when
the component count exceeds a few components.

Check out Pelican Parts if you need one, as they many sources for rebuilt parts.
If you can't find it on the web site, call Tom Gould at Pelican.

[ChrisL]

It's been a few years, and I gave all my receipts to the purchaser, but I'm pretty sure I got it used from a recycler. It was $60, as I recall.

Chris.

[Por_sha911]

Loren-can you suggest any reason that I can't try shooting coolant on the board to see if that affects it at all?
Now all I need to do is [get this] find someone with a perfectly working cruise unit. Right. What years will match my system? Anyone? Anyone?
Buehler? Buehler?

[Lorenfb]

"Loren-can you suggest any reason that I can't try shooting coolant on the board to see if that affects it at all?"

This troubleshooting approach is mainly used to find temp-dependent components,
i.e. if a system starts to fail at different temps. Doing this for your problem
doesn't prove/determine anything, except the cruise control might fail @ -20 deg C.

Most all vacuum controlled cruise control units will work, e.g. 928s, & M/B, for
testing.

If all else fails, consider calling Pelican and asking about testing of the unit.
As I've mentioned before, Pelican has many sources.

[Por_sha911]

Well, I finally got tired of fooling with it. I sent the amplifier to Beckmann Technologies in Durham NC (919-381-2700) and they and they totally reconditioned the unit for $162 including shipping it back to me (by comparison the parts junkyards want $125). Plugged the puppy in and it works great.
Beckmann work fast and are professional all the way. I don't know what the problem was but its gone now.

[Lorenfb]

You could have had the unit rebuilt by Pelican, as they have many sources
for rebuilt parts and possibly saved money and also supported their BBS.

By the way, does Beckmann have a Porsche BBS you can B.S. on?

Typical of most: Find the problem answer on Pelican, and buy elsewhere!

[Porsche80]

Does anyone know if you don't have the clutch switch on your cruise control if it will still work?