Hold On

I played with the cruise control issues on and off for several yrs. Re-solders and all. Got tired of jerkin with it and sent cruise control brain box to Beckman Technologies in Durham,NC They do rebuilds on the VDO units for Mercedes. Couple weeks and $170. later, I had a rebuilt unit that has worked perfect ever since. 1-800-742-1021 Have the part # off case available when you call. Kevin

sstrickstein

i repaired mine by removing the "brain box", and reflowing all the solder contacts with a soldering iron. was amazed when it actually worked again! not difficult (and free if you have a soldering iron), just takes some time as there are a bunch of solder points.

That's the route I'm gonna try first.

sstrickstein

Ok so I reflowed all the solder points. No go. Guess one of the resistors is bad. Does the speedometer guy check all those?

928nut

I don't know what he checks or what he doesn't check, that is his business. Obviously, he must have means and knowledge to test the unit and troubleshoot it for causes of failure. Bottom line is, "he'll REPAIR your CC module" for $100 with 1-2 day turnaround from receiving your unit and gives you "lifetime" warranty. I don't know of anybody who can beat that.

For all I know, there may be limitations to what he'll repair, like burnt PC board or butchered board from your re-soldering attempt, but to find out, best to check his web-site or call him. 507-429-4489

928nut

I should have also pointed out that unless you've performed the CC module troubleshooting steps found in the WSM, you're putting the cart ahead of the horse.

You have to determine if your problem is input signal(s) to the CC module or output from the CC module. The WSM guide will verify the input signals to the CC module. If it checks out OK you have a defective CC module. However, if you fail the input signal tests, your problem is elsewhere - may have nothing to do with the CC module.

syoo8

I posted this in another thread, but it seems salient...


The Cruise Control (aka tempostat) amplifier is inside the center console, You remove the right carpeted cover (to the left of the passenger's left knee) and I think there are two or three bolts (and I think they were an odd size, like 7mm) which will release the amplifier.


The amplifier is a silver metal box, with some taupe colored plastic at the end, mounted on top of the tunnel next to the firewall. Remove the bolt on the right side.

The workshop manuals include a check of the servo by bridging terminals 3 and 5 on the plug. "Connect a push button switch on terminals 7 and 12. Turn on ignition and press the push button switch with engine stopped. The vacuum servo should move the throttle uniformly to full load position. This requires vacuum."

Remove the multiple pin plug. Then, with a voltmeter, you can check the pins. With ignition on:

Between terminal 5 and 12: battery voltage.
Between terminal 6 and 12: battery voltage when brake is pressed
Between terminal 8 and 12. battery voltage, unless you pull cancel on the CC lever, at which point it should go to zero.
Between terminal 9 and 12: 0 volts, unless you push the lever forward on the CC level, at which point it should show voltage.
Between terminal 10 and 12: 0 volts, unless you push the lever down (reset), at which it should show voltage.

With an ohmmeter:
Between terminal 12 and ground: 0 ohms
Between terminals 3 and 7: 14 ohms
Between terminals 11 and 12: should alternate between 0 and infinite ohms when you turn rear wheel.

If you check all these and they are "spec" then your CC amplifier is no good. Both of my 928s had this problem. You can either reflow the solder in your CC amplifier (which I have tried several times with no success) or buy a used one. IIRC it is actually a 911 part, and there are quite a few of them on the used market.

sstrickstein

So this am I stopped at 20th st auto parts, Our local Phoenix Porsche parts yard. I purchased a rebuilt cruise amp from Larry( he offered to take it back if that wasn't my issue). I plugged it in and hit the freeway. No change. Still acceleration but no holding the speed. I am going to remove the Actuator today and bench test it. I think a valve or something in it is not working correctly. Are these serviceable with Deoxit? I did spray the connection. I did test the vacuum under the hood and the reservoir and line to the actuator hold vacuum. Kinda Stuck here.

sstrickstein

Looks like I'm using the ol leg cruise control all the way to Anaheim tomorrow. Went through the whole system and cant find a problem.

Hold On

Did you do the part of the WSM test where you use a switch to test the actuator in the fender ? a lot easier to do than removing to bench test. I used a remote starter button with some extended wire leads. Hook it up to the appropriate pins and push the button with car running in neutral. it will pull the actuator and rev the engine, That way you know it works in the car.

sstrickstein

Yeah did the whole checklist. Only thing out of normal is the resistance for the actuator is a little high. Supposed to be 14 OHM and its around 18 OHM. Not sure that's it though.

The actuator works when I hit the accelerate button. It just wont hold speed.

Hold On

Mine did exactly what you describe yours doing. I also checked every connection in the car and in the brain box. It turned out to be one of the components inside the box was internally fried. Apparently a very common problem with these VDO units in the Porsches and Mercedes. I had three spare units that all did the exact thing. Ultimately I had to send it out to a specialist in VDO units who knew what component and replaced it. havnt had a problem in the last year.

sstrickstein

I just borrowed a rebuilt unit from 20th st yesterday. It did the same thing with his rebuilt unit. At this point I may ask Mark Saturday If I can test with one of their units, though I have no reason to believe the unit I borrowed from 20th st was faulty. I'm of the mind that my actuator is going bad.

sstrickstein

So with all the testing and information I have gathered it seems I either have a bad cruise switch-(which I tested and it passed) or a bad cruise actuator. Has anyone ever had the Cruise Actuator or Switch actually fail? To refresh I have tested and ruled out a bad cruise brain.

Barry Chan

What about the actual lever/switch? Can that go wrong?

SMTCapeCod

https://rennlist.com/forums/928-foru...or-repair.html