Cruise control
 

On my 84 the cruise worked for a moment when I first bought it. Now no go. What is the most common issues? I've read the 928 international write up. I'm planning on Dyoxit the plugs. Has anyone run into it being as simple as that? Let me know what you found was wrong with yours.

On my 83 I had a bad vacuum connection. It would engage but slowly loose speed as the vacuum failed. I fixed that connection in the engine bay near the booster, removed, cleaned and lubed the unit with silicone and it worked for a while but now I have to clean the contacts in the engine bay as when it engages it sets off the center console warning light. The unit is in the front driverside panel behind the wheel well panel.

I have good vacuum. My problem seems to be electric.

I have an issue where the vacuum acceleration part works but the part where it maintains constant speed does not in my 81.

Most of the problems are inside of the metal box under the HVAC system that is essentially the brains of the cruise control where resistors wear out and such.

If you have good vacuum next would be to check input signals to the CC module per service manual. If it checks out OK there is something wrong with the CC-module. I would pull the unit out and send it to David Ruberg, the "speedometerrepairguy dot com" in Winona, MN. About $100, quick turnaround, lifetime warranty. Makes no sense to screw around with it unless you know what you're doing.

Just applied deoxit on all the connections in the system. We will see if it helped

So the cruise accelerates and pulls hard but as soon as I let off the lever the cruise cancels and doesn't hold the setting. When I pull the vacuum line off the booster that vacuum has held and escapes. Not sure what to do from here.

Well, if it's working at least some of the time, then I'd think that the tempostat (brain box) isn't totally toast.

Have you done the diagnostic procedure in the WSM?
All the way through?

Joe, I respectfully disagree.

I've had two instances of accelerating 'properly' with a push of the stalk, only to not hold the cruising speed. A new tempostat brain cured the problem both times.

The problem for me was that each 'new' (read:used) tempostat only lasted a year.

I'm thinking of sending my latest tempostat brain to Winona, MN, like others have advised.

I disconnected and clean the connections for the clutch pedal bypass circuit that is under the dash. Now because my car is an automatic there is just two wires bridge together. Since this is the last connection for the repair manual I can only assume that if the cruise does not work after this my only choice will be too have the circuit board of the cruise brain repaired. Has anyone removed the board from the box and done the soldering themselves?

Thanks Scott.

I haven't dug too far into mine yet. I was under the (apparently mistaken) impression that the tempostat was an "all or nothing" sort of thing.

Wrong again :icon501:

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.

Cruise links
 

This was a common concern aback in the nineties on the email listserv, there used to be a mercedes specialist that would address the 'brains' for a reasonable cost...not sure of the status of that...but it was very common subject of discussion.

More recently, someone on here followed posted instructions to re-flow the board using a household overn..

Some additional info, I don't recall whether the unit is actually M-B or just similar. It'll be some time until I go after the one in my '86.

http://www.peachparts.com/shopforum/...-articles.html

http://www.benzworld.org/forums/w201...pair-14-a.html

http://www.cruisecontrolrepair.com/

And of course the 928 vendors probably have rebuilds.

I do not recommend Autoecu.com. They had my 86.5 CC brain three times last year and could never get it to work. They are also very disorganized and it took numerous phone calls to get them to send it back to me the last time, and get a refund. They had a Ebay sale for the service at the time.

This is one issue where there's no good solution in my opinion. I didn't have the patience to reflow 1000 tiny solder joints, but some have been able to fix their brains this way. For some reason, the CC works perfectly in my 87 and it's NICE!

Also,
 

https://rennlist.com/forums/928-foru...rol-issue.html

and,
https://rennlist.com/forums/928-foru...ol-module.html

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

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

My car is an 1984 928S so it has a vacuum cruise actuator.

Just checkin' in on ya...did ya ever get the cruise sorted?

I too, have good vacuum, all my electrical readings are good, and so put in an overhauled Cruise computer. Same as you everything seems to work, just wont work.

maybe some day I'll check to see if the CC in my 79 works. I never use the things, even in a new car. PITA. I can't see how using your fingers is easier than using your toes. I gave up on the things a few years back, going on the interstate through Pittsburgh, PA. not good in busy traffic. I hate having to think when I drive.

it's hard to open a beer, steer with your knees and operate the throttle all at the same time!

Other than that, it came on the car...I'd just like to see the bat rastard work!

No not working. I gave up. And now it doesn't even do the superman up up and away thing.

One of these days, I will vow to make mine work. Determination and perseverance are the keys to success, right? It currently does nothing whatsoever. One strategy is to start checking on the simple things, and find happiness or grief for each thing. Move on to more difficult things. That can be a lot of grief, more than my normal dose of metoprolol can handle.

So, I am considering the opposite approach: come to a thorough understanding of the system and its common failure points, then take measurements, perhaps with a temporary wiring harness to a board with continuity indicators; I bet there are cheap data loggers on Amazon, maybe with an iPhone app. But then there is the brain - ah, the brain! Reflow the solder, replace the capacitors, get one that is "guaranteed to work" in the functional sense, not merely financial.

I've owned the car for six months now, but have not yet dared to remove the cover of the CE panel. Except for the CC, the e-gremlins are all asleep; I do not wish to wake them. Perhaps I should screw up my courage and check the fuse.

I guess I'm lucky my CC works in the 82 and that is ONE system I haven't had to mess with....:roflmao: