944 cruise problem

[Raoul Duke]

I have a 91 944S2 Cabriolet. The cruise control does the exact same thing I had problems with on 2 other transaxle Porsches, which makes me think it's a common problem. When I engange the cruise control the car accelerates until I disengage it or it runs out of revs. I'm not sure what I can check and I don't want to just start replacing parts. Anyone have any ideas? Thanks.

[Eli K]

You use cruise control????

[nize]

on the canada roadtrip two weeks ago i had the cruise control set at 120mph for a while...

[Raoul Duke]

Originally posted by Siberian
You use cruise control????

It's the only way I can keep it under a hundred!

[bryanthompson]

Yeah, i use cruise control... it's called holding it to the floor

[Porsche-O-Phile]

No, that's still driving manually. Cruise control is putting a brick on the accelerator pedal to hold it to the floor!

[Raoul Duke]

So, any ideas where the problem could lie?

[Porsche-O-Phile]

Brick isn't heavy enough?

[Clowd]

I wonder if the cruise control in my car works?

[Raoul Duke]

Originally posted by Porsche-O-Phile
Brick isn't heavy enough?

Too heavy - the car accelerates until I disengage it. It acts like the Cruise module isn't getting data from the VSS, but a mechanic told me that if the VSS had failed the speedometer wouldn't work. Short of replacing the module as a guess I don't have any other ideas. I wondered if this was a common problem, because I had a 79 928 that did the same thing, and a friend with an 83 944 had the same problem also.

[Don 944 LA]

I hit the cruise button once.. nothin happened..

Since I always drive as fast as I can, I think that the cruise control would be a bad idea.

It's VERY far down the " To Do " list

[Raoul Duke]

Originally posted by Don 944 LA
Since I always drive as fast as I can, I think that the cruise control would be a bad idea.

Unfortunately, the local constabulary has a different idea of how fast I can drive, and it has something to do with those signs beside the highway. I need the cruise control to ensure that they don't take my Porsche away.

[Raoul Duke]

.

[spuggy]

Well, the good news is that your servo, switch and wiring seem to be in good order...

The bad news is that it's almost certainly your cruise control computer that's not functional. A lot of people apparently have success re-flowing the solder joints, as they're pretty horrid to start with, and don't improve with age.

The computer is just above the hood release handle under the dash, you could try unplugged and re-plugging the connector a few times, in case it's just oxidation on the plugs/contacts. A 10mm nut driver works well, IIRC.

There's a detailed test procedure in the factory manual for the inputs etc. but the most inconvenient one to test is probably that the speed sensor is putting out a signal - on a coupe, the easiest test for this is to go through the "sunroof unlatch procedure" as if you were going to take it out and then drive away and listen for the sunroof latches to lock when you reach a certain speed (mine does it at about 3 MPH).

More bad news is that your old unit won't be acceptable as a core for rebuilding once you open it (which involves bending tangs out of the way to remove from the case). But professionally re-built and re-tested units run about $400 anyway. IIRC, some MB cars of the era are the same Bosch system, if not the same part. There's a lot of MB owners with the same problem, and most problems - although not all - seem to be fixed by a good session with a desoldering pump and soldering iron, followed by re-soldering the joints. Fair amount of information on the web about it.

The good news is that these units are available very reasonably on Evil Bay - think all the late (series 2 and up) ones are the same - mine cost me $60, and works perfectly. Don't bid too high unless it's guaranteed to work - you might be bidding on one with the exact same problem you've already got... They're also thrown out by almost anyone building a race or modified car, for obvious reasons, so outfits like Speed Force and Renegade tend to sell them too.

Guess I should try re-building my old one sometime, just to test the theory.

Must admit, I do find cruise useful on long journeys to stop the speed edging up all the time - these cars weren't built to drive anything ending in '5' that only has one other digit in front of it... And it's kinda cool to wind it up for a little burst of action like diving through a closing gap in traffic up ahead, only to change up and back off the loud pedal when you're clear and have the car just settle back into that long-legged rythym again.

[Raoul Duke]

Thanks for the response. I figured this was a common problem, so I'll look for another computer without feeling like I'm shooting in the dark.

[spuggy]

Does it actually never stop accelerating? Even if you change up?

I initially thought mine just accelerated, but realised that it would stabilise about 15-20 MPH above the speed I set it at, and then gradually "creep" up.

[Clowd]

Originally posted by spuggy

The computer is just above the hood release handle under the dash, you could try unplugged and re-plugging the connector a few times, in case it's just oxidation on the plugs/contacts. A 10mm nut driver works well, IIRC.

Is that what that silver box is?

[spuggy]

Originally posted by Clowd
Is that what that silver box is?

Yup, that's where it is on a series 2 car.

You pretty much have to lie on your back with your head in the driver's footwell to get it out. One of the easier things to get at in that area.

On the series 1 cars, it's buried in the central console somewhere. They're not interchangable, the plug is two separate (and keyed) pieces on one and single-piece on the other (forget which is which).