Cosmo Kramer

I got my cruise working, while diagnosing I found a white wire unplugged under the driver side dash. Hooked it up and voila - cruise control. Took it for a drive and now I see why the PO disconnected it!

If I push set while the car is coasting, when I hit level ground it accelerates to 10 mph past the set point before leveling off. When I climb a small grade, it slows down 10 mph below the set point. On perfectly flat road, set it and it holds fine. Basically if you set it on the freeway with some very gentle rolling hills at 70 mph, the car will vary between 60 - 80 mph and hover in the middle on flat ground.

Vacuum holds fine in the reservoir and the servo. If I resolder all the joints in the brain will it fix this or does it have a chip burned out somewhere on the board?

6mil928

Ted make sure and get this resolved. The cruise control is on my list too.

zoltan944

That is exactly how my last '85 was and it was supposedly working correctly. I sorta assumed that the cruise was maybe vacuum related or something and did that? I have no idea, all I do know is I didn't use it that much! lol

Ill give my new '85 a try and see what it does and let you know

Cosmo Kramer

I followed the instructions on John Pirtle's site here. Very good diagnostic information and I was one of the lucky ones that only had one screw on the passenger side holding in the cc brain. If you have one on the drivers side it will be a bitch to get out.

Please let me know how it works on your other car. I manually pumped up the vacuum manifold with my vac tool and it held, jumpered the plug as per the instructions to get full throttle and it initially dropped about 5 hg to open the throttle and held with no leaks so I think I am OK there. I have read some bits and pieces through a search that there is a comparator chip in the brain to regulate the speed which can go bad, so maybe that is it.

Dennis Wilson

If it hasn't been used in a while, the cable from the top of the vacuum unit to the throttle may be sluggish and could cause our problem.

Dennis

Fabio421

I have heard of others resolving this exact symptom by resoldering the connections inside their cruise box. There is a write up here somewhere for the proceedure. It's been years since I've seen it.


Edit: Found it!!!

https://rennlist.com/forums/928-foru...cruise+control

Cosmo Kramer

Are all the ones indicated the joints that need to be re-flowed?

Fabio421

Yes, but YMMV.

SharkSkin

Those were the ones that were found by inspection on that particular board. Odds are, the ones causing issues on any particular board will be different. These boards were originally soldered using a "wave solder" method, where a wide "wave" of molten solder is pumped upward and the board is carried over that wave to solder everything at once. Issues where solder joints are defective tend to have a random distribution when using this kind of process(with some exceptions, usually due to large differences in mass over the surface of the board -- not an issue here IMHO). FWIW I have a lot of experience with this process, I was responsible for setup/maintenance/repair of a couple of wave solder machines through most of the late 80's when I worked at an electronic assembly house, making motherboards, VGA cards, etc.

zoltan944

Well, I gave my car a shot and the new car doesn't work very well either. I have the car torn down a little for tune up and fixing anything I see. I will see what happens afterward, but it doesn't seem that good at this point.
Looks like we need to do some serious soldering sessions!