Always great counsel from Kitty and Dave - I trust and hope you'll be able to diagnose/repair without much time/effort/$.
My experience repairing failed CC units on a variety of cars over the decades has been filled with a lack of success. Back around 1981, I added a Sears aftermarket unit to a 1980 VW Jetta. What was unique (back then) about this unit was it was fully electronic. Not vacuum operated - but rather an electro/mechanical servo to actuate the throttle via cable. It worked exceptionally well - much better than any vacuum operated cruise I'd experienced. About 6 months after I installed that on the Jetta, the factory CC on my '77 911 went belly up. Diagnosis led to the need to replace parts that, at the time, added up to about $800. I got to thinking -- I ended up buying another Sears universal CC unit (about $59 at the time, IIRC), and basically got the Porsche OEM cruise stalk to control the new unit. So, inside the car, everything looked OEM. And I hid the servo so that under the hood in the back, you'd be hard pressed to tell anything wasn't OEM. It worked WAY better than the factory cruise unit did.
Point -- I discovered that, in almost every case, it was easier (and almost always less expensive) to simply put a universal aftermarket unit in and drive it with the factory switches than it was to try and figure out what was wrong with the OEM unit and then hope I could actually find/afford the parts to fix it. Took the same approach on my daughter's 240 three decades ago using a Rostra Universal kit (edit - it might have been a Panasonic unit back then). Have also done the same trick on a Toyota MR2, Honda Accord and BMW E30 over time. Back around 2002, I used a Rostra Universal kit on my 5.0L powered 240 and more recently used a Dakota Digital unit compatible with electronic gas pedal (made by Rostra for Dakota, I discovered) when I ditched the 5.0L in exchange for LS power in the same 240. Everything was actuated by factory cruise switches. The Rostra units for cable throttles were "programmable" with some switches and will accept being controlled by different types of OEM controllers (as well as letting you set other operational parameters). So, if it turns out you have a hard time figuring out what's wrong with yours, or you figure it out, but getting it working is a struggle, there are options for getting you back into business in a way that keeps things looking pretty much original and gives you brand new CC for your 30-ish year old car. Looks like the Rostra units are going for around $200. FWIW
Good luck!
|