Cruise control problem

[U12RACE]

I recently purchased my first diesel, a '91 R1500 Suburban, to tow my race car and for daily use. My problem is that sometimes, while driving with the cruise control on, the engine will suddenly slow down (coast) and the service engine soon light will come on. If I press the accelerator to bring the RPM up, the light will go back out and the cruise will begin working again. It acts like it is just losing vacuum. I can't find a vacuum leak and when I hook a hand vacuum pump to the cruise control diaphram, it doesn't lose vacuum. My guess is a possible vacuum pump problem. Is there any way to test the pump, or am I barking up the wrong tree? Thanks - Kyle

[moondoggie]

Good Day!

See if one of your friends still has their old vacuum/pressure gauge from the good old days when you could fix your gasser car. This could be tee'd in to monitor vacuum.

I doubt that's it, however. If you're getting a SES light, the PCM probably drops the cruise. I don't know much about your vintage truck, but when I had a weird grounding problem with my 95 pickup, it gave an unrelated code 35 & the cruise is dropped as soon as the SES lights.

You might want to post your question on the 6.2L Diesel or 6.5L Turbo Diesel forum. Dr. Lee knows all, but he doesn't own a truck like yours - his is a primo 1984 C10 with probably the nicest 6.2 ever built. If you post in those forum(s), there are probably thousands more of us who will see your topic, & hopefully help.

Blessings!

[Dr. Lee]

My 1984 truck had the vacuum actuated factory cruise control on it, and it went out on one of my trips from Miami to Montana. I had to drive 80% of the round trip with my foot on the pedal, overcoming the relatively stiff return springs. Not fun.

Checking into it, it could have been the $200 vacuum module, the $200 speedometer module, or the $200 control module, with no good way to figure out which. Even if I did, I would have had two other 20-year old components ready to give up the ghost in the future.

So after a little research, I put on the SCS Frigette aftermarket electronic cruise control. The install was relatively easy, and it has worked perfectly ever since. If you like this truck and are going to keep it awhile, look into the SCS unit.

If you don't want to install it yourself, check around with several of your automotive air conditioning repair shops. SCS Frigette is big in aftermarket A/C, and the technicians will know the brand and be able to handle the install.