91 Suburban Cruise control question

[kwin]

My new to me 91 Burb runs great, very happy. Only issue is the cruise control, when I set it it drops about 20 km before it maintains a speed.

Any ideas where to start?

Cruise Control is completely foreign to me.

Thanks

Keith

[kwin]

So I borrowed a scanner and tried to do some digging, got lots of good info on the burb everything works the way it should. One thing I notices is that road speed as displayed by the speedo vs what the pcm sees is off. When the Pcm says I'm doing 30 i'm actually doing 26.

THink this could be the root of the issue?
I have the scanner for a few days to do more investigating.

Keith

[Ski-me]

They should both (theoretically) be the same b/c they both recieve a pulse from the same sensor. The speed sensor (on T-case) sends a signal to the DRAC and then it gets sent out to the ECM, Speedo, cruise and ABS.

[cal30_sniper]

As far as I know, there is no correlation between the speedometer and cruise control. Cruise should be it's own self-contained unit. My '90 speedometer didn't even work half the time. It had a bad ground internally, but the cruise always worked. I'd check all your vacuum lines and stuff. I've had GMs that the cruise would drop 2-5 mph before it caught after you set it, but never 12.

-cal30sniper

[Ski-me]

Start with vacuum hoses to be sure everything is hooked up correctly and not leaking. Because of the age, it might be good to try and swap in new tubing anyways. Check the plunger too. Maybe that is leaking or not working correctly.....and also check the amount of slack from the plunger to the TBI unit. If I recall correctly in Chiltons, there should only be about 1/8" - 1/4" slack and almost tight. If you have a lot of slack, maybe this is where your speed is dropping so much. I've not heard of a speed drop that much after setting. Mine use to drop maybe 1-2 mph but that's it.

The correlation between the speedo and the cruise is the DRAC box. It converts the signal from the speed sensor into pulses and then sends it out to various devices separately. So if the speedo ground is disconnected, the DRAC still sends out a signal to the ECM, Cruise and ABS.

I converted my 89' cable speedo to the 90-91 speedo and it works great now! I did have to re-pin the cluster connector but now it works awesome! I also placed a 7 pin selector in the DRAC to accomodate for larger tire size and axle ratio. 35" tires and I'm dead on with my speedometer reading.

[1BadAction]

It sounds like a vacuum issue, check valve or leaking hose. Possibly a bad servo, but they seem to hold up fairly well. Another possibility is the actuator to throttle body linkage, if it's not adjusted correctly the servo will not have enough throw to hold at higher speeds.