Cruise Control Surging - The 1947 - Present Chevrolet & GMC Truck Message Board Network

izzy_Britton · 05-03-2013, 11:45 PM

I just purchased an 86 single cab SWB 305/700r4 cream puff, 81,000 original miles.

She has been sitting for at least 5 years i am thinking closer to 7 years. I just got it smogged, passed with flying colors. All of the vacuum lines were checked by the smog tech and everything was plugged in as it should be. all the lines look good and there is no dry rotting or visible cracking. I have tested MOST of the vacuum lines to check for leaks using starter fluid or brake parts cleaner, and everything seems 100%.

To the issue:

While driving down the freeway at 70ish MPH, when the cruise control is engaged the speed seem to remain pretty constant at the set speed. However the issue is two fold. While at speed and holding, the truck surges or lurches enough that it can be felt and is quite annoying. the second part is that after several miles. say 3-4 miles, the speed slowly starts to decrease. stepping on the throttle pedal slightly will kick the trans down a gear and the truck will take off again until it reaches the set speed and the whole cycle begins again.

for giggles this past weekend, I set cruise at 70, and didnt touch the throttle pedal for about 30 miles. the speed decreased to an unsafe speed and i exited the freeway around 40 MPH. Once on the off ramp i stabbed the throttle pedal quickly once and it took off to gain speed again but i had to stop at the sign at the top of the ramp.

There is a vacuum line going to the vacuum pot on the left inner fender well, it is intact. what else controls the cruise control function?

this is the 38th c/k truck i have owned in the past 20+ years. this is the only one that has/had a functioning cruise control.

Any words of wisdom would be greatly appreciated. Thanks in Advance

Izzy

**hatzie** · 05-04-2013, 10:03 AM

Vacuum leaks... at the brake pedal switch or the servo diaphragm??

The 1968-1998 GM Cruise Control training and service manuals are in my sig. The 84 Cruise III manual will give you troubleshooting detail.

izzy_Britton · 05-08-2013, 12:56 AM

Quote:

Originally Posted by hatzie

Vacuum leaks... at the brake pedal switch or the servo diaphragm??

The 1968-1998 GM Cruise Control training and service manuals are in my sig. The 84 Cruise III manual will give you troubleshooting detail.

Thank you Sir!

I will delve deeper into this in the coming week. I now have a start and thank you for the manuals. much help.

-Izzy

05-03-2013, 11:45 PM	#1
izzy_Britton Registered User Join Date: Apr 2013 Location: Nor-Cal Posts: 632	Cruise Control Surging I just purchased an 86 single cab SWB 305/700r4 cream puff, 81,000 original miles. She has been sitting for at least 5 years i am thinking closer to 7 years. I just got it smogged, passed with flying colors. All of the vacuum lines were checked by the smog tech and everything was plugged in as it should be. all the lines look good and there is no dry rotting or visible cracking. I have tested MOST of the vacuum lines to check for leaks using starter fluid or brake parts cleaner, and everything seems 100%. To the issue: While driving down the freeway at 70ish MPH, when the cruise control is engaged the speed seem to remain pretty constant at the set speed. However the issue is two fold. While at speed and holding, the truck surges or lurches enough that it can be felt and is quite annoying. the second part is that after several miles. say 3-4 miles, the speed slowly starts to decrease. stepping on the throttle pedal slightly will kick the trans down a gear and the truck will take off again until it reaches the set speed and the whole cycle begins again. for giggles this past weekend, I set cruise at 70, and didnt touch the throttle pedal for about 30 miles. the speed decreased to an unsafe speed and i exited the freeway around 40 MPH. Once on the off ramp i stabbed the throttle pedal quickly once and it took off to gain speed again but i had to stop at the sign at the top of the ramp. There is a vacuum line going to the vacuum pot on the left inner fender well, it is intact. what else controls the cruise control function? this is the 38th c/k truck i have owned in the past 20+ years. this is the only one that has/had a functioning cruise control. Any words of wisdom would be greatly appreciated. Thanks in Advance Izzy __________________ WHO_DCTC - Wifes 05 Z-71 Build - Now defunct as she decided she wants an Escalade too Cream Puff - LS'd Square - Slow going on the back burner Tripple Threat - 3 Wrecks for the price of one build thread Project Delongdium - Step-Sons Plastic Clad 2003 Avalanche - DONE and Delivered.

05-04-2013, 10:03 AM	#2
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,977	Re: Cruise Control Surging Vacuum leaks... at the brake pedal switch or the servo diaphragm?? The 1968-1998 GM Cruise Control training and service manuals are in my sig. The 84 Cruise III manual will give you troubleshooting detail. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful.



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