Surging Cruise Control

[green73]

I have been fighting this since I got the Typhoon. Well I say fighting, but I only recently tried doing something about it. I just installed the full "Sportmachines" vacuum line kit thinking the lines were the issue. Nope. Not it!

Here is what it's doing. Cruise set at between 60 and 65 vacuum will fluctuate between -4 to -12. Cruise set between 55 and 60 it fluctuates much less, and set between 65 and 70 it fluctuates the same as between 55 and 60. The mph doesn't change, but I can feel the engine changing load due to the vacuum. This only happens with cruise turned on and set. I am thinking now that it is the module. Here is a thread I found searching here that sounds similar to my symptoms: http://www.syty.net/forums/showthread.php?t=4698&highlight=cruise+control+vacuum

Anyone else have any clues?

I have the supplemental factory manual on the way, and I understant it has a good procedure for troubleshooting Cruise Control.

Thanks in advance.

 

[ItsMyTy]

Re: Surging Cruise Control

Cruise will never work very well on these trucks due to the low compression and turbo setup. At higher speeds the engine will not be making much vacuum due to the low compression. When the cruise keeps speed up an incline, the engine may go into power enrichment or even boost. Then the cruise backs off to compensate for over accelerating.

Adding to this is the small factory turbo is instantly in boost at highway speed.

I found that cruise worked better on my typhoon with the larger turbo, as it took longer to build boost.

Early '90s cruise control does not interface with the ECM for smooth control like today's cars.

 

[green73]

Re: Surging Cruise Control

I have given thought to the low compression, turbo thing as well as it being an older design. If it was fluctuating more and more as I set it at higher speeds I would 100% agree with you. The vacuum is fluxing on very flat ground (Rosamond Blvd/dry lakebed crossing onto Edwards AFB). Less noticible at 55/60 and 65/70, and is prominent at 60/65 (Where I really want to be driving actually :rant.

 

[sy605]

Re: Surging Cruise Control

My Sy does the same thing except my cruise just shuts down completely after boost comes in while going uphill. As soon as I'm on flat terrain again it works.

 

[green73]

Re: Surging Cruise Control

GOT IT FIXED!!!!

I took the Cruise Actuator out to see just how it worked. Found some slack in the linkage to the throttle body. I tightened it one full "pin" setting (If you look at the bracket on the actuator you'll see how it adjustable by 1/2 "pin" diameters), leaving no slack at the throttle body, but it isn't pulling it either. Night and day difference. Tried all the speeds mentioned before. Vacuum stayed at about -5 with zero fluctuation. It is set instantly now too, whereas before it would lag and drop speed before actually engaging. :tup: WHOOHOO!!

 

[neildemo]

Re: Surging Cruise Control

We should Vault this. Nice find. Mine was doing the same but after I replaced vac lines all was good. My other truck had similar situation.

 

[BoostedSUV]

Re: Surging Cruise Control

nice fix :tup:


I was about to suggest checking the vacuum ball for leaks and for the one way valve operating. Inadequate stored vacuum would make the cruise go haywire too, especially with a higher lift cam etc. glad you got it sorted out!