Cruise control slams gas to floor

[schoward]

1981 sc

Whenever I set cruise, the accelerator gets mashed to the floor. Thought it was the brain so had unit rebuilt. Tried the rebuilt unit and an additional used unit i picked up and exact same thing with both brains. Sent rebuilt unit back to rebuilder they said its fine. Trying second brain seems to indicate they may be right as same thing happens.

Any ideas? Hoses? Wiring?

Thanks in advance.

[KTL]

I suspect it's the vaccuum actuator in the engine bay. That thing is what manually holds the throttle open when you use the steering column stalk to set the cruise. If it can't hold a constant vaccuum, I think it keeps pulling on the cruise control cable.

[sky1jord]

Might be that whatever the required feedback is (RPM or speedometer output) is not getting to the control unit.
I let my son drive my '88 some time ago, he had the cruise control on, then he shifted to neutral and let out the clutch, and the motor red-lined in no time, trying in vain to re-establish the pre-set road speed.

[Venetian]

I had the identical problem and replaced brain with rebuild...problem solved.

[TRE Cup]

One very important thing to check on the cruise control cable in the engine compartment. Probably not your particular problem, but one to check nonetheless:
The hex shaped plastic that clips to the angled bracket next to the throttle body. The cable sheath can break loose from this and when accelerating, then backing off, the sheath pops loose and sets itself against the hex section. Involuntary acceleration is the result.

you can fashion a retainer of the hex and sheath by using several tie wraps, thereby eliminating this hazard

[cali4sun]

Launch Control

[schoward]

Thanks for idea on vacuum pot. Been years since ive been back in there. Found the main small vacuum hose crumbling apart and pot itself sitting loose with nut that attaches it to bracket MIA. Will replace hose and properly mount pot and see if any help.

Cable tie to engine with the hex seems ok.

Brain seems to get a speed reading as wont set/engage below 30mph which is exactly correct

[83-911SC cab]

Would you mind telling me who did you have rebuild your brain for you and how much it cost..if you would rather PM me instead saying here that would be OK..I think I need mine done..seems like everyone I talk with about their used units can't guarantee if it worked or not.

Thanks,
Gregg S.

[Venetian]

Loren at Systems Consulting(I think thats the name) did mine for $125.

[schoward]

ASI in rogersville tn did mine. Gregg, pm sent.

[kodioneill]

The ecu get's it's tempo signal from the speedo the wiring could be wrong.

[83-911SC cab]

Thanks for the tips on rebuild companies gents.. I shall mess with this once I get home from Vacation.

Gregg S.

[JINKATIN]

KTL is spot on, had same issue. Vacuum actuator hose was disconnected.

[schoward]

All:

Resurrecting this thread to see if any additional ideas on this somewhat common problem.

I continue to have the issue I outlined above with the always accelerating cruise (ie set it and slams gas to floor). 'Resume' gives same effect once set.

Background: Once upon a time my cruise worked ok using a used brain I got from Parts Heaven - but after noticing increased frequency of this issue, I sent brain for rebuild, as it would either not work or slam gas to floor, or sometimes work just fine. Upon receiving it back, it ALWAYS and consistently did the full accel. Sent brain back to rebuilder, they said it was fine (but couldn't tell me what the issue was that they repaired in the first place). I wasn't too confident in their response. I opened brain unit to see lots of solder but components looked all original (capacitors, etc).

I then purchased another used brain which was supposed to work and tried it. Same result (full accel/gas to floor all the time).

I have tested and replaced vacuum pot with a used unit - both pots tested ok for ohms and both give me the same continued issue/no change.

When swapping pots, checked the cable - looks ok and is secure.

Replaced all vacuum hoses back to intake including the 'T' and decel valve - all snug and secure- same result.

Thinking maybe the speedo, but not sure - Cruise does only set above exactly 30mph which is correct. Below 30 mph it does not respond (as expected). This tells me the signal and interpretation is probably ok.

Given the background paragraph above - I am back to thinking its the brain. But after trying 2 of them including a rebuild I wonder...

Any additional ideas? I now see why most write these things off, but I feel I am close.

Thanks - And Happy Thanksgiving.

[kodioneill]

Check the speedo pulse with an analog ohm meter don't assume the speedo is giving the correct output.

[schoward]

Thanks for perspective - considering just pulling the speedo and sending for full checkup, clean and calibrate since it is a few mph off (high) like most. Shop said they can test output pulse for cruise while they're at it...

[schoward]

Posting a follow-up to the above to put 'on record' here in case anyone searches or has the same problem and comes across this post.

In the end it was the Brain, despite having it rebuilt previously, and re-checked and supposedly tested again. And second used brain I tried had the same exact problem as the rebuilt one which really threw me off.

Per advise above (and thinking brain may be ok and problem elsewhere), I ended up sending the speedo out for calibration, a new bezel and also a check of the output pulse for the cruise. I learned that the repair place does a check of the output pulse on a scope and replaces the IC if the signals don't look right, so lesson was learned that the speedo IC can go bad and perhaps affect the cruise control. They replaced the IC for good measure, but unfortunately it did not make a difference in my case.

I then had my extra Cruise brain refurbed, as it was original/untouched. Cruise now works with second rebuilt brain.

Cruise works well at highway speeds. Above 50mph it is dead solid like the speedo needle is frozen even when ascending long hills, etc. You can tell the brain is very quickly adjusting throttle to compensate keeping speed dead solid. Below 45-50 it seems to do the drop and light surge as many of them do at these speeds, but not enough to worry about esp. since no real use for cruise at that speed for me anyway. And I'm kind of done sending these units all over the US for repair and driving myself crazy.

In the end, it works, but I now feel if you can't fix these systems with a reasonable try, I say pull it. I really like everything to work, but these brain units are just not the best and many have struggled to end up with a mediocre cruise system. On my next 911, if the cruise doesn't work, the whole thing is getting pulled right away. Spend the $ on other things. Just my opinion.