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| Engine revs in cruise conrol. (Page 1/1) |
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john carter
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OCT 02, 12:13 PM
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What would cause the engine to rev out of control when I actuate the cruise control? TPS? The trans kicks down and its off to the races! 87 GT
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82-T/A [At Work]
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OCT 02, 01:44 PM
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| quote | Originally posted by john carter:
What would cause the engine to rev out of control when I actuate the cruise control? TPS? The trans kicks down and its off to the races! 87 GT |
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I can't say for sure, but I know that cruise control is based off of vacuum. I'm assuming they would have designed it in a "fail safe" kind of way, and not one in which it causes the engine to race...
Sounds like maybe there's a vacuum line disconnected, or it's not properly identifying when and what speed it's supposed to reach.
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cartercarbaficionado
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OCT 02, 01:46 PM
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| quote | Originally posted by 82-T/A [At Work]:
I can't say for sure, but I know that cruise control is based off of vacuum. I'm assuming they would have designed it in a "fail safe" kind of way, and not one in which it causes the engine to race...
Sounds like maybe there's a vacuum line disconnected, or it's not properly identifying when and what speed it's supposed to reach. |
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they did not fail safe these unfortunately. it should be able to see speed and gears on an automatic
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john carter
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OCT 02, 04:54 PM
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I've checked and re-checked my vacuum lines. No check engine light. Replaced the vacuum can and the diaphragm. Dump solenoid is good. Stalk was replaced. I agree that the cruise speed is not being found. The engine screams trying to find the speed setting.
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cartercarbaficionado
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OCT 02, 07:26 PM
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| quote | Originally posted by john carter:
I've checked and re-checked my vacuum lines. No check engine light. Replaced the vacuum can and the diaphragm. Dump solenoid is good. Stalk was replaced. I agree that the cruise speed is not being found. The engine screams trying to find the speed setting. |
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have you checked the safety switches under the dash?
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john carter
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OCT 02, 07:41 PM
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I'm not aware that there are safety switches. Where are they approx? Thanks for your help
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john carter
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OCT 02, 07:51 PM
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I guess you mean the sw on the brake pedal. Yes it works. while the cruise control is reving up, I touch the brake pedal and it dis-engages immediately. I did a test with the key in the "on" position, I can hear the dump solenoid clicking when I press the brake pedal.
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Vintage-Nut
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OCT 02, 08:08 PM
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| quote | Factory Service Manual:
Since any problem in the system is either vacuum, mechanical, or electrical, the technician should perform a few initial checks....first eliminate a vacuum or mechanical problem by starting the engine and using finger to feel for source vacuum at the servo, and by visual inspection of vacuum release valve, throttle linkage, vacuum hoses, etc.
IF preliminary inspection reveals no solution, and the system in inoperative, use the diagnostic information... |
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As you believe that the vacuum and mechanical are 'good', did you perform the electrical ISOLATION TESTS under the system diagnosis section?
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Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles[This message has been edited by Vintage-Nut (edited 10-02-2024).]
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fieroguru
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OCT 02, 08:42 PM
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Vacuum is the force that pulls the throttle with the cruise diaphragm. If it is going WOT when cruise is turned on, it has too much vacuum.
There is likely an issue with the cruise servo where it can not bleed the excess vacuum off.
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Vintage-Nut
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OCT 02, 09:08 PM
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| quote | fieroguru:
There is likely an issue with the cruise servo where it can not bleed the excess vacuum off.
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Using fieroguru's tip, read "D: Vent Circuit Open Test" and "J: Vent Circuit Short Test" in the system diagnosis section.
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