XJ6 & XJ12 Series I, II & III 1968-1992

Cruise/speed control acting odd

 
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Old 09-05-2014, 09:26 AM
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Default Cruise/speed control acting odd

Hi everyone,

When I push the "set" button on the end of the turn signal stalk, my 86 XJ6 accelerates instead of holding the speed. The only way to stop it is to turn off the master switch by the gear lever. Has anyone seen this before? I'd like to troubleshoot before randomly replacing parts.

Thanks for any ideas you may have!
 
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Old 09-05-2014, 09:59 AM
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My guess is that one of the little valves inside the servo is stuck. Perhaps the vent valve? This would mean that engine vacuum keeps pulling the servo (and hence the throttle) further and further rather than stopping at the correct point.

There is a speed adjustment/trimmer on the cruise control module but it's designed to adjust out minor discrepancies and not a gross over-acceleration.

Here's lots of reading:

Cruise Control

Cheers
DD
 
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Old 09-05-2014, 02:01 PM
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And, there is another way to interrupt the CC. Tap the brake pedal.


There are two mechanical/electric switches just behind the brake pedal. One activates the brake/stop lights at the rear of the car.


The other is normaly closed allowing the CC brain to operate the servo and apply throttle. Tapping the brake pedal opens the relay circuit and the cruise does not send throttle command signals.


The other interrupt is on the shift post. No throttle commands relayed if in N or P.
possibly reverse as well. One of the micro switches found there.


I spent a lot of time under the scuttle in that area restoring the brake light feature.


My lump uses one for the converter lock feature of it's sophisticated transmission.


Errands in the Jaguar today. All but one mission accomplished. Jaguar purred.


Carl
 
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Old 09-05-2014, 09:07 PM
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Ok thanks guys. Doug, if it stops raining this week I'll pull the servo out and clean it up. Sticky valve makes sense to me.
 
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Old 09-12-2014, 04:02 PM
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Curious to know if you have dived into the cruise control issue. My car has the same problem, when I press the set button on the turn lever stalk, it continues to accelerate, no matter what speed the car is going initially. I believe it would accelerate to the governed speed.
I had the bellows unit out last night, checked the electric valves for freedom of movement and for presence of rubber seals on plunger ends, and re sealed the bellows to end plates. It was rather clean inside.
I tried adjusting the two pots inside the cruise ECU today with no luck. I am stumped! Any ideas?
John
 
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Old 09-14-2014, 07:31 PM
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I had an "aha" moment this evening. I removed the servo/bellows unit again and removed the servo end from the bellows. The symptoms the cruise was having pointed to the vacuum - side servo not closing the vacuum port. I removed both servos and tested for freedom of movement - check. I made sure the rubber seats were still intact - check. Then I inspected the servo housing. Patentcy of ports and vents - check. Then I looked at the thin copper springs at the lower end, the vacuum side was bent. I may be onto something... By the design of the plunger on the servo, I surmised that the copper strip fit into a groove above the plunger. After carefully straightening the spring strips, I replaced the servos and re sealed the bellows.

I just went for test drive and the cruise functions perfectly! This was a case of mechanic's error, me being the mechanic. I have pictures of the copper strip, but I do not see an option to upload pictures to a thread on Android. I'll check it out on Firefox later.
 
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Old 09-14-2014, 07:35 PM
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Far out . Good sleuthing!

Cheers
DD
 
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Old 09-14-2014, 08:58 PM
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Thanks for the tip John. I death in the family has kept me away from my car these past ten days.
 
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Old 09-15-2014, 02:16 AM
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have you checked the rubber bellows (the black rubber "accordion" part) for any cracks at the lower "valleys" ?
 
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