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Old 14th April 2024, 17:14   #1
daveo138
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Default Cruise Control Not Working

Diesel Auto.

On the way to Gaydon this morning, I tried to set cruise, but it wouldn't engage. The green light comes on when I press the main cruise switch, but nothing else.

Having looked in the archives, I have seen mention of switches for the clutch pedal and brake pedal. Obviously, I don't have a clutch pedal. Is the switch for the brake pedal the same as the brake light switch, or is it a different one?

I've checked the fuse for CC.

I do have TOAF, but have never really done anything with it. Is it likely to help me diagnose the issue?

Thanks in advance.
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Old 15th April 2024, 08:16   #2
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(In France sorry for the mistakes)

Edit : Sorry i just see in your case it's a diesel so a bit different but maybe same cause.

When i baught my 75 (kv6 2.5), cruise control didn't work.

I had light on the console but no light on the dashboard.

After reading forums, I spent time to look on brake pedal (auto box), checked all fuses, looked in the motor bay actuator assy, and behind the steering wheel buttons or console button (i had others), try with toaf, etc., but nothing.

If it was the brake pedal switch your brake light should always be on normally.

It was the cruise control unit (ecu) and its interface behind the glove box. I solved ti with good advices from a french member who guided me on possible causes. I baught them second hand on ebay and all is ok now (wich one had a problem i don't know i replaced both).

Maybe same thing for you if you can try with an other owner for a test.

(Same thing for my keyfob not working, it was simply the receptor out of order, near the hand brake, after searching for a moment too. Bought second hand, plug and play and then perfectly ok)

Last edited by 900xj; 15th April 2024 at 09:08..
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Old 15th April 2024, 09:36   #3
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Merci bien.
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Old 15th April 2024, 09:59   #4
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I might be about to spout a load of rubbish here, but I think the way the cruise control works is that there's a rubber "cup" thing on the throttle body that uses pressure somehow (?) to pull a plastic rod that physically actuates the throttle. That's why you can feel it in the pedal under your foot when it kicks in.

When mine hasn't been working, it's usually because that cup has come off the throttle body and isn't forming a seal anymore. Putting it back on has fixed the cruise control twice for me.
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Old 15th April 2024, 12:00   #5
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Hi Dave
It maybe just a simple problem that the switch has slipped on the pedal, have you tried pushing it back into place?
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Old 15th April 2024, 12:11   #6
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Yes, look a the brake lights. If always lighted, bingo : switch.


"rubber "cup" thing on the throttle body" : On diesel too ? Or only electronical parts on CDT(i) ?

Last edited by 900xj; 15th April 2024 at 12:16..
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Old 15th April 2024, 12:37   #7
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Quote:
Originally Posted by TBennet View Post
I might be about to spout a load of rubbish here, but I think the way the cruise control works is that there's a rubber "cup" thing on the throttle body that uses pressure somehow (?) to pull a plastic rod that physically actuates the throttle.
You're not talking rubbish Tim; that is indeed how the cruise control works on my V6. The rubber "cup thing" contains a diaphragm to which on one side is applied a partial vacuum from an electrically driven vacuum pump.

However as 900xj says, the diesel's system is entirely electronic. RAVE says that it adjusts the fuel injector delivery to maintain the set speed.

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Last edited by SD1too; 15th April 2024 at 12:41..
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Old 15th April 2024, 13:43   #8
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Quote:
Originally Posted by Arctic View Post
Hi Dave
It maybe just a simple problem that the switch has slipped on the pedal, have you tried pushing it back into place?
Hi Steve

Does it use the brake light switch for cruise?

If so, my brake lights are working as expected.
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Old 15th April 2024, 14:02   #9
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The diesel cruise control ECU is usually pretty bulletproof - do your horn buttons work? Could be the rotary coupler failing. Had this on Mrs Noc's auto but it was the resume button wire that failed. Didn't bother fixing it as she doesn't use cruise but then the horns failed so I replaced the coupler and job done.
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Old 15th April 2024, 14:18   #10
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The diesel cruise control ECU is usually pretty bulletproof - do your horn buttons work? Could be the rotary coupler failing. Had this on Mrs Noc's auto but it was the resume button wire that failed. Didn't bother fixing it as she doesn't use cruise but then the horns failed so I replaced the coupler and job done.
Thanks Mike. I did wonder about that. I tested the horn last night and it worked OK.

The radio buttons stopped working about 18 months ago. I noticed it shortly after removing the interior to dry out the carpets, but don't know if something was disturbed when removing the console, or whether it was just coincidence.

Is there an easy way to test the rotary coupler other than replacing it?
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