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I can't remember whether I asked specifically about this subject, so I will risk asking again !!

I read, on the RR P38 facebook forum thingy, that both the diesel and petrol P38's use the brake light switch to help control the cruise control. I wonder if someone can confirm this ? My CC isn't working although the switch lights up. And I know that the diesel doesn't use the vacuum tubes, like the petrol vehicle. But, perhaps the brake light switch could be the problem ?

Pierre3.

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Correct. Clutch also on the manual.

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Ah-ha, I see. I have an automatic so the clutch pedal doesn't come into the equation, fortunately. But, I would then ask the question, if the clutch pedal has a switch for the CC does that mean that manual P38's use a different brake light switch, with presumably no CC switch built into the brake light switch, than automatic P38's, which use only a brake pedal ?

Or do manual P38's have a CC switch in the brake light switch and a seperate one under the clutch pedal ?

Pierre3.

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Can't help much further being new to the p38 myself. However, I did read a post somewhere with a unreliable CC issue. A following motorist noticed the brake lights flickering on a p38 in front, CC was dropping out on that vehicle. Brake switch adjustment or replacement, can't recall which, gave reliable CC operation again.

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Pierre,
I can't say with certainty in relation to the P38. While I've had to replace a couple of the vacuum tubes, so far I've had no issues with the switches so haven't needed to look.

But, I'd be very surprised if the brake pedal didn't have a CC switch. You can imagine the scenario - trundling down the motorway at 70mph, and you need to brake. I'd expect this to disengage the CC immediately. You may not bother/need to use the clutch initially, but the CC would need to come off with the brake. Certainly this is my experience of how more modern cars work.

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On the petrol models there are actually two brake switches. The brake bulb output from the normal brake switch is connected to the cruise ECU, then the brake vent valve also has an additional switch which is also connected to the cruise ECU. Then the vent valve itself acts as a redundancy mechanism to dump the vacuum.

For petrols theres a very good diagnostic guide in rave, which steps you thru checking all the sensors and things. Might be worth looking to see if the same exists for the diesel version.

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OK, thanks for the replies, guys. I think that I will leave things as they are because the brake light switch works the brake lights perfectly. I don't want to screw up something that is working in the hope of fixing something that I don't really need.

The CC on the diesel seems to be a bit of an unknown beast, so I will leave well alone !!

Pierre3.

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There are 2 brake switches on the diesel.
Becm checks one against the other. One will be open circuit and the other closed circuit with brakes not on and the values change when brake is applied.

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Tanis8472, thanks for your information. I will have a look at the brake switch this weekend, weather permitting.

Pierre3.

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Pierre3 wrote:

Tanis8472, thanks for your information. I will have a look at the brake switch this weekend, weather permitting.

Pierre3.
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