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elbekko

What's going on with my cruise control?

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'99 Thor/Bosch P38. When I set the cruise control on the motorway, it'll constantly fluctuate in speed. If I set it to 120 km/h, it'll drop slowly to 115 km/h, then accelerate again, and repeat this process constantly.

I checked the vacuum system yesterday, manually operating the pump, and it kept the actuator perfectly steady over multiple hours, so that can't be it.

Any ideas?

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Posted (edited)

A bit late but as no one else could be bothered.

Have you checked the vacuum pipe, is the pipe work and pedal dump valve/valves 100 percent from the vacuum pump to the valve/ valves via the actuator ?

Edited by teabag
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The vacuum system seems to check out, we ran the pump manually to get some movement in the actuator, then left it while doing other stuff and we could find no sign of a leak, everything stayed in position. But when using the cruise, there is a very annoying and regular fluctuation of the speed. Almost as if the closed loop system is hunting instead of settling.

Filip

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To clarify, all the vacuum hoses are connected together, so operating the pump tests all of them simultaneously.

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As the P38 has the same kit as my Disco1 u could try this test after locating ECU and unplugging the connector.

The test is simple and I’d start  with the vacuum pump test first, the bellows will collapse right back into it’s housing if the vacuum is good through out the hose system any  right to the end of the hose which the brake pedal vac dump valve, and of course the clutch pedal valve if u have a manual gearbox.

The speed sensor test may be useful, but I believe the voltage is pulsed, it’s something I didn’t bother to do based that my vehicles speed sensor was okay because the EFI system and speedo reading didn’t have any issues. All the previous tests had a positive result so I replaced the ECU and so returned back to a functioning Cruise Control.

 

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