P38A Electrical fault when braking

RR P38 1998, 4.6 V8 HSE petrol

When depressing the brake pedal, the dashboard, rev, etc, goes out. However, it sometimes also does it without pressing the brake pedal.

I have removed the black brake light switch (with three connector pins), and the revs drop down or are very erratic.

At the same time, there are a lot of other faults messages shown. Not sure if that is related to the above issue, as I think that there is something that discharges the battery.

Thanks

Søren

Huge guess without much to go on, i wonder if your fuse box is ok?

What happens when you fit a known good fully charged battery?
Have you fully charged your existing battery with a battery charger?

I think on the petrol, there is a vacuum pipe for cruise control associated with the brake switch, if that is leaking, could it affect the revs and/or idling?
^^^^^^Battery voltage is a good call.

+1 on check battery first. Also check the ground connection behind right footwell panel.

Datatek said:

I think on the petrol, there is a vacuum pipe for cruise control associated with the brake switch, if that is leaking, could it affect the revs and/or idling?
^^^^^^Battery voltage is a good call.

Click to expand...

The vacuum is from a pump, not from the manifold, so any leaks means it simply stops working, engine speed is unaffected

kermit_rr said:

The vacuum is from a pump, not from the manifold, so any leaks means it simply stops working, engine speed is unaffected

Click to expand...

OK thanks.

SorenH said:

RR P38 1998, 4.6 V8 HSE petrol

When depressing the brake pedal, the dashboard, rev, etc, goes out. However, it sometimes also does it without pressing the brake pedal.

I have removed the black brake light switch (with three connector pins), and the revs drop down or are very erratic.

At the same time, there are a lot of other faults messages shown. Not sure if that is related to the above issue, as I think that there is something that discharges the battery.

Thanks

Søren

Click to expand...

Instrument cluster comes out easily enough, although be real careful with the screws as the plastic breaks for fun. There's only one connector but might be worth a bit of electrical contact cleaner in the connectors.

That's assuming the revs drop on the dial but the engine still sounds fine. If the engine is really dying I'd be checking the earth straps around the engine bay and the battery terminal connectors are tight. I made a spacer from some copper pipe for someone's battery terminal clamps.