Noisy relay

[ecurie]

My 1979 Rolls Royce Silver Shadow has recently developped following fault :

With the automatic transmission in D or R, engine running and no throtlle (so foot on the brake or letting the car creep forward without touching the throttle) there is a loud rattling sound coming from behind the speedometer. I dismantled part of the dash and removed the speedometer, but it still rattled. Now I could hear the noise came from a relay (one of its wires going to the speedometer). When touching this relay, you can really feel it vibrating. It is clear that the relay is clicking very fast (like a machine gun).
When I give it some throttle the rattle immediately stops. Just the same as when I put the transmission lever in P or N : then the rattle also immediately stops.
Furthermore this fault is only present after having driven the car for a while and everything has warmed up nicely.

I now wonder : is this a faulty relay or is it a faulty component that sends a signal to the realy causing it to rattle ?
First I thought it might be a poor earth or contact, but wouldn't the relay than also ratlle with all systems still cold ?
As the fault only occurs when stationary with the transmission in D or R am I correct in guessing that the fault (if it isn't a duff relay) will be with the speedo transducer (there is no speedo cable, it is an electronic system) or its electrical connection to the relay ?

Benoit

[Cambronne]

I'm no Shadow expert but I reckon there are some tests you could carry out.

From your description, it's obvious that the relay is being activated wrongly by some feed whose characteristics change with a bit of throttle or selecting P or N while everything's warm. This seems to suggest that the relay should have a continous live feed under these circumstances. Or it's dying.

This seems to suggest the following...

* The feed to the relay is being partly interrupted by something that's capable of feeding it properly when it's cold.

* The relay is poorly and can't hold the contact except when the live feed is stronger, i.e. more revs, higher battery voltage.

Could you try these tests?

Use a (preferably analogue) multimeter to measure the output of the feed to the relay?

Attach a small bulb (e.g. instrument light) to the feed to find out if it flickers?

Disconnect the relay feed and replace it with a flying lead connected directly to a known good live feed.

All things being equal, a wobbly meter needle/flickery bulb means the transducer or the wiring to/from it is at fault. More flying leads could confirm which.

If the relay holds in with its own known good feed, it's probably fine. If it buzzes, it's poorly.

Just using old fashioned logic here. If you try these tests, don't short out the flying leads.

[History]

Does the relay have a brown yellow wire going to it.

If so that wire goes to a pressure sensor in the oil filter head.
These sensors start to fail which causes the relay to chatter. The sensor is Std Joe Lucas part.

My Shadow 1, the sensor failed and a Austin Metro one fitted and is still ok 20 years later.

This bit is covered in the Australian Rolls- Royce Owners Club Forum with photos.

Regards
Bob Reddington.

[ecurie]

I managed to solve the problem.
It was indeed, as Bob suggested, a faulty oil pressure sensor.

Thanks for your help.