Lost volts on the instrument panel

[almcl]

Out in the XJS yesterday as the other Jaguar was in for some work and as I set off to come home at dusk, I noticed the voltage on the gauge hovering just below the 13 volt mark. It isn't usually in this position and the battery, while not brand new, is reported as being at least 80% capacity by my smart charger. So drove home with headlights on and air con on low setting, noticing that the indicated voltage was initially a hair's breadth less than the middle of the 13 volt line, gradually recovering to a hair's breadth over.

We got home alright, 25 miles of mixed main road, dual carriageway/motorway and town streets and once on the driveway at home with lights off and engine idling, I put a volt meter on the battery terminals: 13.6 volts! Quite happy with that but now I wonder why the gauge on the dash is under reading? I remember being able to adjust the voltmeter on my Series 3 XJ6 but doubt a similar facility exists on the facelift XJS. I also thought the instrument lights looked a bit dim - but that may just be comparison with the newer XF which is the one I normally drive at night.

This morning I went and checked again. At switch on the gauge indicated barely 11 v while a meter on the battery said almost 12v.

At idle after start the in-car gauge is just over 13 v while a meter at the battery says 14 v. Almost a volt down in both cases.

So, anyone any thoughts on where the missing volt(s) might have gone? High resistance earth connection for the instrument cluster, possibly? Where would one look for that I wonder?

All thoughts welcome.

[almcl]

Sorry! This was meant to go into Elelectrics.

Are the mods able to move it?

[davidr]

Moved as requested

[ptjs1]

Al,

I'm amazed your car has been able to get through to its current age before the instrument cluster started under-reading especially the voltmeter.

Most XJS minor instruments tend to be wayward because of deterioration of the ribbon cable connections and particularly the earth points.

Remove the cluster and clean up all the connections, partic the earths. Also consider running an independent earth to the cluster.

Also check and clean the earth bulkhead points on both sides of the car.

Good luck

Paul

[almcl]

Many thanks, David!

Also Paul for the thoughts.

Last time I had the instrument cluster out (a fair few yaers ago now) it didn't look too bad (I thought) :

Is the earthing points on this that are recommended for treatment, or where they attach to the body?

[ptjs1]

Al,

I can't remember which of those small nuts on the circuit board are earth points. However, if you look at one of the plugs that connects to the circuit board, the earth feed is the black wire. So if you bridge a ohm meter from that point to each of the small nuts in turn, you'll establish the earth points.

Then you can undo one and put the eyelet of one end of a new wire to that point and connect the other end to any part of the body with a decent contact. (I think there's an eyelet near the A-post, or else some point on the body around the dash area). Put a joint in that wire if you like so the cluster could easily still be removed in the future without having to undo the wire on the cluster on the body.

I would fully expect that all your minor gauges will now be more accurate.

The other earth points to check are your two bulkhead -ve points that are covered with rubber boots. (The other 2 near them are +ve points). These are notorious for starting to build up corrosion. Undo the nuts and thoroughly check for integrity.

Hope that helps.

Cheers

Paul

[almcl]

Thank you very much, Paul.

I'll have a go at that once the storm is out the way!

Cheers!