Bad earth affecting instruments

[Concrete-crusher]

Hi can anyone suggest where to look for a bad earth that affects the instruments.

When I switch on the headlamps both the tachometer and the voltmeter drop to zero.

Everything else seems fine.

Steve

[oldelanman]

Poor earths can cause some wierd problems. I would start by checking and thoroughly cleaning the main earth points. If your wiring is original there are only three and each uses body mount bolts.....

In the engine bay it's the bolt which screws into the LH suspension tower, it also secures the bottom end of the bonnet hinge support/radiator mount bracket. This is where your headlamps will be earthed.

Inside the car it's the two bolts located behind the bottom of the dash where it meets the tunnel (there may be a connection on both) the bolts also secure the bracket which the dash is bolted to. This is where instruments will be earthed as well as the heater motor, cigar lighter, electric windows (if you have them) courtesy light switches etc. You need to remove the under dash trim pieces to get at them.

In the boot it's one of the two bolts at the front centre of the boot floor - also where the main battery earth is connected. This is where all the rear lamps are earthed. To clean this one properly you need to unscrew the nut inside the boot then undo the bolt from below the car and clean of the contact area of the chassis under the bolt head as well. When refitting tighten the bolt first then put a spanner on the bolt head to prevent it unscrewing while you refit the earth connector and tighten the nut inside the boot.

If that doesn't fix your problem you need to work back from each earth point and clean the bullet connectors where the earths from the various components are joined.

[ricarbo]

Inevitably the problem is corrosion. After cleaning the suspect connection it's a good idea to try to stop this happening again, so I smother the parts in Vaseline before reassembly. Since Vaseline is non conductive this seems counter intuitive, but it is squeezed out when the fixings are tightened, permitting the newly cleaned metal to make good contact. Thereafter the residue keeps away moisture and air, eliminating corrosion and also conducts heat across the materials, reducing any 'hot spots'. I always use it on general connections, including bullet connectors, lamp holders and inside switches as it also improves the smoothness of the switch action. It just improves everything and I can't imagine why the car makers don't use it on a universal basis.
I suppose on earth connections you could substitute 'Copperslip' (PBC grease), but since it is conductive, it would be unsuitable in other applications such as switches, whereas I cannot think of any application for which Vaseline would be unsuitable.
Regards
Richard

[mbell]

This job is in my to-do list. I've bought some small packets of battery terminal grease to use. Not an expert but would expect it to have the right kind of properties to ensure a good connection and avoid corrosion problems for a decent length of time.

[Concrete-crusher]

Having looked for the earth connections in the engine bay I can,t see them. Does anybody have a photo showing the front strut connection.

Cheers Steve

[oldelanman]

It should be on the LH radiator support/bonnet hinge stay bracket.....either on the lower mounting bolt itself or on a separate bolt through the bracket as shown here....

[Concrete-crusher]

Thanks for the picture , can the bolt simply be undone without anything falling off ?

Thanks Steve

[oldelanman]

Thanks for the picture , can the bolt simply be undone without anything falling off ?

Thanks Steve

The bolt at the lower end of the bracket passes through the mounting bobbin in the body and is screwed directly into the chassis so nothing will fall off if you remove it.

If your earth cable is connected to a separate nut and bolt on the bracket as mine is you will need to clean that as well as the bracket mounting bolt.