Suspect ground - burning out speedo spring

Hiya folks,

I'm going to be spending the weekend hunting down a ground prob that has presented in a kind of funny way. Any suggestions would be appreciated.

The set-up:

I have a 76 MK1. A kenwood head unit was installed at the time, drawing power from the indicator circuit, and earthed (or so it seems) to the earth point at the fusebox.

From what i can tell, everything appears as it should.

What happens:

While driving, at night, with high beams on, fans and radio going, suddenly the headlights (and dashlights) will go very dim, radio cuts out, and when stopped,the car will not idle. When selected, indicators run fast, after a few seconds of the indicator relay  buzzing away.

At the same time, the speedo has gained an extra 30km/h.

Result:

A permanent result of this is the retention spring in the speedo has warped (which gives us the extra 30 clicks) such that the ground path for the dash is now through the speedo cable.

Disconnecting the speedo cable appears to return everything else to normal.

I've had the speedo repaired (twice   ) and have now had this problem twice. Both times everything seemed to work just great for weeks, until I re-installed the radio, and in both instances the problem happened with a relatively high electrical load (radio on, high beams, heater fans and all that)

I suspect a ground problem somewhere. What I'm really wondering about is what could be happening such that the speedo cable is the path to earth, rather than the earth leads from the rosette on the back of the speedo?

(The speedo repair dude said that the likely cause of the warped speedo spring/coil was that it was conducting, due to bad earth, and eventually heated up to the point that it 'sprung'. The scorch marks he showed me have me believing him… - which means the speedo was the earth path the whole time, no?)

Both times this happened before,I thought I'd fixed the prob. (Although I now think I was fixing what went wrong after the problem)

Both times it was the same head unit. I am going to replace that one. (Don't like it anyways)

I would like to sort this one out before I have the speedo repaired for the third time.

Anyone have any suggestions on most likely bad earth points to check out? I'm desperate enough that I'll try just about anything…  

Sorry if this was long winded - figured the more info the better,

-Trev

Hi, I am not sure that instrument panel in a 76 is similar to later Mk 1's..the complete panel is earthed through a black wire on the multi socket that connects the panel to the loom. Check at the panel end with a meter that this has a sound earth, otherwise the panel bulbs and instruments will earth via the speedo cable. Check also that the printed circuit film on the back of the panel is not damaged, particulary the strip going to the black wire contact of the plug. The other issue, and probably the easiest to check first is to make sure the engine has a good solid earth. You could think because it starts etc that all is well but you must have somewhere developed a voltage difference between the battery/engine/ inst panel. I will try to find the earth point for the panel and be in touch again.

Keep smiling. J.

I've so far checked the ground from each wire from the rosette on the back o the speedo. All have good continuity, and the main ground from this is good. (Using cont. tester - haven't checked resistance…)

The black ground wire from the instrument panel (on terminal six of the connector) is also good. The ground circuit that extends to the inst panel bulbs from terminal is also good. There doesn't appear to be any damage to the printed circuit film that can't be explained by 31+ years of degradation. (In other words, the plastic film is a tad brittle on the edges, but no cracks or breaks)

I'm going to give the connectors, spades and strips a good clean anyway. Can't hurt, right?

The radio head unit, and the wires supplying it have been removed completely, and I'm in no rush to install another one. When I do, it will be done very carefully. I can't really hear the radio over the slightly-larger-than-is-practical exhaust anyway.

All that's left is to check is the ground on the engine/gearbox.

I have read in the forums that some people have a ground strap connecting the rocker cover to the chassis - I don't have this. Should I? If so, where are the points I should have grounded?

In addition to that, what are the primary ground points for engine/gearbox? I'm going to check the condition of the straps and clean the connectors, but I don't want to miss anything.

A very big 'ta' for your help, Jonamat. Any more, from you or anyone else, would be very much appreciated.

-Trev

Trev, Primary ground for the engine…battery ground to body RH engine bay and then a hefty braid to gearbox. Also Braid strap vw 357 971 537 from rear RH cam cover screw to lower fixing bolt for coil on bulkhead.  

Cheers, J.

I've cleaned up the terminals/connectors on the battery strap at all points (gearbox mount, chassis and battery). I will get a strap for the rocker cover soon 'for insurance' (I read somewhere that that particular strap started appearing on later mk1s, correct?).

I also cleaned up all the points in the dash, and at the fusebox, and put a few connectors back where they were supposed to be (I'm assuming the last owner pulled them out accidentally and put them back in the wrong spot when he did the radio install). These were for some senders.

When I reconnected the speedo cable, everything works! (Before, when I connected the speedo, everything went dim immediately) Now I just need to get that retention spring replaced for the third time and everything should hopefully stay fixed.

Thanks for your help Jonamat.

-Trev