Electrical system failure 1956 Mk 1

I was having trouble with my electrical system last year, eventually got fixed:

Reported: “We ran a new wire from the fender ground point to the earth terminal of the battery and everything worked as it should and, no warning light illuminated.”

New problem this year, almost identical to last year:

Last summer I got my 1956 Mk 1 electric issues sorted out (reconnecting unlabeled wires behind the dash, reconnecting wires to lights and several other places, re-configuring the fuse block to put the wires where they belong, fixing ground problems.)

I recently removed the speedometer. The only wires I disconnected were 4 ground wires at an attachment point at top of panel and pulled the indicator light bulb holders for ignition and headlights from the speedometer. Speedo has now been reinstalled. Ground wires and bulb holders reattached. Instrument Panel is back in place.

I thought it prudent to put a test light between battery ground post and ground strap to make sure everything is still working. Test light indicates electric system problems. Any suggestions to remedy the following faults would be greatly appreciated:

Door activated courtesy lights: dim when door opened, dim test light.

The following happens when I have key on:

Starting carb Solenoid clicks, test light dim, IGN light starts bright, then dim.

Activate fan switch: no fan, bright test light, IGN light goes off, petrol gauge goes to 0.

Activate interior light switch: bright test light, dim interior light.

Activate panel light switch: nothing.

Activate horn: weak sound, bright test light

Activate headlight switch, position 1 & 2: bright test light, nothing at sidelights or headlights.

At the fuse block, key off, I have 12.5v at A1, A2: with key on I have 8.5v at all fuse block connection points.

As I mentioned, some of the above electrical faults are the same as last year. I’m going to check the ground wire I added. Any suggestions are appreciated


I figured it out and everything works correctly.

The problem was: the secondary ground wire, that was added last year, was not touching the ground point at the battery.

The secondary ground wire is attached to the ground cable at the clamp, when the clamp was removed to install the test light the ground to all of the electrical systems was lost.

I still don’t know why the ground is bad in this car, will eventually figure it out.

It is not unknown for older vehicles to have a failure in the main earth cable…or be missing an earth strap or wire…are you sure your vehicles has the correct original earth wires?

the way to chase the issue is to perform a “voltage drop test” with a DMM on the earth circuit

There is some excellent Youtubes that show how to do this…voltage drops for each part on the earth circuit should very low, in the order of 0.1V

having a new main earth cable made, and adding earths should fix any issues

Thanks for the information on the “voltage drop test.” I have never heard of it, been watching the videos to learn the technique.

I assume the ground wire I added to solve the electrical faults should be disconnected to find the real problem. Should I use the test light between ground post and ground cable to protect the fuses?

As for ground cables: battery to body is new braided, bell housing to body and oil filter block to frame is original braided. All were removed, cleaned, reinstalled to touch bare metal.


If I may, owning a 52 xk120 and 52 db2 if you have the original POSITIVE GROUND set up.
Did you replace any of the ground bolts with STAINLESS bolts or use the original and they are not cleaned.
Moisture and condensation KILL an old system. Also try a HEAVIER insulated ground wirefrom the battery and to the body!
When im done, I ground
Engine to body
body to frame
ALL lights have designated Hard wire ground not just body of bezel to car
I ground EVERY light source and DOUBLE THE THICKNESS of those wires.
HUGE difference, remember the old positive systems are a sort of closed loop, rusty frame or body doesnot help
good luck

1 Like

good point the, I assumed the OP has converted to - earth

although principles of voltage drop test remain the same…its just a bit more confusing if you have + earth

hard to assume what level of electrical knowledge posters have

from the list of symptoms, I would first suspect an earth connection behind instrument panel area may be the problem

while adding additional earths is something I have done, ideally, its is best to detect an eliminate any existing fault, as it can create its own voltage drop, with undesirable consequences

I agree good luck!
It has to be spot on or the system is terrible

Thanks for the additional info.

“Did you replace any of the ground bolts with STAINLESS bolts or use the original and they are not cleaned?”

I cleaned, removed rust, etc. on all of the contact surfaces. Cleaned the bolts, did not replace with stainless.

Could there be an issue with the panel attachment points (3 bolts), causing a ground problem, I think the left/right brackets are painted.

“I assumed the OP has converted to - earth”

Have not converted, still at + earth.

Electrical knowledge: not much. I can say I ran a ground wire from ground battery post to the ground clip on the rubber terminal block holder on the left fender and everything started working but, I have no idea why it worked.

As far as I know, the left fender, firewall and cable to the battery should have provided the ground that was needed.

to fault find, I would disconnect your wire, and establish the where the problem exists using the voltage drop tests. Painted main earth junctions are very suspect

You may need to watch a few youtubes to understand, but its quite straight forward really

positive earth wont make any difference

the easiest way to understand is the electrical circuit is as a whole bunch of small resistor in a string, and they are like taps on a hydraulic system

the problem is really exacerbated on later model vehicles, they have so many earth connections

Should I use the test light between ground post and ground cable to protect the fuses while doing the voltage drop test?

not in my opinion, as it would introduce a voltage drop of its own

the vehicle should be tested “as is” so the fault can be found

I cannot see why a fuse would blow, unless you introduce a direct short, or their is a fault

If so, and fuses blow, they are sacrificial

this is one case where a test light, which i advocate for auto fault finding is not used

dont use the Ohms setting

you are looking for tiny voltage drops of ~0.2V, only a high impedance DMM will show this accurately enough

After watching some videos and reading the Lucas Fault Diagnosis Service Manual I performed the following voltage drop tests that I believe are correctly done:

Voltage at the right tail light, results:

Battery: + to - with tail light off reads 12.68v

Battery: + to - with tail light on reads 12.43v

At tail light: Earth wire to tail light hot wire reads 11.83v

Battery earth to tail light earth, reads .00v

Battery Hot to tail light hot wire reads .49v

Right Headlight: (I checked the earth and hot sides at the right inner fender bullet connector block)


Battery: + to - with headlight off reads 12.53v

Battery: + to - with headlight on reads 12.15v

At connector block: Earth wire to HL hot wire reads 11.16v

Battery earth to HL earth, reads .01v

Battery Hot to HL hot wire reads .94v

Do these voltage numbers sound OK? The Lucas Fault Diagnosis Service Manual says 10% voltage drop is allowed.

Would I get better or more accurate results by going to the HL bulb?


To me imho
You have to clean all the grounds
Your saying each circuit works but drops
Not to off but drops
These systems were terrible then now with 50 years and crude they resist
Remember your ground is positive
You shouldn’t have a negative post wire from battery to fender…
Negative is your hot
Good luck

Are you referring to my comment in post #8 above:
“I ran a ground wire from ground battery post to the ground clip on the rubber terminal block holder on the left fender.”

I used the wrong terminology. “Earth post of a positive ground system” would have described the set-up.

I just discovered, when looking at my parts car wiring, the cigar lighter is wired differently and, I can’t find a resource to tell me which is correct.

On the car I am working on the ground wire is attached to the top (when looking at it from the side, panel tilted forward.) On the parts car the ground wire is attached in the middle, power wire goes to the top.

Which set up is correct?


Hi Bruce,. Is your car still Pos earth?
Difficult to tell without pictures but it sounds like your cigarette plug has been reversed probably for a negative earth application so you can run a GPS or similar. This is a bit of a scary way to do it as the external metal body of the lighter is now live. Easy way to confirm is with a multimeter and check for voltage between the body of the CL and and an earth reference (eg gearstick).

The lighter itself doesn’t care re polarity: it just wants amps to heat up…

Yes, my car is + earth. Below is a photo of the parts car CL. Is this correct? Thanks

It certainly looks original. I don’t believe this reversal of wiring on the CL will be the cause of your wiring fault. If this was incorrectly done you would have bigger problems like a fire under the dash.

Guys No DISRESPECT…If that picture is current.
I suggest taking each component apart and CLEAN and sand EVERY contact.
It looks like the dash was under salt water?
Good luck

Thanks for the comment. The photo is of the parts car, not the project. Photo was posted to see if the wiring to cigar lighter was correct.