Battery gauge dies when lights on

Need help trying to figure out what’s going on with my battery gauge. When the car is running the gauge reads fine, a little bit higher than the middle, charging good. Car starts and runs fine so it’s definitely charging OK.
The only time it changes is when I put my headlights on. Battery gauge goes straight down and pins as low as it can go.
Checked the voltage across the battery, reads 12v.
Start the car - meter goes up around12.9v maybe little higher.
Rev it up to 2000rpm still reads around 12.9 doesn’t fluctuate much.
turn on the headlights and it still reads around 12.9 but the battery gauge down to zero.
If there was a short or something wouldn’t the meter reading at the battery die as well.
How do I figure what’s happening with the meter? could it be at fault?
Ray Z

You said battery gauge so I’m assuming a volt meter not ammeter. To test the gauge, put a good volt meter across the back of the gauge while doing your test and compare.

If they are the same not the voltmeter. With car running alternator should be putting out about 14.6 v or so. If it is reading 12 volts, I’d question your alternator.

During the low charge condition pull the bonnet pigtail to see if anything changes.



The symptoms don’t make a lot of sense, which leads me to the conclusion that it’s unlikely that anyone is going to be able to give to an instant diagnosis. My experience of diagnosing electrical problems on this forum is that it is critical that we know what we are dealing with before we start making suggestions. So many of these cars do not have stock wiring any more, so much time can be wasted when basic assumptions are incorrect. So, perhaps you could tell us:

  1. What E-type do you have - model, year (put it in your profile under “my cars” and we won’t have to ask again)

  2. What mods have been done to the electrical system that you know of? Don’t forget things like non-standard alternator, electroniic ignition…

  3. Did this problem just happen, or has it always done this? If it’s a new problem, did you just change something?

  4. Do the headlights actually come on, when you turn them on? Do all the other lights that should come on at the same time too, come on - e.g. side lights, instrument lights etc.

As already mentioned, if your alternator is only putting out 12.9v there’s something wrong. It really should be a little over 14v. Whether this is related to your gauge behaviour I don’t know, but it could be.


1 Like

When turning on the headlamps causes something else to act strangely I immediately suspect a bad ground.

Headlamps are a big draw and if their ground is poor or missing they will ‘find’ a path to ground somewhere else.

Check the ground - especially at the big bonnet plug.

First step is to confirm the voltage in the system to see if your alternator is at fault. If not 14 plus at fast idle with or without headlights then there is a problem with the alternator or regulator depending on your model.

This problem is on a 1969 Jaguar xke 2+2 4.2L.This has been an ongoing problem.
It doesn’t have electronic ignition. I had the alternator taken in and checked last year trying solve this problem and it checked out OK. It’s original. In my attempts to fix this problem I replaced the regulator and the wiring alternator harness.
None of this has helped. All the lights actually work. So I figured it wouldn’t be a short or something would blow. I.E. fuse, wires, etc.
I eliminated the alternator because I had it checked, and if it wasn’t working the battery would die. I take it out for drives and the battery always gets charged and works. It’s all original except those I mentioned and they were changed in the hopes of fixing this problem.

Can you post a picture of the back of your voltmeter?

I suspect that you have the headlight switch somehow connecting to the downstream side of the voltmeter.

1 Like

That’s the most straightforward explanation. For example, were the VM grounded to the headlight rather than directly to ground, it would pick up ground through the headlight filaments when the are switched off, but pick up 12V when they are switched on–providing 12V on both sides of the VM, or zero volts across it.

1 Like

Agree with Andrew. The VM is likely connected to the wrong place, e.g. the light switch somewhere. The wiring diagrams I have show the VM connected to the instrument voltage regulator via green wire for 12v and grounded via black wire.

Right–the input of the IVR, not the output.

1 Like

I again took my alternator into a shop and had it checked out. They tell me it’s fine in good shape.

If both of you are right (Paul/Andrew) and the problem is the VM somehow grounded to the headlights. I guess I could check that simply by removing the ground (black) on the back of the battery meter and connect that post directly to a ground.
I don’t think it could be an incorrect connection on the other side of the meter. The green goes to the regulator. If that was wrong the oil gauge would have a problem as well because both battery meter and oil have the same connect to the regulator.
Hope this doesn’t sound too confusing.

Good plan for ruling out the ground connection. But to make clear…

Both the oil gauge and the voltmeter should go to ignition +12V. One point for that is the input pin on the regulator. But they don’t really connect to the regulator in the normal sense of that phrase. They could pick up 12V anywhere. The IVR has no role at all.

You want the voltmeter to read battery voltage (when ign switch is on) and not the pulsing regulated output of the IVR. Similarly, unlike the fuel and temp gauges, the OP gauge is powered directly from the battery because its voltage regulator is contained within the OP sender. Neither uses regulated voltage.