Low voltage possible

I am running an alternator and a gauge indicating volts and it barely shows 11 volts. I always drive with the headlights on and if I turn them off I will eventually show a positive charge on the gauge. I have manually checked the output of the alternator and the voltage at the battery with the engine running and they were well within specs. I could run a wire from the battery into the cabin while driving to double check the voltage while driving and noticing this low charge condition. Perhaps the gauge is off? I have a brand new battery in the car.

Thoughts?

Gerard

I took this vid for oil pressure , but you can see the volt gauge on start up , swings to charge , but after a few miles the needle stays quite still at the bottom of the gauge !
I still have a dynamo , fully rebuilt a few thousand miles ago !

If your battery is new , and the regulator is working as it should , would there be much need for high volts to show on the gauge ?
What you need is a flat battery , start the car with jump leads , then look at the gauge :grinning:

Should be simple. If you test the voltage at the battery with known good voltmeter you should see circa 12 volts with the engine off. Start the car and the reading should be close to 14.0volts when the engine is a little above idle. That will confirm the alternator is working.

Then repeat at the connections to your voltmeter on the dash.
Regards

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Same setup on a G. My voltmeter says 11,5 at idle/700. Does not vary with rpm. On a second G, the reading is 12,5 at idle and the needle moves between 11-13 depending on rpm.

did you resolve the issue with voltage ?

For the most part the car shows a charge on the gauge. If I run ALL accessories then it will barely be in the charge zone. I think I need to either rewire the entire car or very tediously clean all of the wire connections. I do always drive with the headlights on so that contribute to a significant draw. Looking around for some plug and play led low amperage headlight bulbs if anyone has a suggestive?

Gerard

if you run all the accessories, lights heater etc you will barely get a charge. that’s normal in my experience. it will also show discharge on tickover

yes what phil said ! …the generator is current limited by the regulator if headlamps and other stuff are on all the time the generator could be close to maxxed out .(this is in the 1966 mk2 .yes .?) …what that means…priority is given to protecting the generator (is it 25 or 30 amp generator ?) the regulator wont allow you to exceed the set current limit for your gen…if you exceed that limit the battery will have to supply the extra current …the ammeter will show negative(discharge) and you will end up with a flat battery …make sense ?

sorry I just went back and noticed you have an alternator …thats a different story…it’s all about voltage (so long as it’s current “rating” is up to the job)…should have about 14v at about 2000 revs…at the battery …almost regardless of load if not can you tell me ?

14.2 Vat the battery when running at idle. Gauge inside is a voltmeter and dips WAY down on full load of all accessories. I always idle for a minute at shutdown with accessories turned off and the meter shows a charge again.

Put a pair of wires between the battery and a DVM inside the car to check if the panelmeter is correct.

It is possible that the brushes on the alternator are not making consistent contact, but then you would be seeing the panelmeter swinging wildly as they make and break contact.

Also put a clamp on Ammeter at the battery and see how much current is being drawn with all the accessories on. If that’s above your alternator output then that’s your problem.

Good advice, prolly will check this out soon. Alternator tested VERY good at old school shop. and I have a 2 gauge wire going from alternator to Battery. You’ll recall I had that problem with a small burning wire scenario and subsequent use of a fire extinguisher which may still have left residue sapping energy from the circuit. It’s always something…

Gerard

I agree …sounds like your alternator is being overloaded
if you can’t source a “clamp on” ammeter you’ll just have to measure each load in turn then add them up I guess…can you do that ? this is on the '66 mk2 yes?..do you know what alternator it is ?

Our XJ6 does the same. A few years ago after truing up the slips rings and rekitting with bearings and brushes my son and I took off for Melbourne (about 900km). As we moved into evening I noted that the arse fell out of the volts with the headlights on (showing “off charge”). I suggested to my son that we might be overnighting at Nana’s place (mother-in-law) as I would have to investigate. We did stop at a service station and I checked the volts - 14.2V at idle with headlights on. We went on to Melbourne and Tasmania (on a boat) and returned without issue. Still have not investigated further, volts dip for air conditioner, head lights, rear window heater. A very low priority. Paul

this is becoming perplexing …when you stopped to check did you turn engine off then re-start… then check ? …and when you read 14.2 at idle with lights on (with another meter ?) what did the dash voltmeter say ?..after resuming travels did the volts continue at “off charge” ? …when you say volts dip for aircond/rear demist etc do you mean all at once or each …? …thanks

Did not stop and restart. With the multimeter in my “go” kit - best of the many small ones that I have. Dash voltmeter was in the “off charge” range - about 12v on the XK6 gauge (red zone). With any significant load it indicates “off charge”. Any of or all as brought into circuit - still better than 14V at the battery. My alternator is rated at 45 amps. That trip was two years ago and I have never had to charge the battery. - a new Australian made Century - good quality. Paul

thanks for the info paul …looks like some xj6 voltmeters don’t exactly tell the truth then …I wonder how many …