I posted a couple of months ago about tach twitches, which still happen when the revs are low (under 600, which is a bit too low I think), but I've also noticed that having the aircon on and the headlights on (especially at the same time) can send the ammeter down under 13, which I believe means it's draining the battery/the alternator can't keep up with demand. It also often twitches down when the engine revs (tach) twitch up, but only very slightly. I've included a very short (20 second) video to show you what I mean, edited to highlight the issues:

I don't recall seeing the ammeter drop below midway before, though admittedly I was more concerned about the other side of the instrument panel (which is behaving very nicely these days, thankfully). The car's going in for its biannual legally-required inspection next week, so if there's anything I need to tell them about this, I'd be grateful for the heads-up....

 

I don't know about your inspections, but I would start by double checking the dash gauge. Simply place a volt meter across the battery terminals (set to 20v) and see if you're getting 13+ volts at idle. Then check again with your load test, you should still get 13+ volts at 2000 RPM. Next check the amps at 1000k RPM's, my alternator for my 94 is 120 amps for instance. Just going by your dashboard your not long from an alternator failure, probably just the brushes our the built in regulator so you can rebuild these yourself, or have it done by a shop.

 

Thanks muchly. I'll start off with that.
I should add that the low ammeter is only at idle, and on the move it doesn't generally drop below the middle mark.

 

It's actually a voltmeter. Not that important, my OCD just forces me to mention it...

An Am-meter will have 0 at the middle and ± 100A (or whatever max current is...)

BR Leo

 

Thanks for that, Leo. No sarcasm. I actually wondered whether the correct term was voltmeter or ammeter, but searches for the former showed lots of hand-held measuring things, and searches for the latter showed lots of in-dash dials, and I didn't see any specific details on the difference.
So being a voltmeter not an ammeter, that means that the needle to the left of the middle doesn't mean it's discharging/running off the battery then?

 

I'm not sure I see a problem .

I dunno about your 1992 car but my various older Jags (and other cars) of 80s vintage had 65 -85 amp alternators which behave exactly as yours....struggling a wee bit at idle speed if loaded by a combination of A/C. headlights, rear defogger, etc. Tickle the engine speed up just a bit and the volts would pick up instantly.

Later cars (such as my 1995 XJR) had higher output alternators.....I wanna say 115 amp....and fared much better.

I agree, though, that the dashboard voltmeter should be checked against an independent meter. The voltmeters in the old Series IIIs were adjustable to compensate for error....but I'm not aware of this handy feature existing on other models.

Cheers
DD

 

Doug - that's precisely my question, indeed - is this a problem? I don't recall noticing it before--normally, at the lights, my attention is focused on the temperature gauge on the other side. I only noticed the voltmeter dropping when I noticed the tach jerk slightly whenever the car would jolt a wee bit (or perhaps I should say, the car would jolt when the revs jumped). I suspect (hope) the revs jerking might be the ECU compensating for an idle set a little too low, but the low voltmeter with loaded electrics was a cause for concern, hence this post.
Actually, according to Jaguar Classic Parts at least, it should be the 115 amp alternator....

 

A volt-meter shows voltage. High readings means the battery is being charged, low means the alternator cannot keep up with the demands.
A fully charged battery voltage will be at appr. 12,6V potential, so that's what you ideally should see on your meter with ignition on and motor not running. Normal charging voltage will be around 14.0-14.5V, but in idle this will drop due to lower performance of the alternator.
You shall start worrying if the needle is constantly low (bad alternator) or constantly high (charging regulator defective or battery dead (high resistance, not accepting the charge)). Especially the high reading is bad as it will fry other components...

The Am-meter shows the current flow. Positive means charging, negative means discharging. The am-meter is quite uncommon to have in cars as you basically will need to have it connected in series with the alternator or battery.

 

Then I'd be tempted to have the alternator output checked.

Over the years, on various cars, I've come across a few alternators that were indeed functioning....but doing so below specification. Just a couple weeks ago I had an old Mustang that showed only 12.9 volts at high idle. Had alternator tested; 40 amp alternator but only producing about 20 amp output. Had alternator rebuilt and, presto, 13.8 volts at idle, right on the money!

Cheers
DD

 

I agree with getting it checked. But also worth cleaning the engine earths, which go from the LHS engine sandwich plate to the front suspension cage to the LHS lower chassis rail. Grant, and my experience proves it too, recommends fitting another earth direct from the alternator bracket to the chassis frame.

 

Oh yes, I'll definitely see about checking the alternator.
I've read your thread, Greg, about Grant's earth idea. Sounds easy enough (will carefully re-read it of course, in case it isn't). Taking it back to bare metal, using grease, etc. First task, get a volt meter (not a voltmeter) and check power.

 

The dash gauge can be a bit flaky too. I was noticing low voltage on the gauge on my car and was worried enough about it that I stopped mid trip on a run from Calgary to Victoria (~1000 km) at a tool store and bought a voltmeter. It showed everything was fine.

 

Volt meter and am[ meter. Good tools to have.

I made a patch cord for my HF sourced VOM
can connect to my cars via the cigar lighter outlet. I can read volts in real time under various load and rpm conditions. Much more responsive than the voltmeter in my car.

The last car I recall owning with an ammeter was our 65/66 Ford Mustang fast back. I kinda liked it.

When I got my first car in 46, it was a 23 Ford T. The only instrument on it's dash panel was an ammeter. It's generator could come up with 15 amps!!! Good enougth!!! Only draw was the seldom used starter and the headlights and single tail light.

OTH my LT1 boasts of a hefty 140 amp alternator. Lots of draw in it's opulent donor car.

Carl.

Carl

 

Hi someday, I have one more thing for you to check

DC volts at battery should be between 13.8 and 14.4 engine running, this means the alternator is charging. This MUST be measured at the battery, not the cigarette lighter or alternator.

AC volts should be less than 0.5volts with the engine running. If it is more then a diode in the alternator has failed.

If both these are ok then your alternator is good. In your car with 115amp alternator you should not have much movement in the gauge, Check all the connections from the alternator to the battery, clean them and make sure they are tight. Dirty or loose connections will cause a voltage drop when loaded ie lights etc are turned on.

 

Thanks for the additional tips, Warrjon. I'll check those as soon as I can (today's raining hard, so it'll have to wait even if I did have a voltmeter handy). I'll also try to trace the wiring from the alternator to the battery. How many of the connections from the battery to the alternator are under the car? (Is there in fact a wire running directly, or do they branch off a single connection once the main lead gets to the engine bay? I actually have no idea, come to think of it...)

 

From the alternator, a brown wire goes to the starter solenoid. The starter is on the right side of the engine, under the exhaust manifold and not at all easy to get to. Impossible from above, and easier from below once the exhaust downpipe is removed. From the starter another brown wire goes to a stud on the right side of the bulkhead. From the bulkhead stud, a heavy wire runs back to the battery in the boot. I believe this wire is black and inside the car, but I'm not 100% sure. Similar wires I have pulled from saloons have been black

I would expect the wires at the alternator and bulkhead stud are under a rubber boot to insulate them. The nut at the alternator is probably metric, the bulkhead might be in inches.

If you are removing the rubber boot and/or nuts , either disconnect the battery negative cable first, or use only non conductive tools to displace the boot/nut. One slip or touch in the wrong place and you run the risk of turning your tool into an arc welder. Definitely a sub-optimal outcome where you run the risk of fleeing, phoning, and fire fighting.

 

Hmm. Great care is required, indeed. I'll make sure to disconnect the battery after I've measured the voltages. I'll check what connections I can, but getting underneath and removing exhaust pipes is almost certainly beyond what I can manage in my current situation.

 

I had to do it to change a starter. I have a hoist to get the car in the air and even then it was an awful job. One of those "don't do it unless absolutely necessary" sort of things.

 

Definitely something to leave for the professionals, then....

 

The big wire runs under the car in the transmission tunnel there are no connectors under there so it'll be ok.

I would check the terminal post on the fire wall RH side (mine was not tight) and the battery connections in the boot, make sure both of these are tight with no corrosion.