96 XJ VDP LWB (X300)

The issue, with engine running:
No Load/P: In-dash charging indicater shows >13v, load tester reads 13.9v - 14.25v
No Load/D/Stationary: in-dash drops
Lights/AC/P: in-dash drops <13v
Lights/AC/D/Stationary: in-dash drops <13, load test 12.4
Lights/AC/D/Road: in-dash shows normal charge, drop when load increases (windows/sun roof)


What i've done already:
Battery tested at good at 14v
I recently replaced my original alternator with a used alt from my jag mechanic, same problem. I replaced the used alt with a new alt, same problem.

I figure there must be a diode somewhere in the system to cause the problem. Does anybody know of this issue, or have that specific schematic for the charging system.

 

If nobody knows anything about the charging system maybe somebody knows where i can find the wiring schematic for the system.

Where are all the pros?

 

I believe you can find the schematics for the whole car at the top of this forum page. Here's a direct link to the thread: https://www.jaguarforums.com/forum/x...ams-faq-34888/

 

I don't really understand the problem/question.

I would expect the volt meter to drop a bit if the idle speed is reduced (such as going from "P" to "D") and/or if circuits are powred up to create a load.

Are you saying the voltmeter dips and doesn't recover or ???

Cheers
DD

 

Re: your battery test, getting 14V off a lead-acid (when it's not being charged) seems a bit odd. Did you use a proper battery tester?

 

Battery voltage is not the issue. Alt and batt test fine
P/no load. Charge is 13.5-.9v, 14.25-.3v @ 2K rpm (no load)
D/no load. Charge is 13.5v, pretty normal
P/Lts,AC. Charge is 12.9-13.1v
P/2Krpm/Lts/AC. Charge at 13.5-.9v
D/Idle/Lts/AC. Charge at 12.4v
D/2Krpm/Lts/AC. Charge at 13.5v

Under load the voltage does not recover unless rpm is up. Basically, as long as i'm driving 2K rpm it's charging under load. That's not normal.

 

Mine drops like that which seems normal as long as it recovers once my rpm's come up if yours is not doing that did the check the voltage regulator????

 

Isn't the V. reg built into the alt? I haven't seen it as a seperate component in any of my previous jags

 

Concern. Cause. Correction.

Confirm these in that order.

Is the concern that you see the voltage reading on the instrument cluster below 12.6 volts at idle with electrical loads present ?

When you see the low voltage on your instrument cluster, is voltage the same at the battery when read with test equipment and what is the reading? If it's not the same at the battery and the cluster, how much different is it with and without loads? You may have a voltage drop at the cluster caused by a bad connection in the circuit or a guage reading incorrectly.

Are you experiencing any starting problems like slow cranking speed or having to be jump started with cables or battery booster pack?

Once we confirm the concern, we can work on confirming the cause.

 

The readings are from the tester. Between the 1st and 6th post i thought i'd made it clear.

 

Forgive me, please. English is my second language.

If the concern is that your dash and your tester at the battery posts show 12.4 volts at ilde in drive with electrical loads, that's not too far from normal. Especially if you have no starting issues like slow cranking speed or having to be jump started.

If I had this issue, one test I would do is a voltage drop test at the alternator. With a quality voltmeter, the voltage between the negative battery post and the case of the alternator should be less than .2-.3 volts.

The voltage between the output terminal of the alternator and the positive battery post should also be no more than .2-.3 volts.

Both of these tests should be done while the low voltage concern is present.

If you have more than .5 volts on either voltage drop test, you have excessive circuit resistance. There may be bad connections like juction terminals that pass current through the firewall or maybe in the trunk.

I remember fixing a couple of these at the firewall.

Good luck. Feliz Navidad

 

It is not clear to me where you are reading the voltage, but I agree it does seem to drop off under load more than I might expect. You did not mention the idle engine speed. If the alternator output is low, it might be low rpm, or would be either a loose belt or one phase of the alternator having a bad diode. But it could also be a voltage drop in the power circuit since the no load voltage seems a little high. It is probably NOT the regulator since the the voltage is high enough when the alternator can make current. The regulator is built into the alternator