OK, good ! If it didn't work at all that would be a problem

I'm a bit mystified by the over-voltage 'lightening bolt' being on...unless low voltage will also trigger it. But we'll see what happens after you get the alternator fixed and pursue mysteries only if needed

Cheers
DD

 

Yeah - mine is an '88. Don't understand what the module does but it gave the same symptoms as above and took a bit of remove/substitute/replace effort to track down the problem.

 

Well, got my regulators, set up the remote mounting of the regulator outside the alternator, installed the alternator, and... nothing. Still no charge. I've managed to track down some XJ40 alternators but they're coming out of cars that are being parted out, so can't say for sure if the units would be functioning. Time to bite the bullet and buy a new alternator, haha. How do I go about taking you up on this deal?

 

ThunderRoad,

Any updates on fixing your Lightning Bolt lamp problem?

I have a Lightning Bolt lamp staying ON after starting the engine. Looks like similar to the issue you described, no?

My working hypothesis is the alternator going south. Was planning to replace the alternator (with the same type/model, but from another XJS which did not have issues with Lightning Bolt lamp on the dashboard.)

Did I understand your last post correctly that after replacing the alternator you still had the dashboard light on?

 

Howdy v1rok!

My update on my alternator issue is that it has been fixed, by replacing the alternator altogether with a new unit.

After replacing the regulator and reinstalling the original alternator, the issue persisted with no charging. I believe that the diodes in the alternator itself had failed. Who knows how old the original unit was or when it was replaced in the car, regardless, it looked like it was well used.

If your alternator light is on while the engine is on, what does your battery gauge show? Is it over charging or under charging? I would recommend the steps mentioned earlier in this thread for basic troubleshooting. It could very well be that the regulator has gone, or that the diodes have failed, or could even be a wiring issue.

Check the wiring to the alternator to see if everything that's is supposed to be connected is connected, ie. wiring. If that's all good, then you may need to remove the alternator, and test it, either by taking it into a parts shop that may have a tester, or using a multimeter on diode mode to check if current is strong, going only one way, or going both ways. If the current is poor, or the current is going both ways, then it could be a diode issue, which will mean either rebuilding the alternator or replacing it. There's a few good videos on YouTube of how to test an alternator at home.

If the diode test is all good, then could be the regulator that has failed. If that's the case, just get a replacement which is pretty cheap, reinstall it (or modify it to be remotely installed in the engine bay to make replacement easier in the future), and that should be all good.

Keep us posted on your progress, mate! Hopefully it's a simple issue to fix!

 

Measuring rectifier diodes with a DVM on diode mode, these large diodes quite often will show as open circuit in BOTH directions as the DVM does not have enough current source to turn the diode on in the forward direction.

If the diode is short circuit then the DVM will show this, but this is NOT a typical failure mode for this type of diode, these usually go leaky or open circuit. A DVM may not show this type of failure.

The best way to test large diodes is to put them in series with a 6volt battery and incandescent bulb. In the forward direction the DVM will measure 0.7volts across the diode, reverse the diode and the DVM will measure 6.0volts across the diode.

 

The ALT lamp (lightning bolt) is turned on when the battery voltage is higher than the ALT voltage, or the ALT voltage is higher than battery voltage.

So the lightning bolt can come on with either a failing alternator or failing battery.

Under NO circumstances disconnect the battery with the engine running, this can damage the ALT and other electronics such as ECU.

You need to check voltages before you replace anything.

Battery voltage with battery disconnected should be around 12.8volts and not less than 12.6volts. At 12.2volts a Lead Acid Battery is only 50% charged.

Charging voltage should be between 13.5 - 14.5volts, depending on the state of charge of the battery

 

Thanks. Yes, will do the easy tests first before attempting replacing the alternator. This weekend most likely. Will update

 

I'd take a hard look at the the battery cables to see if they have good grounds and absolutely NO corrosion from a leaking battery; it seems obvious, but these things are sensitive to battery issues and under 12.5 v on some cars will not work. However, if terminals are great and cables are good, then at idle you should have 13.5 -15v if you turn electronics on.