When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Is there a way to test the diode on a 1998 VPD alternator?
I have a battery drain problem, and all the long tried methods to isolate it have failed so far.
tia
What happens when you pull the positive cable off the battery with the car running?
I would not recommend removing the battery cables while the car is running. While this test works on older cars without a hitch, modern cars are more sensitive to such things. Damage could occur to the computer modules.
I would not recommend removing the battery cables while the car is running. While this test works on older cars without a hitch, modern cars are more sensitive to such things. Damage could occur to the computer modules.
Removing the battery cable is a standard diagnostics procedure (extract from the Workshop Manual, PDF Page 1912):
Yet you chose to question me?
Been doing this quick "alternator test" for decades and on cars much newer than these X308s. If it causes damage to anything it's a Jag only problem.
Last edited by 60Gunner; Sep 19, 2021 at 01:20 AM.
Oh, my apologies.
Glad to be proven wrong by someone with more knowledge on the matter.
You were right, in principle. Without additional special protection, the voltage of an alternator having a standard regulator will go high (could be as high as 80 Volts !) if the battery is disconnected while the engine is running. I remember that for the cars of the 70's, disconnecting the battery on a running engine was a no-no as the alternator voltage would go high and may damage things. However, on cars of those days, it would mostly be blown bulbs and perhaps a radio. Most modern cars, due to so much electronics, had to have special protection circuits added to prevent the alternator voltage rise if the battery is disconnected, otherwise an accidently left loose battery cable could cause all electronics to blow-up (a major disaster).
Up to about 1992, Jaguar had external devices to prevent alternator voltage rise when the battery is disconnected and from about 1993 on, the protection was included in the alternator's voltage regulator.
In any case, I would not disconnect the battery with the engine running on any modern car without first checking that this is o.k. to do.
Essentially a 1 amp draw is huge! Do you have anything that’s aftermarket installed in the car?
If your answer is no, in my experience of current draws in these cars, these are the most common components of failure for excessive draw.
Body processor module (BPM)
Security locking control module (SLCM)
Door module. There’s 1 in each door.
Excluding the alternator, what have you done so far to try and diagnose or narrow down your excessive current draw? Do you lock the car door with the key and all the locks pop up and down? There is also a timeframe in which all the modules go into “sleep mode” after you lock the car. I find that 30-60 minutes is the norm and that’s when the ma number for the draw is at the most accurate.
Last thought. How old is the battery and is it a good quality one? Sometimes when I perform a draw test I need to use a new battery just to rule out the possibility that the battery itself isn’t the issue of giving me a false draw number.
Last edited by Addicted2boost; Sep 19, 2021 at 04:49 PM.
Essentially a 1 amp draw is huge! Do you have anything that’s aftermarket installed in the car?
If your answer is no, in my experience of current draws in these cars, these are the most common components of failure for excessive draw.
Body processor module (BPM)
Security locking control module (SLCM)
Door module. There’s 1 in each door.
Excluding the alternator, what have you done so far to try and diagnose or narrow down your excessive current draw? Do you lock the car door with the key and all the locks pop up and down? There is also a timeframe in which all the modules go into “sleep mode” after you lock the car. I find that 30-60 minutes is the norm and that’s when the ma number for the draw is at the most accurate.
Last thought. How old is the battery and is it a good quality one? Sometimes when I perform a draw test I need to use a new battery just to rule out the possibility that the battery itself isn’t the issue of giving me a false draw number.
No after market anything.
Tests across fuses. No relay tests.
Battery definitely NOT the problem.
I use the remote to lock the doors, but sometimes leave it unlocked. If I use the key, the buttons will go down or up.
When you turn the ignition off and pull the key out, do you hear a sharp “snap”? There’s a little door that closes where the ignition key was. If that door doesn’t close when you pull the key out, the modules won’t go into sleep mode because the car thinks there’s a key in the ignition and will cause an excessive current draw.
When you turn the ignition off and pull the key out, do you hear a sharp “snap”? There’s a little door that closes where the ignition key was. If that door doesn’t close when you pull the key out, the modules won’t go into sleep mode because the car thinks there’s a key in the ignition and will cause an excessive current draw.
I do not hear a snap. The door seems mostly closed I'll check again.
Essentially a 1 amp draw is huge! Do you have anything that’s aftermarket installed in the car?
If your answer is no, in my experience of current draws in these cars, these are the most common components of failure for excessive draw.
Body processor module (BPM)
Security locking control module (SLCM)
Door module. There’s 1 in each door.
Excluding the alternator, what have you done so far to try and diagnose or narrow down your excessive current draw? Do you lock the car door with the key and all the locks pop up and down? There is also a timeframe in which all the modules go into “sleep mode” after you lock the car. I find that 30-60 minutes is the norm and that’s when the ma number for the draw is at the most accurate.
Last thought. How old is the battery and is it a good quality one? Sometimes when I perform a draw test I need to use a new battery just to rule out the possibility that the battery itself isn’t the issue of giving me a false draw number.
Any videos on how to replace door modules? The driver side rear door had issues before, also the MPB?
Alternator diode test
