Hello guys,
Some of you may have noticed from my other thread I'm attempting to fix a few (but very annoying) problems on my 97 4.6L.
All my lights were pulsing, so I dusted off my voltmeter and checked the alternator. It seemed to be charging okay, but the voltage was fluctuating too much for my liking so I presumed from reading the results from similar problems on here it must have a dead regulator.
I sourced a NEW one from the net, and fitted it. I got the following steady readings from the battery:

• Idling with lights on full beam.... 13.9V (14.1V)
• 1500RPM with lights on full beam.... 13.9V (14.1V)
• 1500RPM with lights off.... 14.06V (14.17V)

(The figures in brackets is the voltage direct from the alternator)

The problem I have is with the faulty alternator, I never had the battery symbol on the dash, or the "alternator fault" message come up on the display. With the NEW alternator fitted the battery symbol stays on and every time I start the motor I get the "alternator fault" message come up.

Does anyone know why this is happening? I will admit I did all this with a battery still connected so I was wondering if I need to disconnect it to reset the BeCM?

Look for a bad connection on D+ feed from alternator back to BECM.

Hello Wammers,
I was reading a thread where you had a heated debate over this D+ feed.
It got very technical and now I can't find the thread to use as a reference.
Could I ask you kindly to explain how I trace the connection back to the BECM?
I've noticed you have a lot of knowledge regarding p38 electrics so hopefully your the guy to solve this mystery
In your opinion is my new alternator working correctly from the stated voltage values?
Thanks Gary

EssexP38 said:

Hello Wammers,
I was reading a thread where you had a heated debate over this D+ feed.
It got very technical and now I can't find the thread to use as a reference.
Could I ask you kindly to explain how I trace the connection back to the BECM?
I've noticed you have a lot of knowledge regarding p38 electrics so hopefully your the guy to solve this mystery
In your opinion is my new alternator working correctly from the stated voltage values?
Thanks Gary

Click to expand...

Basically the D+ from BECM supplies 12 volts to the controller and switches on the battery lamp on the dash. When the alternator produces 12 volts the polarity is changed which switches off the battery lamp.

**UPDATE**
After a couple of drinks last night I decided I might as well purchase a second NEW Alternator to make sure the first replacement wasn't faulty.
So today I purchased a genuine new DENSO alternator from my local car parts shop.
An hour later it was fitted and .... NO "alternator fault" message and NO battery symbol whilst running!
It seems my p38's electric's are very sensitive to non standard replacement parts!
Thanks to anyone that posted and offered me advice. I have a few other parts that need attention so I will be needing further advice soon.

When I had my alternator refurbed the guy in my local shop said he had checked and was informed he could only use genuine parts to fix it which put the cost up a fair bit compared to pattern parts I had mentioned the set point was critical for the alternator and cars electrics, your pattern part is the likely cause.

EssexP38 said:

Hello guys,
Some of you may have noticed from my other thread I'm attempting to fix a few (but very annoying) problems on my 97 4.6L.
All my lights were pulsing, so I dusted off my voltmeter and checked the alternator. It seemed to be charging okay, but the voltage was fluctuating too much for my liking so I presumed from reading the results from similar problems on here it must have a dead regulator.
I sourced a NEW one from the net, and fitted it. I got the following steady readings from the battery:

• Idling with lights on full beam.... 13.9V (14.1V)
• 1500RPM with lights on full beam.... 13.9V (14.1V)
• 1500RPM with lights off.... 14.06V (14.17V)

(The figures in brackets is the voltage direct from the alternator)

The problem I have is with the faulty alternator, I never had the battery symbol on the dash, or the "alternator fault" message come up on the display. With the NEW alternator fitted the battery symbol stays on and every time I start the motor I get the "alternator fault" message come up.

Does anyone know why this is happening? I will admit I did all this with a battery still connected so I was wondering if I need to disconnect it to reset the BeCM?

Click to expand...

Hi
To avoid frying your new alternator, I would try and solve your voltage drop (battery vs alternator). If your multimeter is good, the voltage drop between alternator and battery suggests a bad connection or corroded wire somewhere in circuit. The voltage drop between the two shouldn't really be there, unless you're running some really power-hungry stuff, like seat heaters or rear screen. Try the following: First clip a good jumper from alternator casing to battery negative and see if the voltage drop disappears. If nothing changes, try the same from alternator + to battery + (careful - you may need to build a jumper of some heavy wire with a terminal that fits the bolt to avoid risking a short if the clamp were to fall off). If you find that your battery voltage rises with either of these tests, you have narrowed down the search. I would suspect the main ground lead or the alternator +. On mine it turned out to be the latter. The alternator + goes from the alternator and to the starter main terminal and feeds back to the battery through the starter cable. I ended up snipping of and isolating the original lead at the alternator end and running a new separate wire from the alternator to the battery. I now have zero voltage drop.

Henrik