I am dealing with a voltage drop on the xk8 and doing a little research to see if I can locate the problem so today I started my investigation.

History: I recently replaced the battery the old one was over 8 yrs old and replaced the alternator because I had the system tested and it showed that the regulator was not working properly or was it.

Last night drove it with the A/C on and lights on and I noticed the voltage dropping at an idle so I began testing today and this is what I came up with.

At idle in “P” and the A/C off & I think the lights were on:

Volts 13.6
RPM 651
Coolant 91c

At idle in “P” A/C & lights on

Volts 13.4
RPM 654 – 652
Coolant 93c – 95c

In “D” A/C & Lights on

Volts 12.3 – 13.0
RPM 602 - 596
Coolant 95 – 96
At 97c the fans went on high (stage 2) and the voltage dropped to 11.7

Should the computer be looking at the A/C and compensation for the additional load or did I get an alternator that is not regulating properly? If it sounds like the computer I will have it checked. If not then I will look for the cause of the additional load or the alternator. I do have an unusual ratcheting sound when the A/C is on (not from the A/C but at the firewall) and have been trying to hunt it down but know little about that system. It sounds like it is coming from the two solenoids related to the heating system but not sure. I began looking at the training manuals but nothing to direct me to the problem.

 

Gus, the voltage should not drop below 12.1V with all that switched on. The fans are quite heavy but the regulator should compensate for it.
May be an idea to take some current readings with a clip on ammeter if you can.

 

At the present time I am charging the battery. My next step is to begin taking amp readings but that is a time consuming process. I wanted to make this post to get additional input to see if I overlooked something. We can only hope for a quick fix!!

 

As any alternator output drops off rapidly at low RPM, it could just be that the reduced idle speed in D is enough to take the alternator output below the point at which the regulator can do its job.

 

And that was a concern. The ECM sees the A/C and headlights on and it compensates with a higher idle but when the fans go to the second stage the RPM’s go down. Is this normal? That is the question or is something on that is adding to the problem.

Edit When put into "D" the RPM's change.

 

From all I have read the ECM is to control the RPM’s. I have the idle RPM’s set correctly.

 

Fig 3 of the attachment shows that the fans do/should have an input to the idle speed control system. Further detail of how to interrogate the system to see what is actually happening in your situation might be difficult to find!

Obviously the transmission system also has an input to the engine speed control but I can't see anything on whether that control might include an input to idle control on selecting D

 

I think you are correct in saying it might be difficult to find.

After a little dialog I realized I did not check the idle speed in “D” first then the A/C & Lights. I will do that on Friday. Funny how you see what you did not do after you begin to talk about it with others. Got to love the forum….

I will read over your text and see how I can work with it.

 

Gus,
I've just taken some readings off mine, and they're not far apart from what you are getting (engine warmed up, measuring at the connector under the hood):

At idle in “P” all off:

Volts 14.24
RPM ~600

At idle in “P” A/C & lights on full beam:

Volts 12.78
RPM ~600

At idle in “D” A/C & lights on full beam:

Lowest voltage was 12.14
RPM ~590

I don't think the fans went to full speed even though it was idling for 30 mins or so, so I didn't replicate the last part of your checks.

The engine rpm is remarkably constant once it's warm despite the various loads applied so the ECM is doing its job OK.

I don't have any issues with starting (or module memory loss) so I'm guessing all is good. Battery is about three years old.

Are you having any issues other than the system test?

HTH

 

I am short on time but it looks like we are about the same. I got my readings from the cars computer OBDII. What is taking place is that I can see the voltage deteriorate on the cars gauge and I got an ABS failure. I will give an update on Friday I have an obligation all day tomorrow but will check this thread.

 

The specifics escape me at the moment, but some folks have posted recently about faulty replacement alternators of one brand or another, and having to switch brand for a working unit...