Expert: Rob Painter - 7/25/2011

Question
QUESTION: I have a 95 STS.  Recently it started to act like the engine was intermittently being shut off while driving (slight dip in the tach, but no indication of lights dimming).  It started occasionally, 3-5 times in 10-miles, then gradually increased to 30-40 times within a couple weeks.  Suddenly, one night, I started the car, turned on lights, started to drive away and instrument info went to zero, tach went to zero then instrument panel went extremely dim.....  IP Voltage had been showing 12.7 or so, best I can recall, but it suddenly dropped to below 12V.  I shut off the car and tried to restart... no go battery too dead to engage starter and voltage continued to drop.  Installed new battery.... all systems worked fine for about 10 miles.  Charging was about 13.8V and slowly came down to about 13.1V.... then voltage dropped to 21.7 and the car started to go through the intermittent running and voltage continued to drop.  Replaced the alternator the next day and all was well, voltage ranged from 14V to 13.1V, A/C worked all the time.... All was good!  NOT! After driving for 7 days with no problems, the voltage suddenly dropped to 12.7 and all problems returned.  There doesn't seem to be any charging per the IP Volt meter.  Battery cables clean and ground connections good.  Pulled connectors from PCM incase of bad contact, no change.  Also, no error codes.

Any Ideas?  Is this possibly a bad PCM?

Regards,
Glen

I do my own work, have the service manuals.

ANSWER: Hello,

These can be real easy problems to fix and they can be real hard. The first problem is the parts changing without the fix.

Here is what I have run into with these puppies. The PCM and voltmeter can possibly be fried as an after effect and not the original problem. I have seen dealers changing this by installing a PCM because it was burned out as a result of the original problem after the alternator was changed and the battery twice. They also changed the burned out volt meter at another time. The guy had $1,300 trying to correct the problem and I made a suggestion. The ground! He said they had all been checked. I said, the ground!
A mechanic repositioned after drilling a hole in the frame about 6" away. The car is fixed! Not bad for being thousands of miles away from the car. This is a common issue. Don't know why the ground goes bad, but it does and when it does, it has the same complaints as you have. Since you have the manuals and you do your own work, you know there is not much involved in the charging system and since you replaced the battery and the alternator, you realize that the only other parts are the computer and volt meter (might still be good). That only leaves one thing--the ground. It may look fine, but relocate, before changing the PCM. Check the battery again because a surge will boil the water out.

Let me know how it works out please.

http://www.autotheftexpert.com

---------- FOLLOW-UP ----------

QUESTION: Thanks for the advice, I will try the grounds again and check the ground cable.  Will also consider an additional ground from body to engine.

ANSWER: Hello,

Don't forget to check the battery after you do the ground. Hopefully PCM did not fry.

http://www.autotheftexpert.com

---------- FOLLOW-UP ----------

QUESTION: The parts changing was a quick fix attempt, needing the car ....  Now I have another and can work on the real problem.

Additional work.... I re-did the battery grounds and cleaned all battery cables.  Drove the car and all seemed fine for about 25-miles, the IP DVM showed 13.0V to 13.3V most of the time  (initially about 13.8V for about a mile).  At 25-miles, the voltage started to dip in to the 12.8V to 13.0V, then the A/C compressor shut off, voltage dropped to 12.7V and the engine started it's intermittent shut-off.  Slight at first, then as the voltage started dropping down as far as 11.6V the engine shut-off was more frequent and more severe, but still the duration was a second or less.  When I got home, I got the DVM out and started to take voltage readings. The car restarted with IP-DVM showing 13.3V, my Fluke189 DVM measured:
13.5V from the cigarette lighter socket
13.8V from BAT+ to BAT- & Engine
13.76V from BAT+ to body... (A/C clutch engaged)
Engine-off battery 12.6V

A hour later I started the car and the IP DVM was showing 12.3V and at the battery, 12.6V and the A/C clutch would not engage.
Increasing the engine speed, no change in voltage readings and as the car ran, the voltage started to drop.  Obviously no charging.  Was not able to get a voltage reading on the "L" terminal at this time.  There are no codes set no indicator light for not charging.

I did not mention the error codes:
I052 Keep Alive Memory Error,
P109 Keep Alive Memory (KAM) Reset/PCM has lost primary battery power,
T025 LF Wheel Speed Sensor Fault,
T027 LF Wheel Speed Sensor Continuity Fault (intermittent),
S060 Left Front Position Sensor Fault (intermittent)

Any wiring diagrams of the PCM available?  I'm tending to think I have a bad solder joint or ground on the PBM board.

Answer
Hello,

Well, it's not charging. That,s obvious because you need to be at a minimum of 13.2v.
You said you had the manuals, they should tell you what the codes mean.
With that voltage, I don't care if you have already changed the alternator, it sounds like it is not functioning properly.
As I said before, the charging system is relatively simple. You have the volt meter, pcm, alternator, battery and the cables. There is nothing more to it.
The pcm does not control the alternator. The alternator controls the pcm and voltmeter.
We have established the connections are good. We assume the battery is good because it was replaced.
If you use a voltmeter at the battery and you are reading 12v, and if you use a voltmeter at the alternator, you have an alternator issue on any other car, but to be honest, I don't know what a pbm board is.
Sounds like you are fighting this. Been there--it sucks.
You might try all data http://www.alldata.com to see if they have a manual listed for the car. If so, pay the $30 for a year membership and download the factory manual immediately. wiring diagrams are there and anything else you need.
Keep it simple--New, rebuilt, and used alternators fail. That is why they have a warranty.
I have had cars from Hell with electrical issues that I knew I replaced the alternator and so that could not be the problem. I checked everything pulling my hair out. Guess what? It was the brand new alternator I had installed!
I have been through so many electrical issues I got stuck with because the dealer couldn't fix the car!
I found that if I didn't look so hard at the car for something else, the answr was usually obvious.

as far as your codes, they are coming up because of your charging state. Once you find the problem and fix it, clear the codes and they should not come back.
Good luck!

http://www.autotheftexpert.com