Overcharging 99 STS

[jlinde]

About a year ago I helped a friend with his 99 STS (he had just bought it with what was said

to be a faulty alternator and also maybe a faulty PCM) and changed the alternator for him.

Despite changing to an extra alternator and then to a brand new one the overcharge was still there.

I also changed the PCM to an extra one but that did not work at all -I have later learned that

the VIN# has to be programmed into the PCM - so changed back to original PCM and the engine

worked beautiful as before but still overcharge - 16.5 Volts. I told my friend that this problem was not

for me but for specialists. It became obvious that this car did have this problem even a long time

before my friend bought it and it still has this problem. It has been investigated by at least two

specialists since last spring but the problem is still not solved.

When idling the voltage is 13.9 Volts and the 'no charge control lamp indicator' is lit up.

When for instance the cooling fans started up the voltage went up to 16 Volts and the 'no charge'

went out. Then when the fans stopped when water temp went down the voltage droppet to 14

and 'no charge indicator' was lit up again. The voltage is measured directly over the alternator by an extra meter.

I remember when I was working on this car a year ago

that everything behaved right for just around two minutes after a battery change.

This is just written to explain that problems can be very tricky and we would really appreciate

any kind of approach to solve this problem. /Jan L

[navion]

About a year ago I helped a friend with his 99 STS (he had just bought it with what was said

to be a faulty alternator and also maybe a faulty PCM) and changed the alternator for him.

Despite changing to an extra alternator and then to a brand new one the overcharge was still there.

I also changed the PCM to an extra one but that did not work at all -I have later learned that

the VIN# has to be programmed into the PCM - so changed back to original PCM and the engine

worked beautiful as before but still overcharge - 16.5 Volts. I told my friend that this problem was not

for me but for specialists. It became obvious that this car did have this problem even a long time

before my friend bought it and it still has this problem. It has been investigated by at least two

specialists since last spring but the problem is still not solved.

When idling the voltage is 13.9 Volts and the 'no charge control lamp indicator' is lit up.

When for instance the cooling fans started up the voltage went up to 16 Volts and the 'no charge'

went out. Then when the fans stopped when water temp went down the voltage droppet to 14

and 'no charge indicator' was lit up again. The voltage is measured directly over the alternator by an extra meter.

I remember when I was working on this car a year ago

that everything behaved right for just around two minutes after a battery change.

This is just written to explain that problems can be very tricky and we would really appreciate

any kind of approach to solve this problem. /Jan L

This sounds like a classic case of a high resistance somewhere in the electrical system.

I would start by checking the voltage between the alternator output and different places on the vehicle.

Withe the engine running, first check would be alternator output to the engine it's-self.

Then from the alternator output to the car's structure. Then between alternator output and battery ground. Note any difference in voltage between readings.

Next check Alternator output to the battery positive post. If there is a voltage drop shown there, then you are getting close.

I would suspect that there is a problem with the bonding between the engine and the cars body. Or the battery ground cable could have a bad connection where it connects to the car's body. OR one of the battery cables could have corrosion inside, causing a high resistance.

Any high resistance can cause your problem. Set your meter on a low voltage scale and measure for voltage drop from one end of the battery cables to the other. On the ground cable measure from the battery post to the car's body.

You should have no voltage readings at all checking this way. If you find a voltage drop from one end of a cable to the other, or between the cable and what it is connected to, then you may have found your problem.

Good Luck,

Britt

[BodybyFisher]

Does this 99 have the battery under the back seat? How does it crank?

[jlinde]

Yes the 99 STS has the battery under the back seat.

I think the owner and 'specialists' have checked and added extra wiring to get good ground.

Myself I am not an automobile electrician but I have hard to understand how an alternator can

overcharge when the regulator is within the unit and if I understand it right only needs a little

currant from the ignition in the very thin wire to get started. The owner has thoughts about buying

a pure AC Delco alternator for it. He even put in a new instrument panel (with IPC) to get rid of that possibility of error.

There is no problem to start the car so those parts of the cables feel OK.

And the owner drives a tow truck all day long and helps people with bad batteries all the time.

Maybe the two alternators that have been used to get the car out on the road again are both faulty

-probably not so. I bought a set of 99 Seville Shop manuals but haven't studied these yet

-I have the 93-94 books since before -real good help in many cases. The older books points to

faulty regulators when it is about overcharging.

/Jan L

[BodybyFisher]

The problem with having the battery under the back seat is the length of the POSITIVE wire. Extended cranking can melt and oxidize the wire. Trace the wire from the battery through the firewall to the battery and feel for bumps in the wire. DEFINATELY check the positive cables resistance. The only problem with this thought is, with this type of problem (high resistance) the battery will have a hard time turning the starter especially in cold weather. How does it crank.

A thin wire would MELT immediately trying to turn the starter.

Is this an aftermarket alternator do you know? I agree with Britt, this does not sound like an alternator problem. Has anyone checked to see EXACTLY what the alternator is outputting with a volt meter?