Alternator not charging.. I'm stumped (and Clueless)

[KNS]

Just installed a re-manufactured Bosch alternator - Battery is less than a month old. I also have a voltmeter installed in my tachometer (North Hollywood Speedo modification)

Problem: With the engine running the voltmeter will only occasionally show 13.5 to 14.5 volts at 2000 RPM or more. I can start the car and the voltmeter will show 12 to 12.5 volts. While driving the voltmeter will sometimes rise to near 14 volts and stay there for perhaps five minutes, maybe ten or 15 minutes and then settle back down to around 12 volts. One time down to approx. 11.8 volts while driving.

The alternator light does come on prior to engine start (brightly) and goes out when the engine is started. I have new Battery ground strap, new Trans ground strap and new alternator ground. While checking it in the garage, my multimeter agrees with the in-dash voltmeter at idle and 2000 RPM.

Bentley Manual indicates it could be fan belt (nice and tight), alt or VR. I pulled the alternator this morning and had it bench tested (with it spinning), everything OK there. The man at the shop suggested it could be the red + wires that connect to the alternator. I don't know how to check this and don't want to fry my multimeter. Beyond this, I'm stumped.

Anyone..? sorry for the long post.

[rick-l]

Sounds like a bad rebuild.

[KNS]

Sounds like a bad rebuild...

It is a Bosh reman which I just had bench tested this morning - alternator OK..

[tcar]

What does it show idling w/ headlights on bright, rear wdo defroster on...?

Sounds like maybe VR to me, especially if it's intermittent.

Originally the VR on the '80 was by the rear fuse panel, but almost all have had those unplugged and have the later VR that's integral with the alternator. I have seen a few that are still using the separate VR by the fuse panel. I think there's a pic of the separate VR in the Bentley manual.

[86 911 Targa]

Originally Posted by KNS

Just installed a re-manufactured Bosch alternator - Battery is less than a month old. I also have a voltmeter installed in my tachometer (North Hollywood Speedo modification)

Problem: With the engine running the voltmeter will only occasionally show 13.5 to 14.5 volts at 2000 RPM or more. I can start the car and the voltmeter will show 12 to 12.5 volts. While driving the voltmeter will sometimes rise to near 14 volts and stay there for perhaps five minutes, maybe ten or 15 minutes and then settle back down to around 12 volts. One time down to approx. 11.8 volts while driving.

The alternator light does come on prior to engine start (brightly) and goes out when the engine is started. I have new Battery ground strap, new Trans ground strap and new alternator ground. While checking it in the garage, my multimeter agrees with the in-dash voltmeter at idle and 2000 RPM.

Bentley Manual indicates it could be fan belt (nice and tight), alt or VR. I pulled the alternator this morning and had it bench tested (with it spinning), everything OK there. The man at the shop suggested it could be the red + wires that connect to the alternator. I don't know how to check this and don't want to fry my multimeter. Beyond this, I'm stumped.

Anyone..? sorry for the long post.

Is your Voltage regulator built into the alternator?

Or is it separate?

[86 911 Targa]

What were the voltage v/s load #'s at bench test?

Is your Voltage regulator built into the alternator?

Or is it separate?

You may pm me.

[KNS]

Thanks for the replies.

The problem car is an 84 Carrera, the VR is built in and the guy who bench tested the alternator said the VR was working fine (supposedly new with the reman alternator). I haven't put the alternator back in yet, I'm wondering if there is anything else I can check before I do.


I have a voltmeter in the '80 SC and as soon as the engine is started and the entire time the engine running, the voltmeter reads between 13.5 and 14 volts.

I don't know what else to check on the Carrera..

[911pcars]

Originally Posted by KNS

Just installed a re-manufactured Bosch alternator - Battery is less than a month old. I also have a voltmeter installed in my tachometer (North Hollywood Speedo modification)

Problem: With the engine running the voltmeter will only occasionally show 13.5 to 14.5 volts at 2000 RPM or more. I can start the car and the voltmeter will show 12 to 12.5 volts. While driving the voltmeter will sometimes rise to near 14 volts and stay there for perhaps five minutes, maybe ten or 15 minutes and then settle back down to around 12 volts. One time down to approx. 11.8 volts while driving.

The alternator light does come on prior to engine start (brightly) and goes out when the engine is started. I have new Battery ground strap, new Trans ground strap and new alternator ground. While checking it in the garage, my multimeter agrees with the in-dash voltmeter at idle and 2000 RPM.

Bentley Manual indicates it could be fan belt (nice and tight), alt or VR. I pulled the alternator this morning and had it bench tested (with it spinning), everything OK there. The man at the shop suggested it could be the red + wires that connect to the alternator. I don't know how to check this and don't want to fry my multimeter. Beyond this, I'm stumped.

Anyone..? sorry for the long post.

Since the alternator checks out okay as indicated by the shop, I'd look at the alternator wires leading to and from the battery. With both ends disconnected (bat. and alt.), connect an ohmmeter test lead to each end. It should show continuity, 0 ohms. Wiggle the wire connectors where accessible while observing the meter. Don't forget the alt. wire at the starter/battery junction. Could be an intermittent open circuit from a bad connector. Also check battery ground and circuit connections.

I'd normally suggest a VM to check for excessive voltage drop (excessive resistance or open circuit), but access to alternator connections is impossible on a running engine. The ohmmeter is second best.

Hope this helps,
Sherwood

[KNS]

911pcars,

Thanks. I'll look into your suggestion.

Could a new Voltage Regulator be operating intermittently?

[vintage guy]

Could also be your voltmeter. Try checking voltages with a known good one.

[John W.]

Voltage Regulator could be intermittent especially if a connection is making and breaking. I was also thinking like Chuck that the voltmeter could have an issue. Is your battery staying charged or dying?

John

[James Brown]

I know this might sound funny but it might be running correctly. No ALT light on during normal operation. Where is the volt meter getting it's power from? is it just a convenient power source or is a dedicated wire coming from the battery? Also, sounds like the battery is fully charged so the ALT. will not send power to it. Check with a full load on the ALT like tcar said. Original car did not come with a volt meter (mine does now). Also check resistance from the ALT to the battery like 911pcar says. As long as the ALT light does not come on with the engine running I feel that would be fine.

[KNS]

The voltmeter is incorporated into the tachometer (looks like a 930 turbo boost gauge, North Hollywood Speedometer did the mod). When I put a multimeter to the battery - engine off, at idle, and 2000 RPM, the multimeter matches the reading of the voltmeter in the tach.

I have not driven the car for more than a half hour since trying to figure this issue out, and after parking the car, engine off, the battery will show 12.4 to 12.5 volts. To me, it points to an intermittent VR - but it is brand new, installed on the Bosch reman alternator. It possibly could be normal operation. It is confusing when the Bentley manual is so cut and dry about what it "should" be reading.

Again, thank you guys for all the help and suggestions...!!

[lr172]

My first suspect would be the voltmeter. Your idiot light should come on below 13 volts or so and it sounds like it is. This is a simple device. When volts get over ~13 it goes out. Nothing more sophisticated than that. If your voltmeter shows 12.5 volts and the idiot light is not on, I would not suspect your alt or VR. Just wire up a VOM to your cigarette lighter and go for a drive. When your volt meter shows 12 V, look at your VOM for comparison. Don't go replacing parts till your sure you have a problem.

Larry

[lr172]

Also, a VR will always strive to maintain a steady voltage (typically 13.5 - 14.5) regardless of the state of battery charge. The VR continually adjusts the current output of the alternator by maintaining a steady voltage (in simple terms). In other words, it keeps adjusting the Alt output to keep a steady voltage and the result is a continuously variable current. The only time that your voltage should be below this level is when the alternator is at max capacity and it cannot meet the demand requested by the VR. In these cases, such as idle, heavily discharged battery, etc., you will see voltages below the normal range because the alt cannot satisfy what the VR is requesting.

Larry

[KNS]

My multimeter (from the battery) corresponds with what the voltmeter is reading. They pretty much match. I have not yet seen the battery drop below 12.4 volts.

lr172, I see what you are saying about the idiot light, but why does my multimeter reading from the battery also show less than 13 volts at 2000 RPM?

[ischmitz]

First off a 80SC doesn't have an idiot light. Or are you referring to the alternator light? If the alternator stops charging the battery voltage will quickly drop to 12.5 volts. So it appears that the alternator is charging intermittantly. Also, if the D+ doesn't draw any current the alternator light will not come on.

I would suspect the rebuild as others. A simple bench test does not resemble the vibrations the alternator is exposed to when running on the engine. So a cold solder joint in the internal regulator or a bad brush or partially stuck brush would all show the symptoms you are describing. The brushes are spring-loaded against the rotor contacts. Once one of the brushes looses contact the alternator output drops to zero and the regulator doesn't draw any current through D+ so the alternator light will not come on.

If the voltmeter was bad it would be a real coincidence that it happens to drop just to the voltage that is consistant with the alternator not charging. So I'd see that as a long shot at best.

If the shop that sold the alternator doesn't stand behind its work you are screwed. In that case you need to get the alternator out, remove the brush assempbly yourself and inspect if the brushes move freely in their guides. Also, maybe get a new regulator just in case.

Ingo

[KNS]

ischmitz,

Thanks for the reply, the car is an 84 Carrera, the alternator is a factory remanufactured Bosch. If I bring the car to a shop, can the shop verify the problem with the alt installed? Is there a conclusive test?

[rick-l]

Originally Posted by KNS

the alternator is a factory remanufactured Bosch.

It doesn't say whose factory the Bosch alternator was refurbed in.

My 88 has a Valeo.

Edit: I'll second the take the regulator off and look at the brushes.

[86 911 Targa]

Originally Posted by KNS

ischmitz,

Thanks for the reply, the car is an 84 Carrera, the alternator is a factory remanufactured Bosch. If I bring the car to a shop, can the shop verify the problem with the alt installed? Is there a conclusive test?

Yes,

A reputable shop can load test your alternator.