Voltage Gauge Normal Reading: what voltage?

[wallball]

I just noticed my voltage gauge reading being differently lately than what I recall. When I start the car, it reads around 14 Volts. After running for a while, perhaps due to heating of something, it gradually decreases to around 12.2 Volts. If I turn on the lights, it drops to 11.8 Volts. Not a good trend. I seem to recall that it would pretty much stay up at around 14 Volts consistently. I don't know how old the battery is. Is it reading the battery voltage, or a regulator output? What is this indicative of?

[z3forlife]

I'd start with a battery load test. Any battery store around you should have a good tester. I'm afraid it only gets more expensive after that. Good luck

[Schreier]

When the car is on and the alternator is running, the voltmeter is measuring the output of the alternator.

When the car is off, if the voltmeter is doing anything, it's measuring the battery's voltage.

My alternator will consistently provide 14.2 volts, but drops down to mid-to-high 13s between shifts. I don't recall it dropping much at all when accessories are turned on. And yet I have a battery light on my car, likely because something has happened to the voltage regulator in the alternator.

Is the battery light coming on at all when your voltage drops like that?

[wallball]

When the car is on and the alternator is running, the voltmeter is measuring the output of the alternator.

When the car is off, if the voltmeter is doing anything, it's measuring the battery's voltage.

My alternator will consistently provide 14.2 volts, but drops down to mid-to-high 13s between shifts. I don't recall it dropping much at all when accessories are turned on. And yet I have a battery light on my car, likely because something has happened to the voltage regulator in the alternator.

Is the battery light coming on at all when your voltage drops like that?

Battery voltage with engine off = 11.8 on the gauge, and 12.56 at the terminals
Engine on = 11.8 at the gauge; this is recent; last time I started it it went up to ~ 14 volts and then dropped slowly; this time it did not get above 11.8
Battery light starts to illuminate at around 3600 RPM; below that it's off

I'm suspecting alternator brushes, but not sure yet.

[colb]

I would suspect the regulator pack in the alternator being at fault, had this on mine with low charge being put out of the alternator, it was ok doing daylight runs but after using headlights at night battery was flat next day. Swapped alternator out for a new one, problem solved. May buy a new regulator pack for the old unit and keep it as a spare.
You should see 14v on start up, if you switch on headlights you should see it drop then rise as it senses it needs more power to keep battery charged same for other things like heater blower and wipers, more load you should see alternator putting out more voltage to keep battery charged up.

[wallball]

I'm wondering how to diagnose whether its the alternator or voltage regulator. Big difference in replacement price. I'm to be pumping too much $ into this car. My '89 Mercedes just keeps cruising away with 270k miles on it.

[wallball]

System update: went for a drive today for diagnostic purposes. When I started the engine, the voltage gauge hung at around 11.8 V and after maybe a minute, it set up to ~14 V. Drove the car a few miles. As I drove the gauge reading dropped slowly and then stayed at around 12.5 V for the remaining of the relatively short drive. (I didn't want to stray too far from home in case the car decided to stop working. )

The alternator is a Valeo brand. When I pull up the part number from RealOEM and relate it to an online supplier, Bosch usually comes up. Are the two brands interchangeable? I thought I had read somewhere that the mount was different. When I check for correct fitment at the Supplier, it says the Bosch fits my 2000 M Coupe.

Another thing: parts suppliers sell a replacement voyager regulator assembly that "fits the Valeo alternator." I don't see where this assembly would fi on the alternator. Perhaps under the rear black plastic cover? And do I need to completely remove the alternator to switch out the regulator? And the parts interchangeability question becomes more questionable. Are they the same configuration, or just give the same results?

[Mr Bingley]

1) You need a new alternator; 2) Valeo and Bosch are not the same--the parts interchanges are all wrong on this point; 3) a Bosch can be made to work with the proper pulley, re-routing the wiring to the alternator, and some modification to the plastic back cover and vent tubing; 4) It is a fools errand to pull the alternator and replace only the voltage regulator; 5) be sure to replace the drive belt, tensioner and idler pulleys while you are at it; 6) this is a good time to delete the mechanical fan; 7) do not forget to disconnect the battery before attempting this job; 8) until you replace the alternator, keep the headlights off, limit yourself to short trips, and put it on a smart charger to maintain the battery when not driving, or it will leave you stranded.

[wallball]

1) You need a new alternator; 2) Valeo and Bosch are not the same--the parts interchanges are all wrong on this point; 3) a Bosch can be made to work with the proper pulley, re-routing the wiring to the alternator, and some modification to the plastic back cover and vent tubing; 4) It is a fools errand to pull the alternator and replace only the voltage regulator; 5) be sure to replace the drive belt, tensioner and idler pulleys while you are at it; 6) this is a good time to delete the mechanical fan; 7) do not forget to disconnect the battery before attempting this job; 8) until you replace the alternator, keep the headlights off, limit yourself to short trips, and put it on a smart charger to maintain the battery when not driving, or it will leave you stranded.

Ralph, thanks for the input. Bloody Hell. Photos of the Valeo unit for sale show it with a cooling housing on the back end, which the currently installed one doesn't have. Is there a pipe that attaches to that? Well, my car does't have it. Mine looks exactly like the Bosch ones in the sales photos (which don't always represent the actual unit). Maybe a local shop can get me one of each and I'll have a look at them.

[Mr Bingley]

Here are two good threads on the subject:

https://www.bimmerforums.com/forum/s...leo-difference

https://www.bimmerforums.com/forum/s...or-Information

[wallball]

Here are two good threads on the subject:

https://www.bimmerforums.com/forum/s...leo-difference

https://www.bimmerforums.com/forum/s...or-Information

Thanks very very much Ralph, those threads clarify the issues that I'm seeing, perfectly. We have a rebuild shop here in Santa Barbara. I'm going to talk to them first to see if it's cheaper.

[ZGator]

I love you guys, this post was of great timing for me, as I think my alternator is on the way out -red battery light came on today, flickers off at high revs, I think this is indicative of brush wear. I think I have found a good rebuild shop near me, yea! with core charges and good probability of getting the wrong part online -as these post attest- this has got to be the best way to go.

[Mr Bingley]

Don't forget the FAQ sticky at the top of the forum page--it is CNTL F searchable and contains a link to at least one of those threads. There is a ton of information available in the form of linked threads searchable and organized by subject matter--a very useful research resource. (Interestingly, the member who put it together--a ton of work btw--got into a pissing match with a moderator, flipped out and quit the forum in a huff).

[rf900rkw]

That is not a complete and accurate description of the chain of events.

[Mr Bingley]

I thought it was a pretty good Cliff notes version though--do you think I should have used the Oxford comma?

[rf900rkw]

You only mentioned the blowup, and misattributed the leaving part. Left out was the initial theft, the negotiations, and the ripple effect through other members. The day Ben was made to leave is the day several prolific forum contributors became spectators. Check the date of the last real post in my cooling system thread for but one example.

[wallball]

1) You need a new alternator; 2) Valeo and Bosch are not the same--the parts interchanges are all wrong on this point; 3) a Bosch can be made to work with the proper pulley, re-routing the wiring to the alternator, and some modification to the plastic back cover and vent tubing; 4) It is a fools errand to pull the alternator and replace only the voltage regulator; 5) be sure to replace the drive belt, tensioner and idler pulleys while you are at it; 6) this is a good time to delete the mechanical fan; 7) do not forget to disconnect the battery before attempting this job; 8) until you replace the alternator, keep the headlights off, limit yourself to short trips, and put it on a smart charger to maintain the battery when not driving, or it will leave you stranded.

Purchased a re-manned Bosch with the correct part number, installed it today and all is well; gauge at a steady 13.9 volts. Seems as though some of the "musical" sounds from the car have subsided also. I'm thinking the alt bearings were bad although they don't feel bad in the removed part. Commutator looks bad however.

Thanks for all the great input.

- - - Updated - - -

You only mentioned the blowup, and misattributed the leaving part. Left out was the initial theft, the negotiations, and the ripple effect through other members. The day Ben was made to leave is the day several prolific forum contributors became spectators. Check the date of the last real post in my cooling system thread for but one example.

Sad stuff. Sorry to hear about it. I don't know the details of course, but if it affects the free transfer of essential information about some very interesting vehicles, it's bad news.

[Mr Bingley]

Purchased a re-manned Bosch with the correct part number, installed it today and all is well; gauge at a steady 13.9 volts. Seems as though some of the "musical" sounds from the car have subsided also. I'm thinking the alt bearings were bad although they don't feel bad in the removed part. Commutator looks bad however.

Thanks for all the great input.

Sweet. Did everything fit just fine? Were there any modifications required? Did it come with a pulley? What year year, trim and engine to what part number did you match for future reference?

[ZGator]

I had mine rebuilt, here they were quick, only a few hours and it was ready, new bearings and brushes, cleaned up, etc. Mine was a Valeo 90 amp model. Most ones I see for sale online are Bosch, not sure how interchangeable they are for the non M Z3s. I was not thinking rebuild at all until I read the posts, glad I went that route, it was cheaper than I could buy anything for.

Just finished putting it back together, no more battery light.

[wallball]

Sweet. Did everything fit just fine? Were there any modifications required? Did it come with a pulley? What year year, trim and engine to what part number did you match for future reference?

2000 M Coupe, S52 engine. Got it from RM European who are great as I get stuff the next day with free shipping. Cost me $181 for the Bosch PN AL9414X, came with a pulley Only difference that I could see is that it did not have the protective cable mount rubber/plastic housing, and no place to put it. So the cable attachment studs are not covered. Installation was very easy. Rather than loosening the tensioning pulley (which was tough to access) I simply put a 32mm wrench on the fan nut and rotated it CW and the belt tension loosened enough to pop it off the pulley. Running great. What's next?