Would a weak or failing alternator cause the tachometer to read 1000 RPM too low?

[mancini_mark]

Hi Guys,

Ryan suggested to me to look into a voltage regulator, which got me thinking. When I come to a stop, my lights dim and my fan slows because of the lower RPM's. I think that my alternator is weak. Would this also cause the tachometer to read 1000 RPM too low on the highway? Nothing else seems to be affected.

Thanks,

Mark

[ryan roopnarine]

my tachometer would only go up to 600 rpm, lights were dim, and i could only get charge when i moved off from a light, ie fuel pump would buck and struggle until i got up to speed....this makes your problem a lot easier to solve if it is indeed the alternator....i take it you are in the US? you can do like i did yesterday ( my multimeter was saying that my alt was putting out 15.8 volts, autozone's tester said 13.8 @60A at 2krpm with no loads) and take it to autozone or discount auto parts, and they will test your electrical system on your car.

[ryan roopnarine]

my alternator didnt get "weak", it failed almost immediately because one of the springs crapped out, and made it kick out 8.6 volts...a failing one might be less dramatic than my story.

[Unregistered]

my alternator didnt get "weak", it failed almost immediately because one of the springs crapped out, and made it kick out 8.6 volts...a failing one might be less dramatic than my story.

Mine failed... it basically made the battery charge maybe 70% of its capabillity which eventually came to my car simply stalling. Your tachometer problem sounds capacitor (in the cluster) related.

[ryan roopnarine]

but this was the only time it did it, even on my drive the next day, with the battery voltage at 11.01 (which was determined by a VOM attached to the battery) while driving around, it didn't do it, and it hasn't done it again since the charging system was sorted out, so im thinking that its alternator related. trans program, b-fluid, and all of the rest of the lights were sporadically on. as it hasn't happened since, i think i'll resign it to low voltage.
as you said, in your case, your alternator failed.....mine didn't "technically" fail...one of the springs that sticks out the alternator brushes collapsed, so i think that it was a sudden occurence, and not gradual, as a typical alternator failure is.

Mine failed... it basically made the battery charge maybe 70% of its capabillity which eventually came to my car simply stalling. Your tachometer problem sounds capacitor (in the cluster) related.

[Bill R.]

know what model the new one came out of? If it was a v8 instead of a 6 it
could count the pulses from the ignition for the tach in which case the 8 has 25% more pulses than the 6 for the same rpm.. so on a 6 your rpm might read low by 25%.. I'm not sure if the bmw tach works like this or not but I know the coding plugs are specific to your exact model and options and the wrong plug won't work correctly.

Hi Guys,

Ryan suggested to me to look into a voltage regulator, which got me thinking. When I come to a stop, my lights dim and my fan slows because of the lower RPM's. I think that my alternator is weak. Would this also cause the tachometer to read 1000 RPM too low on the highway? Nothing else seems to be affected.

Thanks,

Mark

[ryan roopnarine]

had to change out the instrument voltage regulator in a bmw? i called either autozone or DAP looking for the alternator reg. locally, and one said that they only have the dash instrument one...that's what i suggested mark change. its funny that they would even have that, unless it fits into a bunch of other cars or something i'm unaware of...

[mancini_mark]

Hi Bill,

I bought a 1993 cluster that was supposedly from a 525. I tried to make it work with my 1990 525 (m20) coding pin, but it was all dead except for the speedometer. Then I bought a coding pin from the guy who sold me the cluster, but who knows which car that came from. So, now everything works, except the tachometer, which reads 500 RPM at idle, and 2000 to 2300 RPM on the highway (roughly 120 km/hr).

Should I bite the bullet and try a 1990 cluster from a local wrecking yard? Truth be told, I wish I could spend as much as necessary to get it back to the way it was, but I'm just about done with my third degree at school, and I'm just coasting through on my last few bucks.

Thanks to everybody for your input,

Mark

[Bill R.]

Hi Bill,

I bought a 1993 cluster that was supposedly from a 525. I tried to make it work with my 1990 525 (m20) coding pin, but it was all dead except for the speedometer. Then I bought a coding pin from the guy who sold me the cluster, but who knows which car that came from. So, now everything works, except the tachometer, which reads 500 RPM at idle, and 2000 to 2300 RPM on the highway (roughly 120 km/hr).

Should I bite the bullet and try a 1990 cluster from a local wrecking yard? Truth be told, I wish I could spend as much as necessary to get it back to the way it was, but I'm just about done with my third degree at school, and I'm just coasting through on my last few bucks.

Thanks to everybody for your input,

Mark

[mancini_mark]

Hi Bill,

My original cluster died, and I tried to solder in new capacitors myself to no avail. Just to be sure that I did it correctly, I tried a second batch of capacitors, but it was still dead. That's when my e-bay quest began. I think my problem is that I'm trying to cut corners. I really should drop a grand or so on this car, but I don't start my employment contract until september.

Thanks,
Mark