m60 alternator output, can someone please check yours?

[JGood325]

Can anyone who has a properly running m60 powered vehicle, with a normal 500rpm idle speed (after it warms up and settles), please check your voltage at idle?

I need the following:

1. Voltage at idle, with engine warmed up and RPM settled, with NO lights, A/C, blower motor, etc. *After* it is warmed up and settles to the lowest idle speed.

2. Voltage at idle with headlights ON and blower motor on high (this is what I am really looking for)


I ask because I'm only seeing 12.3 volts at idle with the headlights and blower on in my m60 swapped e30. I'm on my third Bosch Reman alternator and second battery. Without lights/blower on, it's 14v. If I give it any throttle to raise the RPM, it'll hold 14v with everything on as well. It's almost like the alternator output is not enough at 500rpm, but that is the spec for m60's. My battery voltage with engine off is 12.6v. This is in an e30, where there is a lot fewer circuits running as well.

[ross1]

Can anyone who has a properly running m60 powered vehicle, with a normal 500rpm idle speed (after it warms up and settles), please check your voltage at idle?

I need the following:

1. Voltage at idle, with engine warmed up and RPM settled, with NO lights, A/C, blower motor, etc. *After* it is warmed up and settles to the lowest idle speed.

2. Voltage at idle with headlights ON and blower motor on high (this is what I am really looking for)


I ask because I'm only seeing 12.3 volts at idle with the headlights and blower on in my m60 swapped e30. I'm on my third Bosch Reman alternator and second battery. Without lights/blower on, it's 14v. If I give it any throttle to raise the RPM, it'll hold 14v with everything on as well. It's almost like the alternator output is not enough at 500rpm, but that is the spec for m60's. My battery voltage with engine off is 12.6v. This is in an e30, where there is a lot fewer circuits running as well.

Normal idle speed for this engine is much higher, 700-750 rpm. Incorrect engine idle speed is your problem

[JGood325]

Where is that information from? The tech manuals I found show 600 +/- 50. Additionally, I have found many posts on this forum with members reporting a normal ~500rpm idle speed.

Mine is a hair over the 500rpm mark, but that's on a 34 year old e30 gauge that is going through an RPM signal converter, so trying to eyeball a 50-100 RPM difference is futile. This is a m60 that has been fully regasketed, and every sensor/sender etc. tested and most replaced multiple times with new/reman OEM (while diagnosing unrelated issues several years ago). It runs absolutely perfect and very smooth now, I can balance a penny on the valve cover. I'm not saying it's NOT idling lower than spec, I would just be very very surprised, and can't imagine what I could replace for the 3rd or 4th time that would make a difference with it.

[ross1]

This is my personal recollection of cars previously owned, as indicated by the dashboard tach.
So, what is it at 650?
Asking for full output at 500 rpm ain't gonna happen.

[JGood325]

I'm not asking for full output, I'm asking for it to at least maintain battery voltage. Battery voltage is 12.6. I'm seeing 12.3. That would theoretically drain the battery to some extent over a period of time.

Any increase in RPM immediately (not gradually) bumps it up to the full 14v. That's why I'm asking for someone else with a 540i that idles at ~500rpm (like many of the older posts I found) to check voltage. If they are getting 12.6+ volts with all accessories on, than I know there is an issue somewhere with mine. If they are getting less than 12.6v, I'll let it go, as that's obviously how it's intended to operate. I just find it strange BMW would design a charging system for a 5/7 series that cannot charge the battery at idle with the lights and blower on at the stock idle speed.

For all know, mine is sitting at 600rpm at idle. I'd have to get a scanner on the OBD port to confirm. I don't trust the old cluster + signal convertor to that degree of accuracy. But with the state of tune of this engine, I have no inclination to think there is an issue of some sort, with the only symptom being an idle speed too low by ~100 RPM or so. This thing has been smoke tested, fuel pressure tested, vacuum tested, compression tested, leak down tested, and every single part of the fuel injection and ignition has been replaced multiple times (all while tracking down a bad DME). This 'issue' now occurs with both my known-good DME's.

The schematics indicate the DME has a +12v input from the light switch with the headlights on. Not sure if this is an idle bump feature or not.

[moroza]

No numbers for you, but my blower motor would run at ~85% speed at idle, noticably picking up RPMs with the slightest increase in engine speed. This was an M62 running M60 electronics with a smoke-tested airtight intake and everything else tip-top. 140A alternator is beefy for 2019 and huge for 1995 standards; they probably sized the pulley juust right to reduce drag on the engine, figuring that you'll rarely idle for very long without some RPMs sooner or later.

FWIW, I've noticed similar behavior on other vehicles that ran that way for years with reasonable battery life. As long as it's doing 13.5 or better when actually driving, I wouldn't worry too much.

[ross1]

"Any increase in RPM immediately (not gradually) bumps it up to the full 14v."
Pretty clear to me that the idle is too low.

[JGood325]

"Any increase in RPM immediately (not gradually) bumps it up to the full 14v."
Pretty clear to me that the idle is too low.

Which, again, is the point of this thread. I want to know if e34 540i's are intended to idle at a speed that is not able to maintain battery voltage with all accessories running. I'm just asking for someone to check their voltage quick, not remotely troubleshoot what may or may not be wrong with my car.

[Thomas525]

The alternator is fed a starter current voltage from the battery via the signal wire at the D+ post. This maintains a 13.5volt ish voltage even at idle. This starter current changes as engine rpm changes. When the car is being driven, you should be doing 13.5-14 volts.

You should not be doing 12.3volts at idle. If the voltage increases as your rpm increases, there is a high likelihood that your voltage regulator's carbon brushes are too worn to produce decent contact at idle. Its a simple thing to check. Unplug your battery, remove the alternator's back cover after unplugging all the wires on it, then you'll be able to unscrew the voltage regulator which should now be visible to you. Then one look and you'll probably know (have a picture of a new regulator handy)

New carbon brushes are dirt cheap and can be soldered on after the old ones are soldered out. But if you don't know how to do all that, a new oem regulator should run you about $35-$40 with shipping.

[JGood325]

I understand how alternators work. The exciter wire is hooked up and functioning. The brushes are not worn, this is a fresh Reman alternator (and the previous Reman alternator performed identically). I pulled the regulator weeks ago to verify anyway.

Disregard this thread, I found some able to check voltage on an m60. 12.4v at idle with all accessories on, 500-550 rpm idle.

[Thomas525]

There is no logic for the factory to configure the car to produce such low voltage at idle. when it can be easily configured otherwise. People here would have noticed by now I"m sure if that was the case.

I would feel far better if you temporarily wired the battery's + post to the alternator's D+ post via a separate wire and ran the car at idle to see voltage produced. This would eliminate the exciter wire as a suspect entirely. Second test would be to push the idle to 650 where its supposed to be, by tightening the throttle cable appropriately. Then do both tests simultaneously as a third run.