I've been having this issue for a few years. Sometimes after the car's been sitting in cold/wet weather and battery is somewhat low, the alternator refuses to engage (will do so eventually). I can look at the voltage at the battery when it's active (14+v) and when it's not (12.5 and drops to 11 or more). This last time, it kick back on when I tried steering at a stop light. I don't know if coincidence or the extra load demand finally did the trick.

I've inspected/serviced the alternator a few months ago and brushes still look OK, this almost seems like a brainbox issue that's meant to engage the alternator. Body control unit?

FWIW, I also have a vampire drain that seems to be ABS unit related (highish current draw when vehicle off).

 

The alternator needs voltage from the battery to energize its rotor and its electro-magnets so when the rotor spins electricity is generated.

If the battery is low it may not be able to supply this power. Maybe. I don't think it takes that much power or the battery would never get recharged with the engine running. But the voltage regulator may not be prepared to tap the battery for power if its voltage is too low.

If the battery is tapped and the alternator gets too much battery power the load could cause a cold/damp accessory drive belt and pulleys to experience some slippage.

Or the voltage regulator is just flaky (bad). It might be of some interest to observe in some manner the voltage level that is being fed to the alternator/rotor.

There is another possibility: I had a Porsche 996 Turbo with its louvered engine lid and the somewhat exposed to the elements accessory drive system. In damp/foggy or wet weather upon cold engine start I'd notice the battery voltage gauge reading low, ~12V. I'd hear the occasional chirp from the slipping belt. After a moment or two of just letting the engine idle the accessory drive system would dry and the battery voltage would increase and get up to its normal ~14V.

 

The alternator supplies a PWM signal to the DME for electrical load monitoring purposes, but there is no direct control in the other direction that I know of. Have you put a multimeter on the battery during these events, or are you monitoring over OBDII?

As for the voltage jumping when the power steering kicked in... The power steering pump gets a signal from the alternator to know that it can turn on, and I believe that is the same harness/signal that runs to the kombi for voltage monitoring. The power steering pump controller and alternator voltage regulator also both get power from fuse F39.

Without knowing anything else, my first thought is a dodgy connection or wiring somewhere.

 

Good info. Interesting that the DME also serves as the voltage regulator. Or maybe not. Other cars have a similar setup. So my theory as to what might be going on doesn't hold up in this case.

 

Voltage regulation is handled by the regulator in the alternator.

I’m not sure my theory is any more sensible; it’s really just a slightly informed wild guess with so little information.

 

voltage monitored at battery terminal. Fairly obvious when alt is not engaged (less than 13V). What's the DME? To me it seemed like the steering demand I asked for is what finally woke the alternator if that make sense? I read elsewhere that the jump terminal under the hood sometimes gets loose/grimy, could that explain it? I've had to clean grounds @ the battery box in the trunk and at the passenger's side engine mount in the past but these ground issues don't appear to be the culprit IMO. I'll take a look at F39 or replace it.

Thanks.

 

So just like other cars? Well, most other cars. The PWM signal probably conveys voltage to the engine controller/instrument cluster.

Never delved into actually probing an alternator/voltage regulator to observe what's going on vis a vis how much voltage is being supplied to the rotor and how this changes with RPMs to keep the alternator power output constant.

It was just a matter of if the alternator (voltage regulator) was not producing sufficient voltage replace the system with a new one and get the car back on the road.

 

What year?
Is it a pre-facelift before July 2004?

 

I would check voltage at the alternator positive terminal to exclude any possible wiring issues between there and the battery. I don't know hard that is to reach on the R53. If you have access to a scope, the Bentley has waveforms to check against the alternator output. On older stuff you could do things like bypass the regulator for testing, but I don't know about that with these cars. Maybe someone else who knows more can chime in.

Does the charge light come on when the voltage goes low? I'm assuming it does.

I assume you've already verified that things like the belt and crank pulley are fine since you had the alternator apart recently. I don't know much about the underhood jump terminal since my car doesn't have one.

The PWM signal runs to the DME (ECU), not the cluster. It's measuring current somehow since voltage is relatively constant.

The regulator controls the duty cycle of the field winding voltage to set the alternator output voltage, the same as old external electromechanical regulators do.

 

yes '04

 

Yeah, the battery light comes on when this happens and goes away once alt kicks back on and charges for a while I want to say. Pulleys & belts are fine. I guess I'm wondering if the regulator doesn't get turned on by the ECU via CAN bus or something or if it's just a feedback loop direct to battery....I suppose it's possible that the regulator is intermittently failing, tough to diagnose that one but warrants replacement of unit if so. I think I did a diode check and it was fine back when I had it apart.

 

I’m pretty confident that the DME doesn’t have control over the alternator. I think that energizing the field at startup is handled by the signal from the cluster that also controls the charge light, but the TIS document I have isn’t super clear on that front. I may have just contradicted myself with those two statements though.

Wait a minute. Maybe I’m being dense. Did you not have power steering at all until the alternator kicked in, and does the alternator stay working afterwards until the car is shut off? I think I interpreted intermittent as meaning that it would cut in and out.

IIRC, the power steering pump needs to see battery voltage on the wire from the alternator before it will turn on.

 

i had no PS until the alt kicked on and I want to say the low batt signal went away immediately and didn't have to wait for charge. Come to think of it, can the low batt signal ever be on with an engine running and working alt? I assumed it was based on a state of charge but now think it's just a dumb light based on voltage

Sorry, by intermittent I meant to say it would dip way below its operating voltage range

 

The light just measures the voltage in the charging circuit. It sounds like your alternator is failing. You'll find one of these useful:

Volt meter

 

I admit I don't have much to offer in alternator diag.
The symptoms described, do remind of issues I had many years ago that wound up being a failing voltage regulator - in a Porsche.
it maybe a longshot (perhaps unlikely), but you might want to check the nuts securing / grounding the "ballasts" ....

On my 04 pre-facelift R53 there are 2wo left and right just below each hood release cables

I had posted having a weird voltage issue here a while back and later discovered one of the 10mm nuts was missing causing intermittent grounding issues.

just a thought?

 

Thanks, I'll grab one. What I find odd is it appears to be weather dependent. Things do contract in the cold though so if there's a failed contact somewhere it could be bridging the gap on and off.

 

I'll check it out. I'm in the rust belt so ground straps do get grimy often. I'll throw some grease after cleaning.

 

Ah, okay. The power steering pump probably didn't come on until the controller got voltage from the alternator. In general, no, the charge light won't come on any other time, though there are certain fault conditions in certain topologies that will cause the light to come on when the alternator is still outputting voltage. There are a couple of ways the regulator in my van can fail that will turn the light on during an overcharge condition.

There is one thing that has always caused me confusion with the system in the Mini, but I've yet to have a reason to go probing voltages on my own car. The Bentley states that the charge light isn't connected directly to the alternator, but the TIS document I have shows a typical circuit where they are connected. If I believe the TIS, then it seems that the regulator has one output to the DME, one voltage sensing wire, and one wire to energize the field and provide a signal to the PS pump.

 

so it happened again today (wet out, not cold). I should clarify that the failure happens only when it's been sitting for a bit (overnight at least) and once I get the alternator back it doesn't reoccur at all during the day (it may again if I leave it sitting a few days).