LILBENZ230

Okay, recently had a battery light that would come and go. Had the system checked by two different places, both said alternator failing. Dyno mode voltages showed a max of about 13.5 volts running with lots of dips into the 12s and 11s (which triggered the warning). Battery testing at 12.69v - good. Had alternator bench tested at 13.0v - bad.

Replaced alternator with brand new OE Bosch, of course with new VR as well.

No battery light warning yet by dyno mode shows no difference. Still getting about 13.4 in motion and 12.8-13.0v at idle.

Anyone had this happen?

Glyn M Ruck

I hope it's now OK Matt. Get the battery properly load tested.

Alternator should cut out at 14.2/14.5 volts area. I don't know how accurate dyno mode is for serious voltage checking.

Presume they did a proper VR cut out check when they replaced the alternator?

Sometimes only the VR needs changing at low mileage.

Remember there is two way communication between the alternator & the ECU on these cars to control alternator torque during warm up etc. Quite complex

LILBENZ230

Thanks for your response Glyn!

Battery light returned today. Dyno mode showed 11.5-11.7v up to 90mph. Once stopped at a redlight it suddenly returned to 13.4v

The alternator and VR are both brand new Bosch OE ones. I suppose it is possible that it is defective, but it is strange to me that the symptoms are exactly the same as prior to the replacement. The alternator was replaced by the dealer under warranty at 48,000 miles. It has 67,000 miles now. The car may go days without the battery light.

Battery was load tested at least three times and showed good. Even if it is not good, shouldn't the alternator put out 14.2v when the car is running and at 2500rpm? Is it possible that the battery is bad and that would cause the alternator to randomly stop charging?

I am fed up with it at this point.

Glyn M Ruck

An internal short in the battery can play havoc with charging but it sounds like it's been checked multiple times.

They need to check the harness, all plugs/connectors & earth straps. I can't believe another alternator is fried but the VR might be if there is a short to earth now & again or similar.

They must also be absolutely sure that the correct VR is fitted for your VIN. There are 3 different VR types these days.

There are 3 variants:

MFR (multi function regulators)
LIN (local interconnect bus)
BSS (bit synchronous bus)

BSS and LIN are used depending on the engine ECU type
either ME2.8 or SIM4, these regulators actually communicate with the engine ecu and transfer information between each other, the engine ecu has control over the alternator to be able to control the torque loading for emissions requirments. Especially during warm up.
So you cannot swap an alternator with a BSS regulator with one with say a LIN bus. No communication or faulty communication will take place

LILBENZ230

The alternator and VR are both new and both OE spec. It DOES charge, but not all the time. Sometimes it just quits charging for several minutes. Last night I left my office with dyno mode showing 13.5 volts charging. At the redlight 600 yards from the parking lot, it dove down to 11.7 and battery warning came up. Stayed that way for 5 miles and then resumed charging at 13.4 volts for the next 35 miles before again dropping to 12.2.

I took it back to Autozone last night to have them test it again. The battery tested at 12.58v, so it is good. The altnerator test said "NO VOLTAGE". That was a first.

The car is parked. I'm going to take it to a European specialist shop next week for more diagnostics. At this point, I'm over it. I just want it repaired for trade-in purposes.

Glyn M Ruck

The VR must be correct for your VIN.

samaritrey

I was wondering if you had posted on here about your troubles glad Glyn was helping out. Hope the new alternator next week solves your problem.