My Alternator has at excessive Voltage

[Oceanride007]

My no name Alternator is a 55Amp on a 32yo BMW50 Engine, last year I sent the alternator to auto electrician AE and when it came back it had a external Bosch RE57 14V regulator installed. Trouble is the Voltage at the battery gets 15V, it was maybe 14.4V when I got it back from the AE last year, he is miles away now and is clueless as to why it would be going to over voltage, I replaced the Regulator, no change to OV. So I assume that the Field winding is bigger that the designer of RE57had in mind. I also have LFP House bank. Not likely but would like to connect to LFP without OV event.

Your thoughts are welcome. And a cure.

AE had no idea, local AE has a sign on door looking for a AE. have though of a large Diode on the Battery wire or a Resistor in parallel with the Field winding. But I hate inventing wheels.

Field current varies between 1A and .5 Amp, Field voltage between 5V and 2.3V static Field resistance is 24 ohm.

My first thought is to try a 1 ohm 15W resistor between the DF and the field. Thoughts?

[GILow]

First things first.

The battery may be reaching 15 volts, but what is the voltage reading on the regulator sense wire where it connects to the regulator? It may be lower, thanks to poor contacts or maybe a diode on the feed to the sense wire to trick the previous alternator into producing more power.

[Oceanride007]

Thanks Matt. Around 12.6 Volts. Should have mentioned that.

Actually I anticipate your next comment, am now going to separate the Brown wire from the field. See what I get.

[Oceanride007]

Results: at about 1500 RPM.
at D+ on the Regulator 12.5V
at D+ on the input to alternator from Panel but disconnected 12.5V
at Brown wire disconnected from D+ 15V.

Measured continuity from D+ of Regulator to D+ connector that Brown wire normally connects to.

Maybe to a failed Diode.

[GILow]

Does yours have this little embedded resistor and has it been cut as it has in this photo?

[Oceanride007]

No to imbedded resistor, never had one on either. I think that means I have a lower voltage version.

[GILow]

Ok. I’d run a lead straight fromthe battery to the regulator sense wire (D+?) and see what happens.

I don’t quite understand your diagram but those diodes on the feed path look a lot like some kind of charge modifier to lower the sensed battery voltage and increase the charge rate.

[Wotname]

OK, first things first

The RE57 is a simple fixed voltage regulator - there should be nothing tricky about connecting it to most small / medium (say <100A) alternators.

Your 32 year old 55A alternator is also a simple beast and it should be happy with the RE57 if the AE knew what he/she knew what they were doing. Presumably the alternator was internally regulated before it was sent to the AE and for reasons unknown, the AE has fitted the RE57 to provide the necessary regulation because (again presumably) the existing internal regulator had failed and the AE couldn't source another internal regulator.

Without knowing the wiring of the no name alternator, I am going to assume it now has only 2 wires connected to it. One large gauge output wire (B+ terminal) going to the battery and one small gauge wire (field wire) going the DF terminal on the RE57. There maybe a large gauge negative wire going from either the case or a B- terminal to battery negative.

The other 2 wires on the RE57 are straightforward. The D- is the negative (or earth) wire and this should go the engine block / alternator case / battery negative. The D+ wire should go battery positive usually via an ignition switch or relay contact. In essence this is the sensing lead for the regulator and it should be at the battery positive when the alternator is running.

The regulated voltage is not adjustable and is set by the internal design of the RE57 when referenced to sensing lead (D+).

I don't understand (yet) your drawings. If the regulated voltage is too high, I would check (in order)
1. D- is solid connection to ground (battery -ve)
2. D+ is solid connection to battery +ve (via a ignition switch or engine running relay)
3. Alternator wiring (B+, field & negative if fitted) is solid.
4. Engine negative to battery -ve is solid.

Some detail of the alternator wiring might help with further internet analysis!

[Wotname]

BTW, 14.4V is not unusual - a little high but common enough these days when many are using AGM starting batteries.

[GILow]

Originally Posted by Wotname

BTW, 14.4V is not unusual - a little high but common enough these days when many are using AGM starting batteries.

And, according to my RAA battery guy, the calcium car batteries like 14.4 too,

[Wotname]

Your RAA battery guy is right .

[Oceanride007]

Op here, I got 15V at my Battery, not 14.4V

Today I pulled it apart to check the diodes feeding the Reg circuit, Unfortunately I could not pull it apart to expose the Reg supply Diodes.

I have since found another AE and I left it with them.

Last year I had high Voltage, the AE then could not source a genuine replacement regulator, He just simply took off the first cover to expose the old regulator, snipped the wires, installed the RE57 in its place without investigating further.

All connections appear good. Have continuity between D+ Brown wire and D+ of regulator.

The Brown wire D+ is already at 15V but somehow the reference voltage is in the vicinity of 12.5 V it must be sinking the 15V brown wire voltage. It is my belief that although the Voltage on the other side of the Reg Diodes is in excess of 15V plus .6V the reason for only having 12.5V must be due to one or more Reg diodes have failed open.
Sorry about my earlier sloppy drwg this image is from PDF. Hope it makes clear

[GILow]

Diodes feeding the reg circuit?

That's a trick used to lower the sensed voltage.

Just run a direct feed from the ignition, but make sure that feed is.not under heavy load to avoid voltage drop.

[Wotname]

Originally Posted by Oceanride007

Op here, I got 15V at my Battery, not 14.4V

Today I pulled it apart to check the diodes feeding the Reg circuit, Unfortunately I could not pull it apart to expose the Reg supply Diodes.

I have since found another AE and I left it with them.

Last year I had high Voltage, the AE then could not source a genuine replacement regulator, He just simply took off the first cover to expose the old regulator, snipped the wires, installed the RE57 in its place without investigating further.

Did work OK after the AE did this repair or has been outputting 15V ever since the repairs was done?
If worked OK for some time, when did fail?
Anything else happen at the time of failure?

It is unlikely any failure of the diode packs inside the alternator will cause the voltage to be too high.

Originally Posted by Oceanride007

All connections appear good. Have continuity between D+ Brown wire and D+ of regulator.

The Brown wire D+ is already at 15V but somehow the reference voltage is in the vicinity of 12.5 V it must be sinking the 15V brown wire voltage. It is my belief that although the Voltage on the other side of the Reg Diodes is in excess of 15V plus .6V the reason for only having 12.5V must be due to one or more Reg diodes have failed open.
Sorry about my earlier sloppy drwg this image is from PDF. Hope it makes clear

Where are you reading this 12.5V?
The regulated output voltage of the alternator (say 14.2V) is determined internally inside the regulator. Normally the regulator senses the battery voltage on voltage sensing wire (D+) and adjusts the field voltage (DF) until the alternator output (B+) equals the regulated voltage (say 14.2V).

The only ways the alternator output voltage can reach 15V is if -
1. The regulator is not correctly connected
or
2. The regulator is faulty.

Without seeing exactly how the RE57 was connected to the alternator and the battery wiring, it is impossible to determine if the fault is 1 or 2.

[Wotname]

Originally Posted by GILow

Diodes feeding the reg circuit?

That's a trick used to lower the sensed voltage.

Just run a direct feed from the ignition, but make sure that feed is.not under heavy load to avoid voltage drop.

I am assuming the diodes referred to by the OP are the 3 diodes inside the alternator that provides the self-exciting voltage for the field once the self sustaining alternator rpm is reached.

Without knowing how the RE57 was connected to the alternator internal wiring, I agree the D+ terminal on RE57 should be taken directly to 'ignition' or key switch but not to an idiot light. Typically the D+ terminal on an alternator goes to the idiot light but clearly something is wrong here. If the alternator has being outputting 15V ever since the AE fitted the RE57, then it was connected incorrectly (either to the alternator or to the battery wiring). If it worked for awhile and then failed, either the RE57 is faulty or the external connections are now faulty.