M

mbaldock

New Member
Need some help please. Just fitted new battery and new 120 amp alternator but not convinced its charging correctly. Searched for previous advice but confused (old age no doubt) as some say output should be above 14.2 v. I have 14.08 out at the rear of the alt, 13.4 at the battery when running at 2000 rpm, same at smaller brown and white wire at alt. can anyone tell me if this sounds ok or should the output be higher please.

P38, v8 1997

Thanks
Martin
 
mbaldock said:
Need some help please. Just fitted new battery and new 120 amp alternator but not convinced its charging correctly. Searched for previous advice but confused (old age no doubt) as some say output should be above 14.2 v. I have 14.08 out at the rear of the alt, 13.4 at the battery when running at 2000 rpm, same at smaller brown and white wire at alt. can anyone tell me if this sounds ok or should the output be higher please.

P38, v8 1997

Thanks
Martin
Click to expand...

I don't think they ever fitted a 120 amp alternator, it has the wrong regulator, however if you can get close to 14 volts at the battery it will probably be OK unless you do a lot of short runs. The voltage by consensus should be 14.2 volts at the battery.
If you have a 0.68 volt drop between the alternator and the battery there is a bad connection or the wire is corroded inside the plastic sheath. I would not want to see much more than 0.1 volt drop.
 
Datatek said:
I don't think they ever fitted a 120 amp alternator, it has the wrong regulator, however if you can get close to 14 volts at the battery it will probably be OK unless you do a lot of short runs. The voltage by consensus should be 14.2 volts at the battery.
If you have a 0.68 volt drop between the alternator and the battery there is a bad connection or the wire is corroded inside the plastic sheath. I would not want to see much more than 0.1 volt drop.
Click to expand...

There is an 120 amp alternator listed for the 4.0 4.6. But in this case with a new unit and a new battery he should be getting close to the 14.2 controller output. As you say connections or cables gone high resistance. :):)
 
Thanks guys, did some voltage drop checks and earth side seems okay at 57mV but live side is nearly 600mV!! .... so can see why Im not getting the 14.08 at the battery (new cables coming up) but would this also cause the lower than expected output at rear of alternator please??
 
mbaldock said:
Thanks guys, did some voltage drop checks and earth side seems okay at 57mV but live side is nearly 600mV!! .... so can see why Im not getting the 14.08 at the battery (new cables coming up) but would this also cause the lower than expected output at rear of alternator please??
Click to expand...

Try cleaning the connections on the starter motor.
 
mbaldock said:
Thanks guys, did some voltage drop checks and earth side seems okay at 57mV but live side is nearly 600mV!! .... so can see why Im not getting the 14.08 at the battery (new cables coming up) but would this also cause the lower than expected output at rear of alternator please??
Click to expand...

No, that's down to the regulator in the alternator. It's got the wrong one. Regulators can be specified in 0.1/0.2 volts steps from 13.6 to 14.8 volts. I suspect that you have the wrong alternator for your car, I thought yours should have a 150 amp one.
I wouldn't worry about it if you can get close to 14 volts at the battery.
 
Last edited:
Hi Datatek, I thought I checked this out, the one on there was only 100amp orginally how can I confirm the correct model ... I think the 150amp version was for later vehicles than mine ??
 
Datatek said:
No, that's down to the regulator in the alternator. It's got the wrong one. Regulators can be specified in 0.1/0.2 volts steps from 13.6 to 14.8 volts. I suspect that you have the wrong alternator for your car, I thought yours should have a 150 amp one.
I wouldn't worry about it if you can get close to 14 volts at the battery.
Click to expand...

STC2227 100 amp Diesel. AMR4247 100 amp 4.0. AMR2938 120amp 4.0-4.6. EER5834 150 amp 4.6. Think the 150 runs at 14.5 all the others 14.2.:):)
 
Just checked on a suppliers site and the 150amp version is XA410482 onwards, mine is VA360 etc so looks as if the 120amp version is correct but still confused why only 14.08 at the alt .... maybe its faulty (even though its new)??
 
mbaldock said:
Just checked on a suppliers site and the 150amp version is XA410482 onwards, mine is VA360 etc so looks as if the 120amp version is correct but still confused why only 14.08 at the alt .... maybe its faulty (even though its new)??
Click to expand...


OK so Wammers has confirmed it's the right alternator, V8's are not my speciality. The alternator is probably a recon, it has the wrong regulator or it's faulty.
 
thanks for you prompt replies, its supposedly new (not recon) so I will add a new positive lead direct to the battery and see if that makes any difference. The other remote posibility is my meter might be out by 1.2v I suppose .. hey ho, thanks again
Best
M
 
Sorry if this is a dumb question but, the small brown and white wire on the back of the alternator, I presume this is a feedback from the Battery/ECU (as its the same voltage as the battery on mine) ... whats the purpose of this connection please??
 
mbaldock said:
Sorry if this is a dumb question but, the small brown and white wire on the back of the alternator, I presume this is a feedback from the Battery/ECU (as its the same voltage as the battery on mine) ... whats the purpose of this connection please??
Click to expand...
The Brown/Yellow is a wire to D+ which, via the BECM operates the charge light. Required to start the alternator charging.
 
Last edited:
OK, thanks. It is the only wire there (apart from the starter motor connection) , so guess its a charge light although confused as to why (if its an output) why it isnt at the 14.08
 
mbaldock said:
thanks for you prompt replies, its supposedly new (not recon) so I will add a new positive lead direct to the battery and see if that makes any difference. The other remote posibility is my meter might be out by 1.2v I suppose .. hey ho, thanks again
Best
M
Click to expand...

Just try cleaning the connections to the starter. That is where the alternator output goes to get to the battery.
 
Thanks Wammers, will do. I will post again with the results of the new cable and clean ups as suggested. Thank you to both you and Datatek for the prompt replies.
Best
M
 
mbaldock said:
OK, thanks. It is the only wire there (apart from the starter motor connection) , so guess its a charge light although confused as to why (if its an output) why it isnt at the 14.08
Click to expand...

It senses the battery voltage, the controller then adjusts the current output (Amperage) from the alternator, by adjusting the rotating field coil voltage up or down as required, depending on the draw being applied to the battery by systems, to keep the battery at a constant level of charge. If you have 14.08 volts at the alternator output you should have close to that where the wire is connected to the starter. Maybe a little less at the battery. Unless the starter cable and the charge cable from alternator to starter have internal corrosion and have gone high resistance. The only point of loss has to be where the charging cable attaches to the starter cable. Clean it see what you get.
 
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