My son's T25 CU engine, battery red light didn't glow on the dash. Alternator blue wire was broken, Repaired the wire and battery light glows and goes out when engine running. But, battery volts reading at the battery was 12.08 and stayed the same with engine running and revving. Tried new Voltage Regulator, made no difference. Alternator has a 3 connector plug, so can't easily test at the alternator, or can we? Also confused with wiring at the battery, there is the thick negative cable to the body earth, thick positive cable to the starter solenoid, and another red wire to the positive terminal and that's it. We would like to test the alternator output somehow before condemning it and buying a new one.
The Samba.com and RAtwell.com are two useful resources: This diagram from the Samba shows the alternator connected to the starter motor solenoid, one wire from the battery going off to the front of the vehicle, the other wire from the battery to the solenoid. Internally the alternator has two rectifier systems : a small rectifier that drives the blue wire, and a big rectifier going to the red wires. If the small rectifier on the blue wire works and the big one does not, but the regulator is working properly, the light would be on at zero RPM, go out as you rev up, then come on again as the internal voltage of the alternator goes higher than the battery ... I have seen that one with a loose battery earth clamp too.. But because the light just goes out, it means something probably wrong around the regulator and its wiring. Maybe not the regulator but a cracked cable core inside a crimp, so the insulation holds the wire together. A good trick is to pull firmly on wires going into crimped connectors, but not hard enough to break the wire: if the wire either pulls out of the crimp or the insulation just stretches, that crimp had a problem, either a bad connection or just fatigued off. Dont do that unless you have some new crimp connectors and a good quality crimping tool ! Yes, you can test the alternator - heres a good reference : https://ratwell.com/technical/ChargingSystem.html#testalt
When I fitted the new regulator I noticed a lot of grooving in the copper slip rings. The old regulator had one good brush and the other very worn down.
Yes its possible the brush is not making good contact giving bad regulator symptoms. One worn and one not worn brush indicates the old regulator had a "hung up" spring on the non worn brush and probably is actually electrically OK.. Depending on how much you dont want to replace the alternator, using a needle file on the slip rings until there is a smooth "groove" where the brush runs may fix it by allowing proper contact. Or buy replacement slip rings - they are available for Bosch alternators. Dont know how hard to replace them. If budget no issue, swap the alternator.
Sounds like a new alternator would be best then. My son charges the battery fully and then drives it which is not ideal and may be a factor in the obvious lack of power when driving. Its been like this since he bought the project van 18 months ago.
