Ok,
Fitted new alternator after last one fell to bits. All was fine for couple of months then heading down M5 & rev counter started to die and battery warning light came on. So new alternator is not charging battery, connections on alternator look fine as do the ones on the battery - so what else do i check (i do have a circuit tester though not sure how to use it) any help would be greatly appreciated as bank balance cringes at the thought of sticking her in a garage for diagnostics and repairs. Shes an M reg 2.5 tdi
Cheers in advance
Simon

make sure that the generator is working....between 12 and 16 V according to rev......it's most obvious than the voltage regulator coz the rev counter isnt working too:doh:....see the belt

roughtor said:

Ok,
Fitted new alternator after last one fell to bits. All was fine for couple of months then heading down M5 & rev counter started to die and battery warning light came on. So new alternator is not charging battery, connections on alternator look fine as do the ones on the battery - so what else do i check (i do have a circuit tester though not sure how to use it) any help would be greatly appreciated as bank balance cringes at the thought of sticking her in a garage for diagnostics and repairs. Shes an M reg 2.5 tdi
Cheers in advance
Simon

Click to expand...

Check for a live on the large battery cable connection on the rear of the alternator and check for a battery warning light live though this will be a reduced voltage as the current is passing through the bulb, best thing to do is remove the warning light wire and put it to earth with the ignition on and the light should come on this will confirm the circuit is intact.

Next with the engine running do a volt drop test on the main battery connection and also check the body earth the reason for this is if you check the connections for voltage there may well be enough to work a test light but if there's a poor connection under load the circuit will fail.

You need a milti meter set to DC voltage, with the engine running put the neg clip on the positive battery terminal and the pos clip/probe on the nut securing the battery cable on the rear of the alternator, any reading BELOW 0.5v is fine if higher there is a poor connection on that circuit.

To test the earth connection again with the engine running put the neg clip on the neg battery terminal and the pos clip/probe on the alternator body (scratch it a bit to ensure a good connection) again should read BELOW 0.5v, any higher and you have a poor connection.

Don't bother trying this on the battery warning light circuit as you will get a high reading because the bulb is consuming the current.

If that all checks out, replace/repair your alternator!

Cheers for that, will give it a go.
Simon