Thread: TD5 Intermittent No Charge Light

Got a problem that I don't understand.

The charge light in my TD5 has started coming on intermittently. It is off when the car is cold but after about 10 minutes running it starts coming on intermittently without any apparent relationship to anything else the car is doing.

When it is off the battery volts are approximately 14.6V and when it is on the battery volts are, wait for it, 14.6V.

This is where I get confused. According to the TD5 circuit diagram and all other cars I have worked on the light is simply connected between the diode bridge star point in the alternator and the battery volts. Thus when the alternator is putting out less volts than the battery the light is on and when the alternator volts comes up to the battery volts the light go out. So why does it come when the alternator is putting out 14.6V.

The only thing I can think of is a loose connection between the output of the alternator and the light which is going to ground. If it was just simply loose and disconnecting then I would have thought it would also need to be grounded for the light to come on and if that point was grounded while it was connected to the alternator I would hate to think what would happen 100 amp short?

Also loose and the need to warm up don't make sense.

I have searched the forum and the web but no reference to this. Anybody got any thoughts?

All fixed but still not 100% sure why.

Cut the charge light wire up near the computer and measuring the volts looking back at the alternator, established that the volts were still disappearing when it warmed up. There is nothing between this point and the alternator so the problem is in the alternator.

So out comes the alternator.

Replaced the brushes and the regulator. The brushes were below the limit (less than 6mm). The rotor contacts were a bit dirty and one was a bit scored but these cleaned up to a reasonable level with a bit of emery cloth.

The only other thing I found was that at some time in it's history someone replaced the alternator connector plug. The replacement had three wires so they joined two of the wires together to join up to the two wires in the cars wiring. The alternator has three connections but only two are used. What the third pin is connected to in side the regulator I don't know because there is no details as to the circuit of the regulator so maybe this had something to do with it. Cut the third unnecessary wire off.

Put it all back and the light no longer comes on when it warms up.

I put my money on just a faulty regulator, sure the brushes were worn but I don't think if they were a bad connection it would keep charging.

Notes:-

1. Vacuum line was extremely hard to remove from the pump. The clamp had fused into the rubber. Had to wreck it to get it off. Replaced it with a normal jubilee clamp.
2. Damm the T50 tork screw bolt holding the alternator in. Why not a normal headed bolt?
3. Remove the tork bolt but keep the alternator attached by the top bolts, removing these last. Much easier to maneuver.
4. At the other end of the tork bolt is a nut which is not held in. The moulding of the case holds it while it is tightened up but with the bolt out, it will fall out.
5. Replaced the oil drain return with standard fuel hose. Replaced the clamps with normal jubilee clamp.
6. Serpentine belt tensioner was noisy, Replaced it complete rather than trying to change the bearing. (got it from Allfourx4).
7. The diodes all looked like the connections were good and they where not burnt in any way, but as they are all connected in a bridge arrangement you can't measure them individually without disconnecting them. I don't think so.