Here's the history:

1. My battery went flat and would not restart the car after parking.

2. Restarted from jumper pack.

3. Driven home 50 miles.

4. No restart.

5. Spare battery substituted.

6. Started fine, used intermittently for 2 or 3 weeks, battery weakening.

7. Replaced Valeo 110A alt. with a used and reputedly tested Bosch 140A alt.

8. Exciter plug didn't click home positively.

9. Re-charged original battery refitted, started fine but not charging.

10. . Battery light does not appear on display when ignition turned on and never appeared on display when engine running during the time I've been having issues.

An experienced mechanic friend says that the warning light should come on before the engine is started (as I would expect). He suggested that if the bulb had failed, it would affect the alternator, i.e. no charge. I wonder if the light isn't coming on because the plug isn't giving a good contact and maybe that's why it isn't charging now, but wouldn't explain the original fault, as the plug fitted the old alternator fine.

My questions are:

1. Why would the plug not click home in the Bosch alternator?

2. Can warning light bulb failure affect charging?

3. If the plug isn't making a good circuit, would that stop the warning light working?

1. Latch was probably broken when removing from old alt.

2. Not a warning light bulb, it's an LED; but the wire that excites the alt, the blue one in that two-way connector, helps the alternator 'boot up' when engine is starting. It indirectly results in the warning light coming on, or not doing.

3. Yes, if the blue wire is not connecting to the alt, either at the plug or somewhere on route to it, the warning light will not work.

Generally, a burst of revs (2500, say) will start the alternator charging, even without the blue wire being intact, so try that as evidence that the replacement alternator is itself OK.

Thanks Breezy_Pete, I'll try the revs. If the blue wire is the exciter, what's the other wire in the plug?

There's an intermediate connector pair between the two-way connector at the alternator and the final destination of the two wires. It's a 4-pin plug/socket pair 'front left in engine compartment'.

That's left as experienced sitting in the car facing forward, so on the right side of the engine bay if you are standing in front of the car looking in.

I would try to locate and inspect that connector pair before necessarily blaming the alternator connector, you may find a broken blue wire there instead.

1 minute ago, TinyTim said:

what's the other wire in the plug?

It's known as DFM, Digital Field Monitor. It informs the engine ECU how hard the alternator is being worked, as a percentage of its maximum capability at any given engine revs.

It being broken would not have the same effect on the warning light.

That other wire changes colour at the 4-way connector from brown/red to violet/green before heading onward to ECU.

The blue wire goes onward, still blue into the cabin, to the BCM (Onboard supply control unit).

Edited 17 March, 20251 yr by Breezy_Pete

I've tried revving the engine, but still no output. I presume once the output is there it is still there when the engine is back at tickover? It wouldn't rev past 3500. Is that normal?

Thanks for the detail regarding the 2nd wire. I'll be returning to the problem tomorrow and will check for voltage on the blue wire with the ignition on. Going offline now, but will look later.

42 minutes ago, TinyTim said:

I've tried revving the engine, but still no output. I presume once the output is there it is still there when the engine is back at tickover? It wouldn't rev past 3500. Is that normal?

Yes, it should latch on and generate its own field current once it starts charging.

Yes, probably normal for revs to be limited out of gear, not moving.

Oh dear, I've probably got two duff alternators now! I understand that the usual problem is the brush/regulator pack. I'm thinking of removing the Bosch 140A and taking it and the Valeo 110A to an auto-electrician for testing. At least I'll know then.

Edited 17 March, 20251 yr by TinyTim

Can you see markings next to the two way connector on the Bosch alt?

Should say L and DF, I think, if it's the correct type.

Now it's installed I can't see much at all. It was previously on a Seat Ibiza 1.2 TSI, I believe.

@TinyTim My 2p; the exciter wire can break internally but otherwise look good. Have you continuity tested it yet?

Yes, measure for volts on blue wire contact of alt plug with ignition on.

I will do that. Thanks.

I've measured for volts on both wires on the plug. I got slightly less than battery voltage on one wire and a bit less again on the other. My voltmeter battery is weak, so I'm going to pick up a new one. I'm not convinced the plug is giving a contact. It just doesn't seem to go in far enough and there is no friction at all when it is pulled out. I wonder if there is something in the socket stopping it. Tomorrow I'll take it off again and investigate. i'm retired and have 3 other working cars, so I'm not giving it my undivided attention!

I've checked continuity from the alternator output terminal to the battery, which is fine. I contacted an auto electrician, hoping to get both alternators bench-tested, only to be told that they are 'smart-charging' and can only be tested when installed in the car! I've been too busy today, but tomorrow I'll remove the 140A Bosch and investigate the problem with the plug.

20 minutes ago, TinyTim said:

only to be told that they are 'smart-charging'

Not true, unless the car has start/stop technology, which I doubt, or the replacement one was the wrong type. Where did you get it from? Any photos from listing etc.?

I took the alternator (Bosch 140A) off again to see why the plug wouldn't go in. The reason is pictured below. It fits round the connector within the plug and has one groove on one side and two on the other. The grooves were obviously right for the Valeo alternator, but not for the Bosch. With it removed the plug goes in properly. In the Bosch alternator socket the pins are numbered 1 and 2. The blue wire goes to 1 when the plug is in. If that is correct, then the problem is solved. The shape of the plug being what it is, it's hard to see the purpose of the pink insert.

The insert helps to retain the contacts, I think. checking pin numbers for you now, but I expect that's right.

Yes, I can confirm that pin 1 is the blue wire.

Interesting that the keying is different, I shall try to remember that.

Strangely enough, I was planning to test a Valeo alt in my Polo at lunchtime, and I'm pretty sure the current one in there is a Bosch, so you may have saved me some headscratching if the connector doesn't fit.

Thanks for the confirmation of the blue wire/pin1. I hope that when I refit the thing it will now work! The biggest problem I find with removing and fitting is the tightness of the lugs to get it in position before the bolts go in. Thanks for your advice and persistence. I hope I have saved you some aggro.

There's a trick with the lugs. The steel threaded bush is actually a tight but sliding fit within the inner alloy lugs. Before attempting to fit, you can move these outwards to increase the gap down the middle. Doesn't help with removal, but knowing it can make refitting much easier.

Put bolt in several turns with it off the car, supported around a hole in something that the bush can move into (e.g. open vice jaws), then give the bolt head a few taps with heavy hammer. Only needs to move a mm to make a world of difference.

Couldn't hold it all above open vice and take photo, but hopefully you get the 'drift'.

Might be room to hammer one or more of them looser for removal, depending on how crowded the engine bay is near the bolt heads.