Alternator overheating

[Souleman]

Can anyone help: the air conditioning on my Citroen SM had gone warm. I took it to a a/c specialist who looks after lots of classics. He found a leak in the a/c system and dealt with it. However on starting the car up, with the blowers on full although lots of lovely ice cold air came out of the vents the alternator got seriously hot. So hot in fact that it started smoking the drive belt from its pulley. When the Aircon was turned off the alternator was fine.

I have the car booked in for a classic car specialist to look at on Monday. I would take it to my SM specialist but they are 120 miles away and I want the car ready for a road trip on 10 days.

Can anyone suggest what might be going on? I can’t make sense of it. If the alternator is knackered ( and it is the same age as the a/c compressor at three years when I had the car overhauled) the a/c compressor kicking in shouldn’t make any difference to the extent the alternator gets red hot; there is very little draw from the clutch or the a/c compressor coming in so why would the alternator get so hot? Someone suggested the alternator belt is slack, but it isn’t and I don’t follow how that would get the alternator so hot that it was trying to ignite its belt.

All and any theories gratefully received.

David

[Pete54]

I'm not sure what you meant by the alternator getting so hot the belt smoked. The belts (if you're running the standard ones - the very narrow one for the alternator), are notorious for slipping.

If the a/c has been worked on it may well be the belt tensions are way out. There is a specific method of tensioning them - and they are surprisingly tight.

I've used the Dutch mode from Renard to change the belts - but the power the a/c and alternator consume still means careful alignment and tensioning are very necessary.

There is an internet forum for SM, where there is a wealth of knowledge (even if some of it is American and so a bit off!), well worth seeking out.

[Souleman]

Thanks. I think I know what has happened. The guy who fixed a leak on the a/c system overfilled it with gas - he put 1kg in. I understand that the max should be 750g. The a/c unit jammed on the excess gas and I think you are right the alternator slipped and lost tension.

I have an appointment at my trusted SM specialists on Monday (I should have taken the car to them in the first place - lesson learned) to see if they can fix everything so we can take the car away on Wednesday.

[larrylamb11]

• edit* our posts crossed - sounds like you have your answer.

As the above poster has suggested, a slipping belt is by far the most likely cause and can transfer a huge amount of heat into a pulley.
You really need to break this down into what's actually going on - is the alternator getting hot because it's producing huge amounts of current? or getting hot from mechanical resistance? or hot from a slipping belt?
To measure how many amps are being drawn when the aircon is on use a suitably hefty ammeter connected in series with the alternator output wire. Once you know how many amps it's producing you can start to work out what is drawing the most current by switching things on and off. It may be that you have an electric cooling fan that should be running when the aircon is on and it's seized, drawing huge current. Once you've worked out whether the alternator is operating in normal parameters you can then start to trace why it's getting hot.
If the amp output is normal, its time to look at mechanical resistance and belt tension, both of which are obvious and easy to check.
It's entirely possible the alternator has a bad bearing in it that's getting hot, though you'd normally hear that.... which brings suspicion back to poor belt tension.

Edited by larrylamb11 on Friday 21st June 20:40

[Souleman]

Thanks. Yes a check with the ammeter showed the alternator was putting out a healthy charge, within range even under load - lights, blowers on etc. You are right that the fans come on with the aircon compressor but they are fine. There is a slightly voltage drop with the aircon compressor on but bear in mind this was before it was trying to churn 250g of gas too much. Everything points to it needing degassing and doing again properly and both aircon and alternator belts replacing and tightening. I should know by Monday night - 250 mile round trip to my SM specialists. If they are happy and the car is fine on the 125 mile journey home I think I will be set for Southern Ireland in a few days time.

[spikeyhead]

A current clamp meter is far better for measuring high currents in situ