Signs of battery failing

peelaaa · 5th December 2021, 13:41

Hi,

Car starting okay but noticed lately when pressing window demist button, there is an instant drop in revs for a short while as if battery is struggling. Would that be the battery or alternator failing?

SD1too · 5th December 2021, 14:48

Quote:

Originally Posted by peelaaa

... when pressing window demist button ...

Ian; are you talking about the windscreen demist button (which activates the air conditioning) or just the heated rear window button?

Simon

peelaaa · 5th December 2021, 14:51

Quote:

Originally Posted by SD1too

Ian; are you talking about the windscreen demist button (which activates the air conditioning) or just the heated rear window button?

Simon

demist button

SD1too · 5th December 2021, 14:57

Thanks for the confirmation Ian. The windscreen demist button places a huge load on the engine because it not only activates the air conditioning compressor clutch but the blower runs at full speed and the heated rear window is switched on as well. It wouldn't be unusual for the engine speed to drop slightly under those conditions and doesn't necessarily indicate a problem.

You can check the charging voltage of your alternator by activating the instrument pack diagnostics display no. 9.0.

If you're in doubt about your battery, particularly if you undertake many short journeys or use the air con., HRW and heated seats a lot, give it a boost with a smart charger for as long as you can.

Any help?

Simon

peelaaa · 5th December 2021, 16:19

Thanks Simon, I've got a booster and will check the diagnostics no 9 .
Just hope its not the alternator as had it replaced a couple of years ago and its a big job

bl52krz · 5th December 2021, 19:32

I have read that when you engage the air conditioning on our cars, I presume it is the same on other vehicles, that the ecu recognises the fact you have called for air conditioning, and enriches the mixture to compensate for the load on the engine. Mine does this.

KeithA · 6th December 2021, 08:41

Quote:

Originally Posted by peelaaa

Thanks Simon, I've got a booster and will check the diagnostics no 9 .

Just hope its not the alternator as had it replaced a couple of years ago and its a big job

If it is alternator you might get away with just replacing the brushes (or easier, the voltage regulator pack). Dmgrs sell the VR packs for about £30 and it can be done in situ, albeit a little fiddly.

Sent from my moto g(30) using Tapatalk

Mike Trident · 6th December 2021, 10:45

Quote:

Originally Posted by bl52krz

I have read that when you engage the air conditioning on our cars, I presume it is the same on other vehicles, that the ecu recognises the fact you have called for air conditioning, and enriches the mixture to compensate for the load on the engine. Mine does this.

This the reason you should turn off the AC when you take it for an MOT. You will get a high emissions reading.

The engine tick over dropping slightly is just the load placed on it when you operate the AC, nothing to worry about.

SD1too · 6th December 2021, 12:47

Quote:

Originally Posted by bl52krz

I have read that when you engage the air conditioning on our cars ... that the ecu recognises the fact ... and enriches the mixture ...

Unfortunately David I suspect that what you've read contains a misunderstanding by the writer. The air:fuel ratio (mixture) must remain constant at all times. The extra load placed on the engine by the air conditioning compressor may result in the engine speed increasing to compensate. That would involve extra fuel and air. The mixture won't be enriched.

Quote:

Originally Posted by Mike Trident

This the reason you should turn off the AC when you take it for an MOT. You will get a high emissions reading.

As above Mike, you'll be relieved to know that the emissions won't be adversely affected by the air conditioning being on or off.

Simon

Yorkshire GOC · 6th December 2021, 16:43

Air Con on will increase the revs nothing more.

5th December 2021, 13:41	#1
peelaaa Posted a thing or two 75 cdti 2004 contemporary Join Date: Apr 2008 Location: meath Posts: 1,551 Thanks: 272 Thanked 80 Times in 58 Posts	Signs of battery failing Hi, Car starting okay but noticed lately when pressing window demist button, there is an instant drop in revs for a short while as if battery is struggling. Would that be the battery or alternator failing?

5th December 2021, 14:57	#4
SD1too Doesn't do things by halves Rover 75 2.5 Connoisseur Auto (1999) Dealer launch model. Join Date: Mar 2007 Location: Former Middlesex Posts: 20,344 Thanks: 1,587 Thanked 3,749 Times in 3,181 Posts	Thanks for the confirmation Ian. The windscreen demist button places a huge load on the engine because it not only activates the air conditioning compressor clutch but the blower runs at full speed and the heated rear window is switched on as well. It wouldn't be unusual for the engine speed to drop slightly under those conditions and doesn't necessarily indicate a problem. You can check the charging voltage of your alternator by activating the instrument pack diagnostics display no. 9.0. If you're in doubt about your battery, particularly if you undertake many short journeys or use the air con., HRW and heated seats a lot, give it a boost with a smart charger for as long as you can. Any help? Simon __________________ "Whatever is rightly done, however humble, is noble." Sir Henry Royce.

5th December 2021, 19:32	#6
bl52krz This is my second home Rover 75 cdt club + Rover 2.5 KV6 Conni SE Join Date: May 2008 Location: Birmingham Posts: 11,387 Thanks: 6,587 Thanked 2,262 Times in 1,729 Posts	I have read that when you engage the air conditioning on our cars, I presume it is the same on other vehicles, that the ecu recognises the fact you have called for air conditioning, and enriches the mixture to compensate for the load on the engine. Mine does this. __________________ Great Barr, Birmingham.

6th December 2021, 16:43	#10
Yorkshire GOC I really should get out more....... Rover 75 connoisseur se v6 auto Join Date: May 2014 Location: Hull Posts: 2,063 Thanks: 1,842 Thanked 623 Times in 469 Posts	Air Con on will increase the revs nothing more. __________________ She came off the Longbridge Line on 20-05-2003 The Silver Machine was the 13th of 160 Rover 75's to come off the production line that day and is the 100th of 527 Starlight Silver Rover 75 2.5 V6 Connoisseur SE Auto saloons listed in the build records produced world wide.

5th December 2021, 16:19	#5
peelaaa Posted a thing or two 75 cdti 2004 contemporary Join Date: Apr 2008 Location: meath Posts: 1,551 Thanks: 272 Thanked 80 Times in 58 Posts	Thanks Simon, I've got a booster and will check the diagnostics no 9 . Just hope its not the alternator as had it replaced a couple of years ago and its a big job