Questions about voltage

[Kenny Han 1]

Hi guys, I'm having some questions regarding battery voltage drop with air conditioning on.

My car has a 1.9L 8v engine with twin weber 45. Car starts and idle at 13.6v, with fan kicks in at 13v, headlight on at 12.8v and when air-conditioning is on it drops to 12v (without ICE turned on).

Appreciate if anyone with air-conditioning model can advice if this is ok.

Thanks!

[ian010778 21]

Most cars should read around 14.2 to 14.4 with no load and when a load is introduced the voltage should drop a little and recover.

 

If not, I'd be looking at the alternator.

 

 

Ian.

[Kenny Han 1]

28 minutes ago, ian010778 said:

Most cars should read around 14.2 to 14.4 with no load and when a load is introduced the voltage should drop a little and recover.

 

If not, I'd be looking at the alternator.

 

 

Ian.

I got my alternator rebuilt recently. Not sure if I need an uprated alternator but I will get it checked again. Thanks.

[stewal 8]

I have ac: it may not be your alternator: when you engage a/c the load will cause engine speed to drop and alternator output to fall.

 

where are you measuring the battery voltage: it should be on the battery direct and are the measurements you are giving at tickover? If they are at tickover but go up when you rev up I would say its ok.

 

the standard alternators are quite low output when compared with modern cars.

[petert 586]

In a standard GTi with AC there is an "idle up" solenoid on the throttle body, that lets in more air when the compressor turns on. As you have carbs and I assume a very basic ignition system, ie no ecu., there is no way of increasing your idle speed when the AC is on and thus maintaining the correct charge voltage. More modern cars have an idle control valve, which maintains a constant idle (and thus charge voltage) regardless of load. Enjoy the primitive world of carbs.

[Kenny Han 1]

On 6/29/2020 at 8:27 PM, stewal said:

I have ac: it may not be your alternator: when you engage a/c the load will cause engine speed to drop and alternator output to fall.

 

where are you measuring the battery voltage: it should be on the battery direct and are the measurements you are giving at tickover? If they are at tickover but go up when you rev up I would say its ok.

 

the standard alternators are quite low output when compared with modern cars.

The voltmeter is installed at the ignition key. The voltage only go up by 0.1 or 0.2 volt when i rev up lol

 

On 6/29/2020 at 8:42 PM, petert said:

In a standard GTi with AC there is an "idle up" solenoid on the throttle body, that lets in more air when the compressor turns on. As you have carbs and I assume a very basic ignition system, ie no ecu., there is no way of increasing your idle speed when the AC is on and thus maintaining the correct charge voltage. More modern cars have an idle control valve, which maintains a constant idle (and thus charge voltage) regardless of load. Enjoy the primitive world of carbs.

That's good info Peter. Many thanks!

 

Not sure if anyone has installed an alternator with higher output but I might try that. Can't find any info on this in the forum.

[stewal 8]

From experience of fitting a voltmeter to my car if your are reading voltage anywhere other than accross the battery direct you will not get an accurate reading just an indication.

 

By taking your reading from the ignition switch, fusebox or any other ignition switched source you will be reading voltage after it has been dropped accross all the wiring to that point as well as after any load on that circuit.

 

from my experience of connecting to the fuse box it can under read by anything up to 2 volts - even more if its tapped into a circuit with a heavy load like the HRW or fan blower.

 

in the end I connected mine direct to the battery BUT THIS MUST ONLY BE DONE VIA A FUSED RELAY - I would never directly connect across the battery terminals without a suitably fused isolator like a relay. 

 

By using an ignition switched relay, I now read accurate voltage accross the battery all the time the ignition is on but unless you are up to speed with 12v systems I would ask an auto electrician to do any work. 12v won't electrocute you but it can burn you or your car (I still have the burn on my wrist from accidentally shorting out the alternator to earth via my metal watch strap whilst changing my 205 oil filter 30 years ago.........so any wiring you do must be done with care.