Proper way to test the Alternator?

Tidybuoy · 10:57 PM

My car is new to me. The prior owner told me that there has always been a voltage drain that can't be found and if the car sits, the battery will only last about 9 days.

I've been driving the car daily and recently installed a new Bosch battery. This week, the engine cranked slowly and so I put it on the charger and it needed to be charged.

This does not sound like an undiagnosed voltage drain as I've been driving the car and the alternator should be charging the battery. The alternator light comes on every morning for a couple of minutes and then goes out.

Just for curiosity, I disconnected the battery today while the car was running and the engine kept running fine but the headlights instantly dimmed somewhat.

My question: what is the correct way to check the alternator and are there any fixes or do I need to replace it. My car is a '74 and generally in very good condition. Also, when looking at alternators on Pelican, it looks like there may be an upgrade to 75 amps. Does anyone know about this and is this something I should consider - I eventually want to convert to power windows; if I can find them.

Thanks.....Vern

Porboynz · Porboynz's Garage

Couple of things spring to mind here. Never disconnect the battery on a running Porsche, or any car for that matter. The voltage spike from a good alternator can fry anything in the electrical system that costs more than $5. (Moores law, the fuse will be protected by the expensive electronic device) Equally the alternator might vaporise its own rectifier diodes when you remove the battery load, but if the engine kept running you are still getting some sort of voltage output from the alternator. Next is the alternator light, it should extinguish as soon as the revs rise above about 1200rpm, often lower than that. So I am betting the alternator is sad. Easy test, voltmeter should read 13.8V plus across the battery at say 2000rpm. Even easier test, rev the engine from idle with the headlights on an you should see them get brighter. Best test, a suitable 100amp shunt that allows you to measure the charging current, maybe best for an auto electrician to check this one. I guess you did disconnect the earth lead from the battery before hooking up the charger, right? In summary I think you have an alternator thats not charging correctly and maybe an excessive standby load as well if the PO is right about the battery going flat after a few days parked up. Get it sorted, eventually your new battery will need replacing if you keep running it flat.

Eagledriver · 10-12-2011, 09:30 AM

To check the alternator: start the car, rev to 2000rpm, let car idle, measure voltage on battery terminal (or any other source such as the cigarette lighter). The voltage should be at least 13.5 volts and not more than 14.5 volts. If you have this voltage, your alternator is working fine.

-Andy

911pcars · 10-12-2011, 09:36 AM

Quote:

Originally Posted by Eagledriver

To check the alternator: start the car, rev to 2000rpm, let car idle, measure voltage on battery terminal (or any other source such as the cigarette lighter). The voltage should be at least 13.5 volts and not more than 14.5 volts. If you have this voltage, your alternator is working fine.

-Andy

In addition, put some load on the electrical system to confirm the charging system is maintaining system voltage and charging the battery if needed (i.e. headlights, blower motor, rear window defroster, etc.).

Sherwood

Brett San diego · 10-12-2011, 10:35 AM

If the charged battery will go dead with just a few days of sitting, there is an excessive current drain when sitting. The alternator is a secondary issue.

Brett

gshase · 10-12-2011, 11:22 AM

I would bet money the alternator is both issues. One of the 3 diodes in the alternator can be bad and cause a current drain and also it could charge at 13.5 volts and not have sufficient amperage to fully charge the battery.

911pcars · 10-12-2011, 11:47 AM

Quote:

Originally Posted by Brett San diego

If the charged battery will go dead with just a few days of sitting, there is an excessive current drain when sitting. The alternator is a secondary issue.

Brett

... or the battery can't hold a charge. Best to test and validate and not guesstimate.

Sherwood

Brett San diego · 10-12-2011, 09:08 PM

Quote:

Originally Posted by 911pcars

... or the battery can't hold a charge. Best to test and validate and not guesstimate.

Sherwood

The OP did say he put in a new battery. Hopefully, that eliminated that possibility.

Brett

911pcars · 10-12-2011, 09:56 PM

Quote:

Originally Posted by Brett San diego

The OP did say he put in a new battery. Hopefully, that eliminated that possibility.

Brett

Yeah, I know. However, my statement still stands. Don't assume too much. You may end up spending excessive cash.

S

ivangene · 10-13-2011, 05:32 AM

since you obviously know how to disconnect the battery...try pulling the ground and then with everything OFF touch the battery, watch for any "spark" = some draw somewhere..start pulling fuses till the "spark" stops...that is the circuit that is drawing power...now find everything associated with that circuit.

I agree the alternator/VR could be the culprit of both issues - but just replacing parts gets expensive fast....then again it might just be some short in some hack radio installation...and wont cost a penny

ivangene · 10-13-2011, 05:35 AM

Quote:

Originally Posted by Eagledriver

To check the alternator: start the car, rev to 2000rpm, let car idle, measure voltage on battery terminal (or any other source such as the cigarette lighter). The voltage should be at least 13.5 volts and not more than 14.5 volts. If you have this voltage, your alternator is working fine.

-Andy

+1

but run it for several minutes...sometimes a VR can work intermittently - re-test a few times...I found mine worked "fine" sometimes...then spiked to 16volts!!!