Any way to test alternator?

[Sheehan879]

Just checking I am not missing something. Battery died and would not keep running from a jump start. Charged it overnight and put it back in. When running the battery is only showing 12.6 v, I am assuming this means the alternator is not charging it. Is there any way to check the alternator or could it be anything else?

Many thanks,

[cheshired5]

Click on the link in my signature for alternator testing videos.
It's important to check voltage output directly on the alternator B+ post to accurately gauge alternator performance not on the battery.

[deeman940]

An easy way to check is to run the engine and shine the lights at something i.e a wall/fence then rev the engine. If the light gets brighter it usually means the alternator is charging. Could just be a knackered battery...

[Skyedriver]

Had something similar a few weeks ago
Battery was checked by me and two others (local garage and Halfords) and was ok. 13.5V and up to 14V
Alternator was shown to be charging but every few days I would have a flat battery.
I swopped the alternator for a spare but I had with no improvement.
Then one day, the car died while parking, wouldn't restart. The jump start pack wouldn't start it.
The AA guy turned up and blamed the battery.
I said no, he tested it 10V. Then touched the alternator - extremely hot.
The alternator was draining the battery.
Old alternator back on and a new battery - bingo - everything ticketty boo.

[stephend]

You should get about 14.4 V across the battery if the alternator is charging it. But cheshired5 is right, you should really check at the alternator, as there might be a voltage drop across the cable to the battery if there's a poor connection, or the cable might even be broken.

[TonyS9]

You should also check the field current excitation wire which is conencted to the warning light and a resistor.

Does it light before you start the engine? If its not charging the light should come on, but sometimes a faulty alternator can produce just enough power to put the light out. It only comes on if the alternator is completely dead.

You have to have alot of resistance in the joints to drop to 12.6V, and this sounds like a poor alternator output. So as suggested check the voltage across the 2 alternator output wires. It does vary with temperature so don't worry if it closer to 13.5v but any lower than this is bad.

The alternator voltage should be the same as the battery voltage, they are directly connected, but with corrosion in the loom this gradually gets worse and the voltage can drop at the battery depending on current load.

Voltage drop with high beam on (from alternator to battery)
<0.1V = very good
0.1-0.3V = Ok, typical for age
>0.3V = Could be better, starting to slow the charge rate
1V = Half charging rate, you may well experience flat battery/no start when doing short journeys.

[Sheehan879]

I am back to square one, the new alternator seemed to do the trick, all the dash lights started to come on including the battery light. Got in today and no lights apart from the seatbelt warning. I checked with a fluke and the alternator is not charging again.

I am thinking instrument cluster, I removed it but all looks fine. Can I test the instrument cluster?

[Laird Scooby]

Something i had on one of my 740s was the connectors in the plugs that plug in the back of the instrument cluster were all dirty and the springiness of the contact fingers had subsided over time.

I removed the cluster, went through each contact "finger" on each plug (where it connects to the PCB on the back of the cluster when the plug is pushed in) and eased it up to make more of a curve (you'll see what i mean when you look at the plugs) and cleaned each one and also the PCB with some very fine emery cloth - don't use sandpaper!!!

Plugged it all back and tried it, job done! Also might pay to check the bulb in the charge (battery) warning light is making a proper connection in the bulb holder. It's a higher power bulb than the others usually to provide the current the alternator field needs but should be a maximum of 3W - don't put anything bigger such as a 5W in as you could easily damage the alternator.

Also check the connections to the alternator.