Not convinced my alternator is working...

[Steve S2]

I have a sneaking suspicion that my alternator isn't working ('72 Plus 2 S130). To test it I measured the voltage across the battery terminals before starting it, then started it and held the car at 2,000rpm and re-measured. The voltage was the same. I'd have expected it to read higher.

Anything else I can do to be certain that my charging circuit is / isn't working before I start shelling out large wads of cash?

[Stuart+2]

Hi Steve.

Yes I would have been expecting around 14.7v assuming nothing is loading the circuit.

You could always risk it and disconnect the battery while it's running.

You must have had something else tell you that there was a problem surely?

Cheers,

[Steve S2]

You must have had something else tell you that there was a problem surely?

I've tried a couple of batteries and they don't seem to hold their charge for very long, plus the voltmeter on the dash never reads much more than 12V. However, I'm reluctant to rely on a 40-year old voltmeter!

[Tonyw]

Steve,

You will need to place a multimeter set to volts accross the battery terminals and if you do not see a rise from engine off to engine on your battery is not being charged, as it has been said 14+ volts from an alternator is normal.

If you want to test the alternator just put the positive probe on the output side of the alternator and the earth probe to earth (engine running) this will provide a quick test to see if the alternator is charging, if you get 14+ volts at the alternator but not at the battery then you have a wiring problem or scale on the inner part of the battery terminal.

Tony w

[45bvtc]

And that's the beauty of an ammeter; it shows real-time current flow from alternator/dynamo to battery/load. A Voltmeter just doesn't.

[RichC]

I like my ammeter too ! just fun to see the control box doing its work and the needle calming down after a good run

[spyzee]

Another simple thing to try is to make sure your ignition light is not glowing slightly when the engine is running. Obviously best done in the dark as it is not always noticable if you only drive car in the light.

[UAB807F]

On the same tack of very simple checks, one of mine was to start up, load the battery by the heater and lights at tickover and then rev the engine. If you're charging you should see the main beam lamps get noticeably brighter.

But these days I just look for 14v across the battery when it's at high tickover, maybe blip the throttle at the same time just in case the fan belt is slipping on the alternator pulley if you've got it quite slack ?

[Steve S2]

Have tightened the alternator belt and checked the voltage across the battery at tickover and higher revs. No change in the reading. Also checked the voltage at the alternator and it didn't differ from that which I read at the battery.

Finally, I measured the voltage across the battery with the engine off (12.63V - I'd charged it overnight), went for a 20 minute drive with the headlights on the whole time and then measured the voltage again when I got back. The result: 12.63V. So the alternator's doing something because if it was doing nothing at all, the battery voltage should have been reduced after running the car and lights for 20 minutes.

Make sense, and should I replace the alternator?

[Stuart+2]

Steve,

Start your car then screw up the idle. Then disconnect the earth lead from your battery. Crude but effective.

If it keeps running then your alternator is doing some work so go from there.

Cheers,

[UAB807F]

I've tried a couple of batteries and they don't seem to hold their charge for very long, plus the voltmeter on the dash never reads much more than 12V. However, I'm reluctant to rely on a 40-year old voltmeter!

At a slight tangent to thinking the fault lies with the alternator, if the main symptom is a draining battery, have you checked the car for current leakage when it's not moving ? With everything switched off do you still have the battery supplying current to the loom, and if so, how much ?

Like you I wouldn't place a lot of faith in the accuracy of an old voltmeter although it should be fairly close. As before on the thread, the best test is 14v across the terminals while you either run at a high idle or blip the throttle. I've only had 2 alternators pack in and in both cases it was pretty obvious - the red ignition light stayed on to give me a clue ! (but I did check with the voltmeter on the second occasion)

If you need a new one, I bought a brand new 17ACR from an Ebay business seller last year for around ?55, it was the cheapest deal I could find at the time. He was selling 18ACR (45 amps) ones for the same price - it was a place called "phoenix-marine-electrics". No affiliation, don't know if he's still selling at the same price, etc, but worth mentioning if you do need one.

Brian

[kstrutt11]

stupid question and no insult meant , but the red light does come on doesent it?

The alternator senses voltage through this so you need a good 12v feed to the bulb and a bulb which works for it to charge.

[bob_rich]

Hi

agree with the last post. If the ignition warning lamps is faulty ( blown so open circuit) then the alternator may not kick in fully. when running the light is out so if it was also out when ignition on but engine not running then it may just be the bulb.
the bulb feeds power from the switched ignition to the alternator control circuit.

hope this helps bulbs are cheeper than alternators!

Bob

[dpo#4]

Dear Steve,

Battery voltage level (12.7v) being present at the alternator B+ (heavy wire terminal) implies no alternator output. Applying power to the field windings bypasses the regulator and tests the alternator by allowing maximum output with ever rising voltage. This has to be a short test, or the resulting 18v+ will damage solid state circuits (tach, radio, alternator rectifying diodes and solid state regulators). So once 15v is attained, end the test.
My friend , who ran his own starter/alternator rebuilding shop, cautioned against removing battery cables with the engine running on alternator equipped vehicles. He had an article from a trade magazine posted on his wall that quoted a maximum possible voltage of 200v(!) if contact was lost, as the regulator could no longer "see" the battery voltage, and tried to fix the situation. I'm guessing the rectifying diodes blow at well below that level, and he did show me a few failed units to which he attributed "cable removal" as the cause. The diodes were fried, with smoke deposits and actual gaps in the diode bodies. This prohibition did not apply to DC generators, which possibly is the origin of this test. I don't know if this caution applies to mechanically regulated alternator systems, but it well might as the voltage limiting strategy is the same, i.e., interrupted field current. The danger with newer vehicles is to the many expensive computers, for which there is a long list of prohibited practices. Paranoid, as always - Ray -

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My friend , who ran his own starter/alternator rebuilding shop, cautioned against removing battery cables with the engine running on alternator equipped vehicles. -

I agree....do not run it with the battery wire off.....I had a Europa and the battery wire got disconnected while I was driving...I think the terminal or wire broke....anyway it killed the alternator stone dead!