Can the alternator be tested while still on the car ? My current alternator works, but, it’s been in the car 20 years.
There’s a local shop that rebuilds them
Any tips here ?

Old school way we tested them in the past was with a load tester.

This is a modern fancy version, all mine just had needles.

https://www.autometer.com/bva-34-800-amp-va...mp;gclsrc=aw.ds

You have to know how to use them.
Basically, you get the engine speed above thermal cut it (usually 2k rpm works fine) and you twist the main knob adding load to the system through a carbon pile that would get HOT during the testing.
You would test how many amps the alternator could hold before it dropped below a certain voltage (usually 12.5) and you would know your total output.

You can sort of do this with nothing more than a DVOM.
Carbon pile testers are expensive, and they don't last forever.
I've done without one for a couple of decades now.

DVOM process. get car hot and idling with no loads on. Record voltage across battery terminal. raise engine speed to 2k, record voltage.
Turn on ALL loads in the car. all lights, all blowers, everything you can tun on.
The only thing I leave off is the wiper system.
Running any wiper system on dry glass can destroy it.
Radio maxed, everything...

Do the readings again at idle and 2K

If all of your readings were above 12.7 you are golden. I'd expect the lowest readings at idle with loads followed by idle no load. If your 2k readings were in the 13-14.5 range I'd be happy.

There's more to it than that but that is a 60 second health check which will usually show a weak alternator.

Hope this helped,
Rick

After thinking about it a bit I realized another good test is with this DVOM.

https://www.amazon.com/UNI-T-Digital-Voltag...65072&psc=1

That unit has a built-in small amp clamp that can accurately measure DC amps-which is a rarity in small amp measuring devices. AC amp meters are 90% of what's available.

You could clamp that around the current wire (large one) coming directly off the alternator. This would allow you to see an output number in amps.

It's cheap, reads AC amps as well and I use the piss out of mine.

This would not allow you to see total power of the alternator.

For that you need an easy way to add a variable load.

You might check at a few of the shops in your area, someone might still be using a load pile tester.

I predict the guy using it will be old...

What does a new one, higher amps, cost?

Thanks. I have a battery tester that goes through a load test. I’ll post my last results shortlly

Thanks. I have a battery tester that goes through a load test. I’ll post my last results shortlly
I didn’t realize it could help with alternator diagnosis

Jim, I found this thread which should help.

http://www.914world.com/bbs2/index.php?sho...p;#entry2862962

Let me know if you need a hand, I'm sure we can get it tested.
Bob

I've full fielded alternators in the past mounted in the car. This can be HARD on other electronic devices in the wiring harness, and I stopped testing this way 30 years ago.

I will not full field an alternator in anything today, especially a 50-year-old German car that might have un-available parts in its wiring systems.

The only "safe" cars to do this with are really old cars that do not have printed circuit boards in them like our 914s.

I'm still not going to do it as I've damaged too many things in the past via a full field test.

How well something is learned is directly proportional to the money, blood, or face lost during the learning process.

And that thread is wrong on one count. If you have an oscilloscope, you can see a bad wheatstone diode pack in the patterns. You will not have 3 nice upper sine waves, one or more will be pointed down.

Mr Hoyland, your car is beautiful! I am envious of everything you've done.
It is an inspiration to me for mods on my car.
Just stunning!

Thanks for all the great information; will follow up.

after ascertaining that both the battery neg cable is cleanly attached to the firewall and battery post and that the rear transmission strap is cleanly and firmly attached to the rear trans tail cover and the bottom of the trunk floor, put a volt meter across the two battery posts, test, and then the two battery terminals and test. When the car is at rest the battery voltage should be about 12.5 and then when the car is started, 13.5-13.9 NO MORE. If the charge is below 13.1 substitute the voltage regulator.

Listen to the alternator for bearing noise while it is running, and also if it is charging well, you can unplug the voltage regulator and the idle will increase

If the red light is on when the key is turned on and SORT of goes out but is still on dimly, a diode is bad in the alternator

Jerry
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This is good:
“In the diagram below, the main components of the alternator are shown. There's 9 diodes, 3 stator windings, and an armature. Pretty much the only component which can be isolated is the armature. The diodes and stator windings cannot be isolated to be tested separately.

If you want to do a bench test, then you have to set up a fixture to spin the alternator while checking voltages and the current capability of the alternator.

The diagram below can be used to do a bench test. Here's what you need to do to do a bench test:

1) Connect a battery between B+ on the alternator and the alternator case.
2) Do not install the voltage regulator.
3) Use the original wire harness to the voltage regulator connected to the alternator.
4) Short D+ and DF to put the alternator into maximum charge state.
5) Connect a GEN bulb as shown in the diagram.
6) Spin up the alternator to about 3k to 5k RPM; you should measure 16V at D+/DF to ground and B+ to ground.
7) The voltages between D+/DF and B+ should be within one volt of each other.

This will give a good indication of alternator health.


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Jim,
20 years???
Mine was in the car since 1987 and still working. Had to replace it because the bearing started howling just before WCR 2022. I would suggest having it professionally rebuilt by someone reputable. I had to install not 1, not 2, but 3
re manufactured Bosch alternators (seems like almost all re manufactured stuff is junk now) to get one that actually charged or not have the dash light stay on.
I'll make sure to bring jumper cables to Idyllwild if she gives you any problems.

Mike

I agree. We have a local shop in Costa Mesa that specializes in alternator rebuilding, several World Teeners had rebuilds there.

I don't know how useful these things are but for $36, I gave it a try. The tiny Bluetooth device connects across the battery terminals and communicates with your smart phone.
They are sold by the Antigravity lithium battery company:
https://antigravitybatteries.com/products/a...cker-lead-acid/
The unit has a memory of 30 days and there are models for regular lead-acid batteries and for lithium ion.
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