Can I test the alternator output by measureing the voltage at the battery terminals when the engine is running?
Thanks

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1983 300SD
200000miles

yes you can.
mine tests at about 13.8v

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

You can test the voltage with this method but to completely test an alternator the device needs to be load tested to determine what the current output parameters are....I have seen many alternators that put out a correct voltage and barely provide enough current for the needs of the demand in the car. This is usually done with a special device either in an alternator shop or on the car with a portable device that provides a large demand on the alternator to see what it can handle...I recently had an alternator in my VW Golf that put out correct voltage but barely provided enough current to run the car at night with the blower fan on...I changed the regulator in it and it still did not perform up to what it should be. I finally replaced the alternator and it then worked as it should...On this forum it is commonly suggested that voltage be checked and if it is up to spec that the alternator is OK...Nothing can be further from the truth....My 2 cents

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83 300CDTurbo 307,000
2005 Honda Element 266,000
56 Nash Ambassador Country Club Special 34,000(under restoration presently)

http://www.peachparts.com/shopforum/car-audio-multimedia/125099-123-install-extreme-dynamat.html

yep it certainly would be ideal to bench test it, but the quick test method at the battery terminals will at least let you know if the battery will get charged.

lucky our diesel engines don't need the batt to keep running....

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1984 300D Turbo - 231k....totalled 11/30/07 RIP

You can take the car to Autozone and they have a portable machine that can test alternators.

Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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All you need is a simple digital volt meter to test an alternator on your car. First check the voltage with the engine not running and all electrical loads off (doors closed, lights off, etc). The v (from now on, v = voltage) should be atleast 12.5 v. Now switch on the ignition with out starting the engine. Make sure the battery light comes on. This is the most missed item when testing a charging system. The battery light MUST work, and if it doesn't it may not charge when started. Start the engine with everything else still off. V should be 13.5 to 14.5 v. Now turn on the A/C with fan on high, headlights on, doors open, but not the wipers (they cause very irregular loads on the system making it hard to get a good reading). V should be above 12.5 at idle and increase to 13.5 or more with engine at 2000 RPM. If all the above is OK, then your charging system and battery are OK. No fancy meters needed. This may not tell the maximun power output of the alternator, but will tell you if it has enough to keep the car happy, and that is all that matters.
There are a lot more things that can be checked with this very simple meter. For instance, make sure the battery is fully charged and with the car off, check the V. It should be 12.5 or more. Now turn on the headlights only with engine still off. If the V drops below 12 V with in a few min, then the battery is weak and needs to be replaced before it leaves you stranded. If the readings while charging with a load are low, switch the v meter to AC volts. If you see more than 1 V AC then the rectifier pack in the alt is bad. Back to DC Volts, if the readings very greatly say 12 to 13 v back to 12 when nothing else is changing, then the brushes are probably worn out. In this case replace the regulator which includes the brushes.
Another thing you can check for is bad grounds. Connect the - lead to the battery - term and the + lead to the engine. Start the car and watch the meter. If you see more than 1 v, you have a bad ground connection at the engine or a bad ground cable. If all is good, move the lead from the engine to the body. Now you have the + lead hooked to the body and the - lead hooked to the - post on the battery. Turn on the head lights and check the meter. If it is more than 1 v you have a bad ground to the body. Start cleaning connections until you can get it below 1 v , the lower the better.
The volt meter is a great tool when working on autos.

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84 500 SEL (307,xxx miles)

In-car alternator test:
Cold start engine, set engine to around 1500 RPM and monitor voltage at battery. Battery voltage should rise from 12.6v to around 15V and then drop back to 13.6 - 13.8V. This will take time, as it has to recover the charge lost overnight and starting the engine. When the voltage drops back to 13.6v, the battery should be >90% fully charged. NOTE: if your battery is nearly flat, or you needed to crank long & hard to start the engine, it will take a long time to recover charge. It is possible to recharge 50 Amp Hours of charge to a battery, using a 65A alternator in less than an hour, if other electrical loads are minimal.
When the battery volts have dropped back to 13.6V, increase the load on the electrical system. Turn headlights on, heater, A/C, rear demister and stereo. In fact, every electrical system in the car.
At idle, the battery voltage will be ~12.0 - 12.6 volts (discharging). Raise engine RPM to 1500 and check that battery voltage increases to 13.6 volts. This indicates that the alternator is supplying ALL of the load AND recharging the battery too.
If your alternator passes this test, a bench test would be unlikely to fault the alternator.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.

Thanks all. I started over thinking this and wound up just getting confused. It made sense that I could test in place but some times you over analize something.

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1983 300SD
200000miles

pmechecnies procedure is very good, but if you are "overanalyzing" then simply look for a little over 12 volts with engine off, and somewhere around 13.5 to 14.5 volts with engine running. If you are seeing these, then in most cases all the rest of it is splitting hairs.

In the days of mechanical voltage regulators and especially generators, all the other checks and adjustments were necessary. With todays electronic voltage regulators a simple voltage test will suffice in 99 out of 100 cases.

Good luck,

THANKS TONY!!! you just gave me a present Mucho Thanx. The battery had been slowly going bad on my Mog. Then I noticed my it was operating up around 16v the other day and I thought this was a bad voltage regulator. Part of me wanted to say this was a new reading and it had not been operating at that voltage all along, but I didn't trust myself. I knew the gauge should show 13 or so after starting but I didn't know it would read up toward 16 if the battery needed significant charging. When I saw 16 I just new it was the regulator and that this was what had being slowly frying the battery.

Anyway..... I was looking at a remove replace of regulator etc. Had to remove part of the cooling system plumbing to do it. Now I don't have too. Installed newer battery, fired her up and the volt meter reads perfect.

Thanx again, you saved me a lot of time.

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"If anyone knows other lessons I need to learn, please tell me. I'm tired of learning them the hard way".
by JerryBro

The Glow Plug Wait: This waiting period is a moment of silence to pay honor to Rudolph Diesel. The longer you own your diesel the more honor you will give him". by SD Blue

My normal daily life; either SNAFUed- Situation Normal... All Fouled Up, or FUBARed- Fouled Up Beyond All Repair

62 UNIMOG Camper w/617 Turbo, 85 300SD daily driver- both powered by blended UCO fuels

LarryBible,
You are correct and I realize now that I went a little overboard with my response. My initial thoughts were, yes you can check your charging system with a volt meter. I guess I got a little carried away with all the detail. Maybe it will help as members learn the basics of the charging system and need to diagnose problems and only have a simple volt meter. In the future I will try to only go as deep as necessary to solve the problem at hand.

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84 500 SEL (307,xxx miles)

Paul,

No, you SHOULD go into the complete procedure when you have the time. It was a good writeup. For most folks, however, they can do it the simple way and have a good shot at knowing which component to replace, while doing full load testing or even a voltage drop test is probably beyond them.

The skill level of our members and guests here at mshop runs the gamut. We have people here that are barely able to check their oil and tire pressure as well as some who could probably design and build a whole car from scratch. We have every level imaginable in between.

One of the toughest things to figure out when helping people is what they will understand and what they won't. Most people probably don't understand ohms law, but maybe they can use a DMM.

Thanks very much for your contribution.

Have a great day,

An excellent post. I did not see it while I was composing my reply, or after, until now. Had I seen it I would not have needed to post.