Alternator Test Quick Question
 

Hey everyone,

I have a quick question, I know on gasoline powered cars that if you disconnect the battery and the engine/electrics die then the alternator is bad.

I dont see why this test wouldnt carry over to the diesel powered cars as well..minus the engine dieing (for obvious reasons)..but you never know.. I am just wondering.

I have my dad's SD here and I checked voltage at the battery, it was 13.6 when the car was running. We put on the headlamps, blower and a few other things and I disconnected the battery...everything turned off (minus the engine). I checked voltage across the disconnected battery terminal and ground..got nothing. I also tested the dist. block on the rail..got nothing.

Safe to say dead alt? correct?

Just thought I'd double check this with everyone...


Also: how is changing the alternator on a 126 617 compared to a 201 103? I've done the 201 a few times. Its a pain in the ass. I'd think the 617 is easier due to the belts and what not

Thanks,

Kris

NEVER disconnect the battery with the motor running, it can damage the alternator diesel or gas.

Leave the battery connected and check voltage with no load(lights off) should be almost 14V, then with a load will probably be about 13V. The voltage should also go up when you rev the motor to about 1500-2000 RPM.

You cant get 13.6 V at the battery with a dead alt. Bosch alts dont like having the battery removed when they are running. Its a good way to toast the regulator. I can speak with experience on that. Dont do it on any car. What is the real problem? If you have a problem with the voltage dropping back to say 12 V when you turn the head lights etc on with the engine revving over 1000rpm. The first thing I would look at is the lead from the alt to the battery, If you can take the back off the plug on the alt, you could test the alt output voltage, should be approx 13.6v. If a lot less, check the alt brushes. If they are good, then the alt needs looking at further.

Noted. I am curious though is there a reasoning behind this? I understand that the sudden change in voltage could damage some electronics such as computers and what not however there does not exist any in the SD.. I'm not questioning your advice just wishing to understand more.

I did test it at idle it was 13.6 roughly.

Further:

The problem originated when my dad tried to leave work today, he said that the glow plug light on the dash did not come on when he first put the key in. He turned it off and then it came on. Tried to start, and nothing happened. She cranked but wouldnt turn over.

So I dove into the gp system. GP's are all near 1.3 to 1.4 ohms. I swapped out the relay box with one from my other SD (stored for the winter) as I believed that the 80amp fuse might have a fracture in the metal. I couldnt find my spares and rather than risk breaking the fuse when swapping fuses with the other SD. I opted to just swap relays.

With the GP system back together, I had him turn the key on and I verified that the GP's were firing up.

So we were going to take it down to the Advance Auto to have them check the charging system but they changed their hours to 8pm instead of the usual 9pm.

I'll go run a few other load tests as suggested. I'll return with the results.

My apologies for the disconnect the battery when running test. I was unaware.

Will return shortly.

You may have damaged a perfectly good alternator by disconnecting it with the engine running! That so called test goes back to the 1940s when cars had generators, generators had no sensitive electronics in them and that was a semi reliable test, it only showed that the gen had SOME output,not that it was up to spec. With an alternator there are diodes in the alternator and more diodes and IC chips in the built in regulator,when you remove a battery cable with the engine running you can and most likely will get voltage spikes high enough to damage those components if at a slow idle you might get away with it, if you revved the engine up with the cable off its very likely you did some damage. I dont know how this so called test has hung on for so many years and couldnt even guess how many alternators and regulators have been ruined by it. Don

Alternator Check
 

here's my method, some will agree, some won't.
With car off, check battery voltage. Should be 13.4 to 13.6 <> on most good 12 volt batteries.
Start car. Check voltage at battery, should be about the same as car not running ( at idle ).
Run engine up to about 1800/2000 rpm's. Voltage at battery should go to around 14.2/14.4 . Shows that alternator is charging . Keep rpm's up, turn on lights and other high amperage items. Check voltage , should be able to maintain at least a 13.4 or so. Alternators do wear out, brushes can go but main problem is a diode that fails and will drain battery .
Hope you don't have to work on this in the cold we are having at present time.......

Thank you for the explanation. I appreciate it. I will pass it on.

Results from the various tests:

Battery - Car Off - 12.43v
Key On - Car Off - 11.65v - No GP Light On
Cranking Voltage - 10.4v
Started Idling - 13.3v -
Started Revving - 13.6v peak
Only Blower High @ Idle - 12.63v
Only Blower High @ Rev - 13.6v
Blower High + Lights @ Idle - 12.2v
Blower High + Lights @ Rev - 13.4v
Blower High + Lights + Rear Defrost @ Idle - 12.0v
Blower High + Lights + Rear Defrost @ Rev - 12.5v

So I am leaning towards maybe alternator is OK, maybe voltage regulator?

Also, the GP light would not come on, it did come back on after I turned the key on and off 3x..then it came on as per usual. I've not been able to repeat the situation where the light does not come on..However based on the voltages I suspect the GP's are running despite the light being on..

oldiesel,
You are correct. The regulator uses the voltage as a reference, when it gets a spike on it. it usually pops a transistor in the reg, you cant replace it as they are not discrete components, Part of an IC.

Approximatly twelve point seven five volts is a fully charged battery without the alternator running I believe. This is with letting the battery sit awhile probably.

There is a change I guess if the battery was read instantly after the engine was shut down it may still reflect what the actual chargeing voltage was. Be careful here.

My normal sequence was always to pop the hood with the car cold. Read the voltage at the terminals. Then start the car and check for the increased voltage to indicate the alternator was indeed charging. Check it out for youself.

James,
Can you get the cover off the plug into the back of the alt? need to check the output voltage of the alt (thick wires). The voltage at the battery is a bit low under load.
The fact that you had to turn on x3 to get the glow plugs to work suggests a connection problem.
With the load & revving, given the low battery voltage, I would expect the charge light to be glowing dimly.
I once had this type of problem, ended up I ran a new heavy wire from the plug to the battery, it fixed several other elec probs that sometimes occurred.

I believe the cover to the alt is off. I can see the regulator. Can I conduct this test at the dist block on the frame rail? That would be easier.

It did take x3 key turns to get the light to kick on for the glow plugs however I believe they were working prior to that, as the one time I had my dad turn the key on and he reported the light off I heard the relay click and verified voltage at the #1 GP..

Which tests would you like me to rerun measuring from the dist/alternator?

I also found this interesting read about how to test a battery:

Results of tests round #2 measured from alternator output:


Battery - Car Off - 12.28v
Key On - Car Off - 11.50v - No GP Light On
Glow Plugs verified to be running, voltage @ GP #1 ~11.05
Cranking Voltage - 10.4v
Started Idling - 13.14v -
Started Revving - 13.70v peak
Only Blower High @ Idle - 12.55v
Only Blower High @ Rev - 13.42v
Blower High + Lights @ Idle - 12.13v
Blower High + Lights @ Rev - 13.33v
Blower High + Lights + Rear Defrost @ Idle - 11.87v
Blower High + Lights + Rear Defrost @ Rev - 12.68v
Blower High + High Beams + Rear Defrost @ Idle - 11.69v
Blower High + High Beams + Rear Defrost @ Rev - 12.36v

It does not seem to cope well with loads. Not sure if thats an indicator of the battery or the alternator.

I have another SD, its battery is currently on trickle charge, it has not been driven since snow hit. (not a winter car)

Not coping with loads is a sign of the charge circuit not being up to spec. Until you have the output voltage at the back of the alt, you cant tell what is the cause.