How to test alternator, cheap wagon style..... - PeachParts Mercedes-Benz Forum

JimmyL · #1 10-21-2008, 08:09 PM

I am not amused by this cheap wagon.

1981 300TD
Wife had to come bring me the battery jumper this morning when I got somewhere in Smash and he wouldn't start. She was thrilled......

Got home and tested voltage on the battery while the car was running and I was only getting battery voltage, no umfffff from the alternator. It had been charging around 14vdc a few weeks ago.
So I replaced the voltage regulator. Old one had little bitty brushes left so I just knew that was my issue. Uh, not so much. Same thing.
So I went to China's original engine and removed the reman Bosch alternator from it and installed it onto Smash. I am still not getting alternator voltage at the battery. {I pulled and looked at the voltage reg. on this one before I installed it and had good long brushes.
So, I now am left wondering several things.
*do I have a 2nd bad alternator?
*do I have a wiring issue?
*could it be something else?

From the connector on back of the alternator, the plug goes over 2 large and 1 smaller pin. With the wagon running is there a way for me, with a voltage meter, to see if the alternator is working correctly by coming off the pins on the back, thereby taking the wiring to the battery and wherever else it goes out of the equation.
I haven't ever fought the ol' no charging issue, and certainly expected it to work after I replaced the alternator {albeit with a used one}
I would appreciate a good plan of attack.
So much for working out of Smash.......

mobetta · #2 10-21-2008, 08:35 PM

the wires from the alt go to the junction box on the wheel well, just for of the battery- Have you checked those connections?

I do know an alt needs to be " excited" by a battery to work proper, so I dont know how much luck you'll have with your proposed testing.

bbboomer · #3 10-21-2008, 10:58 PM

Check each connection at both ends. Alternator to junction block, battery cables, etc. especially the ground. Make sure all connections are clean (shiny) and tight.Try connecting a jumper cable from the alternator housing to the ground post on the battery. If that fixes your problem you have a bad ground, either from the alternator to ground or the engine to ground (most likely). There should be at least one (probably more) ground straps from the engine to the frame/body. any one could be the problem if the ground jumper fixes the problem.

79Mercy · #4 10-21-2008, 11:14 PM

check all the grounds.

and correcxt me if im wrong but if the battery light on the dash doesn't come on when the key is in the glowplug postion the alternator will not charge. I think I remember hearing that somewhere.

AdvisorGuy · #5 10-22-2008, 10:53 AM

I've never gotten a close up inspection of that wiring but is there a fusible link anywhere in the alternator harness ?

Johnt49 · #6 10-22-2008, 02:12 PM

Try removing the plug from the alt & grounding the small spade terminal. If you ground the large ones you'll have a dead short. This will cause the battery light on the dash to light up with key in run position.. If it doesn't, check bulb & connections on back of inst cluster. You can also connect a test light to positive, then probe the small terminal while it's plugged into the alt, but it's hard to access. This will energize the alternator just like the idiot light on the dash does. Also be sure to verify you have 12V on both large terminals at alt.

Jeremy5848 · #7 10-22-2008, 02:35 PM

Confirming what others have said, the "ALT" idiot light in the dash, its socket, the PC traces on the circuit board (part of the instrument cluster) and its associated wiring MUST be in working order for the alternator to work.

JimmyL · #8 10-22-2008, 03:35 PM

Quote:

Originally Posted by Jeremy5848

Confirming what others have said, the "ALT" idiot light in the dash, its socket, the PC traces on the circuit board (part of the instrument cluster) and its associated wiring MUST be in working order for the alternator to work.

Hmmm, all this did start after I replaced the temp/fuel/oil pressure portion of the instrument cluster. I'm out in the garage so I'll be testing shortly.....

JimmyL · #9 10-22-2008, 05:32 PM

Quote:

Originally Posted by 79Mercy

check all the grounds.

and correcxt me if im wrong but if the battery light on the dash doesn't come on when the key is in the glowplug postion the alternator will not charge. I think I remember hearing that somewhere.

Quote:

Originally Posted by Johnt49

Try removing the plug from the alt & grounding the small spade terminal. If you ground the large ones you'll have a dead short. This will cause the battery light on the dash to light up with key in run position.. If it doesn't, check bulb & connections on back of inst cluster. You can also connect a test light to positive, then probe the small terminal while it's plugged into the alt, but it's hard to access. This will energize the alternator just like the idiot light on the dash does. Also be sure to verify you have 12V on both large terminals at alt.

Quote:

Originally Posted by Jeremy5848

Confirming what others have said, the "ALT" idiot light in the dash, its socket, the PC traces on the circuit board (part of the instrument cluster) and its associated wiring MUST be in working order for the alternator to work.

Well you stinkin' geniuses!!!!!!!

The M****** bulb was burnt out.

I wondered why my orange brake light bulb was on, but I just assumed it was because those wires to the front pads were so shot.
I had the 2nd alternator completely loose and was just fixing to pop a 3rd one in there when I saw your post. Took the cluster out and put it in China and it wouldn't charge there either. Amazing!!!
Reinstalled all the bolts for the alternator and tightened it back up, replaced the bulb with a good one that ohm'd out and I got 14.02 vdc across the battery.

I'm happy but I'm irritated as all wizz about that design. I never noticed that the battery light didn't light up during the glow cycle. It is just one of those minute details that we look straight past every day.
Now I just need another radiator.
A bulb......

Mercy, Jeremy and Johnt49, you need Chad to fix you up with Super Hero status. Could there be an Alternator Man opening......

79Mercy · #10 10-22-2008, 07:46 PM

OMG!!!!!!!! I can't believe I solved your problem over the internet.

JimmyL · #11 10-22-2008, 07:56 PM

Quote:

Originally Posted by 79Mercy

OMG!!!!!!!! I can't believe I solved your problem over the internet.

Mercy, I totally missed the second portion of your post.

I did remove clean and re-attach all the grounds, but my brain just didn't process your second paragraph.
Your contribution should be duly noted, and it is!!!!

pizzachef · #12 10-23-2008, 04:25 PM

Wow...what other butterfly effects do these cars have?
Let's see...leaky door lock valve: engine won't shut off.
Burnt out alternator idiot light bulb...alt won't charge...sheesh

#13 10-23-2008, 04:43 PM

Interesting, thats new news to me.

BENZSTER · #14 10-23-2008, 05:40 PM

I learned something new too, this forum rocks!!

pj67coll · #15 10-23-2008, 06:58 PM

Quote:

Originally Posted by JimmyL

I'm happy but I'm irritated as all wizz about that design. I never noticed that the battery light didn't light up during the glow cycle. It is just one of those minute details that we look straight past every day.

Uh, not all of us. Personally I rather liked watching the light do it's thing. Just emphasized I was finally part of the Mercedes diesel owners club

That's when I still was

- Peter.