1979 300SD, Turbo 617 engine- Alternator voltage
 

I have just had 4 alternators installed and replaced on the car. The last one a Bosch 65 Amp is only producing 11.6 volts, 50 amps. Wiring has been checked, new ground wire replaced, and 2 new batteries installed and replaced.
A top mechanic has checked Ohm's, and originally replaced the alternator with a Quest unit, seeing that it was not doing the job, then replaced unit with a heavy duty Quest. Still not enough charge, based on the car specs he then ordered the Bosch 50 amp, still not enough voltage, last and present unit is a 65 Amp Bosch. At high revolutions still only 11.6 DC volts @ 3500 revs. However the meters do show 50 to 55 amps output.

Any Suggestions?????????? Captain Naselli 561 743-4508

1979 300SD, Turbo 617 engine- Alternator voltage
 

79.300 SD, Turbo 617 engine- Alternator voltage
Please help, any suggestions..............
I have just had 4 new alternators installed and replaced on the car. The last one a Bosch 65 Amp is only producing 11.6 volts, @ 50 amps. Wiring has been checked, new ground wire replaced, and 2 new batteries installed and replaced.
A top mechanic has checked Ohm's, and originally replaced the alternator with a Car-Quest unit, seeing that it was not doing the job, then replaced unit with a heavy duty Car-Quest. Still not enough charge, based on the car specs he then ordered the Bosch 50 amp, still not enough voltage, last and present unit is a 65 Amp Bosch. At high revolutions still only 11.6 DC output voltage @ 3500 revs. However the meters do show 50 to 55 amps output.

Any Suggestions?????????? Captain Naselli 561 743-4508

Answer
 

My first assumption:

The wrong size pulley has been installed on these alternators.

.

Glow plug relay is staying on? Disconnect it and check your running voltage. Easily and quickly done.

Good suggestion...on a couple of occasions while cruising, I've engaged the glow plugs for a half second (just to observe the voltage drop) and the voltage drops from 13.6 or so to 12.2 in an instant.

1. This thread would be better received in Diesel Discussion. Tech Help is meant for spark-ignited Benzes.

2. The combination of amps and volts suggests a big drain on the electrical system, like maybe your glow plugs are stuck on.

Barry12345, above, could be 'spot on'. wish I had figured out my relay was stuck... ...before I went through alt in-and-out 3 times - and finally, replaced a failed battery!

Not saying that's your specific problem, but over time this forum has a habit of saving people hundreds...

...but if it doesn't, your next post is free. :)

Its showing 50-55 amps while its on the car running? No wonder you can't get above 11.6 v.

Just wondering how you're reading current.

Puling that much amperage its feeding something. Almost has to be the glow plug circuit.

About the only way we usually have the ability to measure high dc currents is a cheap meter with a shunt. You can make a shunt fairly easy and calibrate it. Just not used that much so I do not have one. Instructions on making one are on the web. One of those things that is cheap to make of you can buy them calibrated for different meters but usually not the cheapies.

How does the alternator connect to the car's harness?

On my '87 wagon, the harness has a plastic connector holding three wires with the female spade connectors. The alternator has thee male spades. The female connectors had weakened and opened up over time, so they didn't make good contact, and so the alternator output was weak. Cleaning and tightening up those connectors fixed my problem.

The first alternator should have been bench tested at FLAPS or by your mechanic to ensure it was really the problem. Failing to perform that simple test resulted in a wild goose chase, at your expense. This is why I prefer to do all my own work.

Voltage draws
 

Well Skippy, looks like you nailed the problem, and you were miles away. It is the relay which was drawing the voltage and killing the battery. So far I am into about 380.00 dollars, and now comes the cost of the Glow Plug Relay............plus the labor.
If I did not have 38 years into the car, it would be history, but it is what I keep telling my wife that is where I want to be buried in after the"BIG ONE".
Will keep you posted on the final results.............Captain Joe

OP posted in Tech Help before I advised him to repost here. He updated that thread confirming it was the GP relay.

I can convert your glow plug relay to manual, activated by a momentary switch. If you do not like manual, buy the relay new, not used because a used one is 30 years old and could fail on you anytime.