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'80 SD Battery Charging Problem

Started by Abe_Markillie, 16 August 2021, 08:24 PM

Abe_Markillie

I have a 1980 Mercedes W116 with the OM617 and the alternator doesn't seem to be doing anything. I bought it about 150 miles from where I live, drove it back and it ran good for the next couple days. one night I went to get it washed, then went to a friends house and when I came out I noticed the headlights and taillights were SUPER dim. I managed to baby it home and ordered a Bosch 55 amp alternator, then a couple days later I put it in along with new belts, and I did a valve adjustment while I was at it. it didn't solve anything as I drove 5 miles to a hardware store and came out and the battery didn't have enough juice to start it back up. i figured maybe its because i accidentally left the lights on or something. i got it jumped and drove it around and shut it off and it did it again. So i took the battery out and charged it up and took the new alternator, old alternator and battery to an oreillys to get tested. They ALL tested OK. so I did come research and heard that maybe it could be a ground. I cleaned the one going from the negative on the battery and inspected the one going from the transmission to the body. there was no bolt holding the ground on the transmission. I assumed this HAD to be the problem, so I cleaned it, put dielectric grease on it and half-assed a bolt in there temporarily. I checked the battery and it was at 12.93 volts. I started it up and at idle the battery was at 12.16 volts. I took it for a short drive and got back it was almost at 9 volts. Does anyone know what else there could be? the battery light on the dash doesn't come on while its running, but the bulb does work. Anybody have any ideas?


daantjie

Sounds like it could be the voltage regulator.   Super common failure and easy to change out.  Look at the back of the alternator you will see 2x screws holding the part to the alternator.   Remove screws and carefully tilt it off and out.  If the brushes are worn down then this is your failure.
Daniel
1977 450 SEL 6.9 - Astralsilber

Abe_Markillie

The thing is that there's a brand new Bosch alternator in the car, so it should have a new regulator.

Alec300SD

The alternator case might not have a good ground connection to the alternator bracket.

When I upgraded to an AL129X alternator, I ran an auxiliary ground cable directly from a preexisting tapped hole in the rear of the alternator
to the negative battery cable's attachment point on the chassis.

This can be of benefit as marginal grounds can cause all kinds of electrical gremlins.
Simple test would be to temporarily connect a lead betwen the alternator case and the negative battery cable  and see if you get an improvement in charging voltage

Also, how fresh and tight are the alternator belts?
If glazed, they may have been burnished from slipping.
Radial roughening of the pulley grooves with a bit of sandpaper can help stop belt slippage.
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Abe_Markillie

Ok I'll check the ground on the alternator, all the belts are brand new I replaced at the same time as the new alternator.

Also, how big should the ground be to the chassis from alternator?

Alec300SD

I used a 32" 2AWG ready made battery cable from the local auto parts store.
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Abe_Markillie

Update: I found the issue. The glow plug relay has been sticking on, even though the light turns off on the dash after about 5 seconds. So now that the plugs have been lit while the engines running, they're probably toast. Also, is it just the relay or is there more that could cause it to stick on?

Alec300SD

A bad ignition swtich  might allow the glow plug relay to be powered on, but most likely  you have a defective glow plug relay.

You can monitor the battery voltage/charging status (to see if the replacement glow plug relay shuts off) with a simple voltmeter gauge that plugs into the cigar lighter in the ashtray.
An example is in the second photo in the link below.

http://www.peachparts.com/shopforum/3490745-post1.html
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Abe_Markillie

I was checking the voltage on the battery and it was at 10 volts before at idle, then I disconnected a wire at the glow plug relay, and the voltage shot up to 13.6. When the engine is warmed up and running, if I touch the wire back to it's terminal on the relay, it arcs and the voltage goes down. So definitely a relay or ignition switch.