1983 300D, or a 123.133 with a 617.952

Alternator only works right when key is OFF (red light on brightly) and engine is still running.

Off voltage: 12.8
Running voltage, key on with no load, idle: 11.8
Running voltage, key on with no load, rev to 3000 for one minute: 13.2
Running voltage, key off, headlights on, idle: 13.85

Any clues which wire or splice is funky?

__________________
James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

Inspect the regulator.

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83 SD

84 CD

Where did you do your voltage measurements? At the battery post ?
Try redoing the tests after the car has been driven for about 10 minutes, before you shut it down.
What was the voltage doing in the 1 minute while revving after idling? Slowly building?
Do you have afterglow?
How were you able to have the key off & idle?

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

At the freshly cleanedest posts.

Slowly build (approx 0.05 volt/second) to listed voltage, then drop back down almost instantly upon release of revs.

Only per updated last fall (end of '10, for us northerners ;-) Bosch relay, nothing external.

Disconnect vacuum pod at IP and turn off key? Haven't you armageddon-qualified your Benzes?

Actually discovered by accident; pump/IP/switch have not been getting along lately. Some stops are instant, some can take up to 10 seconds. Today shut off switch, car didn't stall, volts from crappy 11.8 to normal 13.85, then car stalled after about five seconds, and volts drop to ~12.7.

That is a known problem (leaking shutoff in IP), but not something anywhere near the top of my list these days.

__________________
James Marriott
2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
www.engineeringworks.biz
1987 300SDL junker 170k
1982 300SD junker, 265k

The key off test is meaningless as you have taken away the reference voltage from the reg.
The 11.8 is just low battery after starting. It came up with charge.
I dont see anything there to concern me.
What problems are you experiencing with the elec system? if any

__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

The key off test does have meaning since it removes elec loads that may have been on (except the head lights). I'd start by cleaning the connector at the alternator.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

the 12.8 at rest reading tells me the battery is either freshly charged, or brand new. if you did not just take the battery off a charger or a store shelf, the alternator must be charging it.
likely the glow plug relay is still on when you are testing the alternator.
start the car, go for a drive, come back, THEN test the alternator voltages. within the first 3 minutes of starting the car, many items in the car could be pulling more amps than the alternator can handle.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!