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  #1  
Old 10-20-2011, 05:23 PM
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odd alternator behavior, W123

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?
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2003 Buick Regal
1983 300D (228k, frau Auto)
1996 Suburban K2500 (192k, 6.5 turbo diesel/4WD towmaster 10,000)
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1987 300SDL junker 170k
1982 300SD junker, 265k
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  #2  
Old 10-20-2011, 07:10 PM
toomany MBZ's Avatar
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Inspect the regulator.
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84 CD
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  #3  
Old 10-20-2011, 07:42 PM
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Quote:
Originally Posted by BoiseBenz View Post
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?
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?
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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.
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  #4  
Old 10-20-2011, 10:56 PM
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Quote:
Originally Posted by layback40 View Post
Where did you do your voltage measurements? At the battery post ?
At the freshly cleanedest posts.

Quote:
Originally Posted by layback40 View Post
What was the voltage doing in the 1 minute while revving after idling? Slowly building?
Slowly build (approx 0.05 volt/second) to listed voltage, then drop back down almost instantly upon release of revs.

Quote:
Originally Posted by layback40 View Post
Do you have afterglow?
Only per updated last fall (end of '10, for us northerners ;-) Bosch relay, nothing external.

Quote:
Originally Posted by layback40 View Post
How were you able to have the key off & idle?
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.
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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
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  #5  
Old 10-21-2011, 03:33 AM
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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
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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
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  #6  
Old 10-21-2011, 09:54 AM
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Quote:
Originally Posted by layback40 View Post
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
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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  #7  
Old 10-21-2011, 10:46 AM
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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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