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Old 11-15-2008, 05:16 PM
Registered User
Join Date: Oct 2005
Location: Dallas, TX (Addison)
Posts: 270
charging issue, alternator, battery

'85 300SD, 269K

the following occurs randomly, as i have not found a particular set of circumstances causing it..

i go to start car, turn key, glow plugs turn on and the lights go very dim and car will not start, as if dead battery.

hook car up to charger for just a minute, and car starts instantly, BUT is takes a few minutes for all the lights and electric to work... then all of a sudden everything brightens up as if some switch turned everything on..

so i hooked multitester up to cigar lighter and noticed that when the car is first started, only get about 11 volts, then after a few minutes all of a sudden voltage jumps up to 12.2 and climbs to 13.5+.. so i thougt voltage regulator, replace that.. no difference.

so I thought alternator... so did the guys at the auto store.. bought alternator (rebuilt bosch unit by duralast).. installed alternator.. same thing still happens.

i have replaced battery at least 3 times in the last 2 years.. cannot find any current draw when car is off by hooking up multimeter between negative battery terminal and negative cable.

this has me perplexed.. any help out there??

R Martin
Dallas, TX (Addison)

86 560SEL, 128K
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Old 11-16-2008, 11:23 PM
Registered User
Join Date: Oct 2005
Location: Dallas, TX (Addison)
Posts: 270
further inquiry..

i have read other threads and some people have found that the battery bulb being out will cause alternator not to charge BUT that is not my case because the bulb lights up when first turning the key and during glow plug cycle.. but it always takes a couple minutes before the alternator starts charging. my multimeter across the battery terminals only shows about 11.5 volts immediately upon starting. then after a couple minutes it goes up to 12.2 and starts climbing until it reaches 14+ volts.

really weird huh?
R Martin
Dallas, TX (Addison)

86 560SEL, 128K
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Old 11-17-2008, 12:58 AM
compress ignite's Avatar
Drone aspiring to Serfdom
Join Date: Feb 2004
Location: 32(degrees) North by 81(degrees) West
Posts: 5,554
Mis-Behaving Electrons

Have you,in your diagnostic frenzies,Cleaned and Polished ALL of your Electrical
Connections? (Grounds[including the Ground Strap from the Engine to the Frame],Etc,Etc.)
'SubSet of the above:Have you Ohmed out your cables?
How is your OVP Relay? (Have you used the FSM test sequence?)

Is there a test sequence for your Glow Plug "PreGlow Relay"?
I ask that because something is almost instantaneously draining your Battery's
power...and the Culprit may be the GP relay.
The time delay in the system showing 12+ volts may be related to the GP Relay's delay in releasing the system.

Or the whole mess could be caused by a bad Regulator,helped along by a bad
connection between the Ignition Switch,Alternator Light(Dash) and the Regulator.

Or let's look at the other possibility ...BAD STARTER.

Still doesn't explain the SLOW increase of Voltage after start.

So I'm gonna go with the starting problems caused by your Starter's excess
need for Electrons.(Has it been on there for 300K?)
And the LAZY increase in Voltage after Start being the GP Relay.

'84 300SD sold

Last edited by compress ignite; 11-17-2008 at 01:07 AM.
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