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I've recently fulfilled a decades-long desire of owning a Unimog and am enjoying my new-to-me 1975 MB4/94. It will be a farm tractor/utility vehicle and I've already put it to use both in the field and around the farm. One problem I have is with the wiring, particularly in the dash. Currently there are no wires going to the charge indicator light and from what I've learned reading this forum that is also probably why the tach isn't working. Although I have the Case service manual with the wiring schematic it isn't clear about the charge light. Can anyone point me in the right direction about how this all hooks up? Thanks
 

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'78 Mog 416.141 DoKa
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Cross check me on this, but the light is hooked in series with the ignition sense (field) line on the alternator and key switched 12V. In a 406/416 instrument cluster, the light terminal (Ladekontrolle) is on terminal 5 of the 8-pin connector. I think switched 12V is on terminal 12 (on the smaller 4-pin connector). Switched 12V is required to make everything else in the cluster work, so if other stuff DOES work, the switched 12V line is present. The wire color for the <light-to-ig sense> line is supposed to be BLUE.

From what I understand, some small current is required on the FIELD (IG SENSE) terminal of the alternator to "tickle" the field and allow the generator to start producing current. When the generator is 'cold', no voltage is present on the field and current flows through the light (current limited by the bulb). When the generator starts, there is voltage on the field line, which, being the same as the voltage on the other side of the bulb, stops current from flowing and the light dims out. If the voltage on the generator field starts to drop (in a overload situation, where the generator can't supply required current), there exists a voltage difference between battery and generator field, which causes some current flow and a partial lighting of the lamp. The lower the generator voltage goes, the more current flows and the brighter the lamp.

The tach doesn't work because the generator can't produce output on the W terminal without the field winding being excited through he lamp. HOWEVER. most alternators will self-excite if spun fast enough. My 416 OEM alternator will self-excite when the engine is revved to about 1500 RPM. Below this engine speed, the tach will not work and the alternator will not supply charge current. Alternator specs vary, but that's how mine works.
 

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Thanks for the hints, Mogasaurus. This gives me more information to add to the wiring schematic and should definitely help with getting things working. It does seem to be charging as I've been using it for over a month now, including some use with the lights on, and the battery is still at full voltage. I'll be able to tell better what's going on when I get the air intake pipe removed as right now I can't see what all is going on behind the dash. Just gotta find some time to work on it!
 

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'78 Mog 416.141 DoKa
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If there are no wires at all on the multi-function instrument for the charge light (on the 406/416 cluster, pin 5 on the 8 pin connector), I suspect someone has done some re-wiring OR the Case version is different.

I just went through this on my 416 - the lamp holder for the charge light was smashed and the light was broken. The tach wouldn't work and I had to rev the engine to get the alternator charging. After fishing all the broken bits out and doing all the contortions required to get the new bulb holder in (hint for future reading - move the coolant tank out of the way and reach in from the interior), the bulb has been replaced, lights properly upon turning the key and the tach reads immediately after engine start.

Good luck on your repairs - careful, it's tight in there with plenty of sharp hooky things to perforate your arm and hand :)
 

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Yeah, I've already met some of those pokey things of which you speak when I was doing the exploratory surgery. I had the same thought about removing the coolant bottle since I did most of the work from the interior. Kind of needed a smaller hand and a second wrist to get in there with the bottle in place. Thanks for all the suggestions - this will certainly be a big help. Right now the farm work is stacked up so unfortunately it might be a while before I can get back to wrenching on the Mog.
 
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