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Old 01-28-2007, 10:00 PM
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Engine block reads 35 ohms to the NEG battery terminal

I was looking around for a good ground to use for a WVO temp sending unit on my 82 300SD and everything I looked at read about 35 ohms to the NEG battery terminal. The meter and leads will zero out at 0.6 ohms..... the various points on the block and engine read about 0.7 ohms to various points on the chassis.

I don't seem to have any electrical issues, essentially everything works...... but I can't believe this is a normal reading.

Where does the NEG battery cable physically attach to the engine/chassis?


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Old 01-28-2007, 10:41 PM
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It should attach right near the battery. On the W123 the cable is about 7 inches long and attaches with a simple bolt. I would imagine the W126 is roughly the same.
Adam Lumsden
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Old 01-29-2007, 06:56 AM
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IIRC on the W123 and others the neg terminal goes to the chassis, then there is a separate ground strap back from the chassis to the engine. On my 300CD that is on the left side, and is clearly visible from underneath the car.

I'd un-bolt that one and clean it up with a wire brush to get it good and shiny and then re-attach it if I were suspecting a less than perfect ground on the block...that's where the engine gets its ground from.
Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz
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Old 01-29-2007, 07:12 AM
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Join Date: May 2004
Location: Flowood Mississippi
Posts: 438
Bad Grounds

I added extra ground cables to the Block,head,alternator,transmission and frame....Even stuff bolted together can lose electrical conductivity over time...Extra ground cables solve a lot of strange problems....You should have almost zero ohms on the above mentioned locations back to the (-) battery post.....
1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)
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Old 01-31-2007, 07:54 AM
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This is NOT a good thing. I would suggest that you remove all ground cable connections and thoroughly clean and retighten them. The problem connection, however, is most likely the negative battery terminal itself. This can be quickly confirmed with an ohmmeter.
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Old 01-31-2007, 08:29 AM
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The first thing to check is to put one ohmmeter probe on the battery negative terminal itself, and the other probe on the cable clamp that is attached to the terminal. So, you have one probe on the lead battery terminal and one on the clamp right next to it. This confirms the clamp is connected well to the battery.

Next you work up the ground in the chassis direction. Put one probe on the battery negative terminal and one on the terminal where the negative battery cable clamps to the chassis. Not the bolt, but the terminal. This confirms the cable is good.

Then move the probe to the bolt holding the cable terminal you just checked to the chassis. This tells you the bolt is connected to the battery cable.

Now move on to the chassis near the negative cable terminal bolt. Etc.

By now you have probably isolated your problem.

Write back if not.

1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles
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Old 01-31-2007, 08:53 AM
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Originally Posted by Ken300D View Post

By now you have probably isolated your problem.

Write back if not.

eh, go ahead and write back even if you do! we like solutions found...

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