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Old 11-21-2018, 06:21 PM
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cleeves cleeves is offline
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W115 300D Glow Plug Relay Issue

Hi all,

My glow plug relay ostensibly died last week. The glow light would come on when I turned the crank, but not when turning to position 2. Glow plugs are new. 0V on glow plug 5 when in position 2 and everything under the engine bay seems to check out.

On my old relay board, I found that when turning the key to position 2, one of the relays inside (small one, bottom left in pic) engages. However, the larger relay inside the board sparks initially (single spark on key turn) but does not engage. I confirmed that the ignition switch was sending a signal to the relay board by checking that pin 4 shows 12V when in position 2, and 0 otherwise. Jumping pins 5&6 turns on glowplugs (and indicator) as does manually pushing down the big relay with a screwdriver. When cranking the engine (pos 3) the relay engages.



So, thinking the relay was bad, I bought a NOS relay from eBay for $230. Same problem! Arg. It's hard for me to believe that the NOS relay is also bad. Any suggestions appreciated.

I suppose my next idea is to put an Ammeter on the ignition -> relay input pin and see if the current changes between pos 2 and cranking. Perhaps the ignition switch is bad after all?

Lastly, I am not interested in the manual setup. I would like to get the car back to it's original operating condition.

Thanks!
Henry C.
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1976 Mercedes-Benz 300D, Colorado Beige / Mahogany
www.hmclever.com
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