Mercedes-Benz Forum banner
1 - 10 of 10 Posts

·
Registered
1985 300TD
Joined
·
68 Posts
Discussion Starter · #1 ·
So, testing glow plug relay on 1985 300td with no responsive glow plug indicator light...

Testing each of the 5 pins in the relay that go to each glow plug.

Turning the key to the glow plug position, I'm only reading about 0.5 volts from each pin, some a fraction more and some a fraction less.

Is this a normal reading?? I would think it'd be more than this if not 12 volts.

Does this mean my relay is fried?

I'm getting 12 strong volts into the relay.
 

·
Registered
1984 300D
Joined
·
5,517 Posts
Make sure you are getting Battery Voltage on both sides of the Strip Fuse. Voltage at the Strip Fuse and the Flat Copper Wire Strip is there all of the time with the Key on or off.
 

·
Registered
1985 300TD
Joined
·
68 Posts
Discussion Starter · #4 ·
Thanks for the replies.

Yes, I am getting 12 strong volts on both sides of the strip fuse.

So I guess I'm looking for a new relay. Found them used on ebay for 50$, but then I searched and found this updated Bosch kit which includes 4 glow plugs for 133$:
1985 Mercedes-Benz 300TD - Preglow Time Relay - BUY 1985 Mercedes-Benz 300TD - Preglow Time Relay

I know it says the kit is updated...but is Bosch the OEM for this vehicle?
Why does it only include 4 plugs if this is for a 5 plug vehicle?
Why is the Hella kit more?

Basically what I'm asking is...is the Bosch kit sufficient for my vehicle?

Thanks!
 

·
Registered
1985 300TD
Joined
·
68 Posts
Discussion Starter · #5 · (Edited)
Opened up the relay and it had about a half-inch of water in there...the bottom half of the circuit board looked like it was pulled out of a shipwreck...I'm amazed it lasted as long as it did.

No idea how the water got there. Suprised there wasn't a rubber gasket around the opening of the relay.

Oh well...new one on the way

Also learned that the relay, functioning or not, needs to be plugged in and powered in order for the car to start.
 

·
Registered
1979 240D
Joined
·
69 Posts
You can search for my post about the Bosch relay kit - I'm happy with it as it also includes the new pin style glowplugs and converts it to parallel operation instead of serial.

However, the cheaper option is a new relay.
 

·
Registered
1977 Mercedes-Benz 300D(non-turbo)
Joined
·
101 Posts
You can search for my post about the Bosch relay kit - I'm happy with it as it also includes the new pin style glowplugs and converts it to parallel operation instead of serial.

However, the cheaper option is a new relay.
Will that kit work on a 77 300D with the filament type plugs?
 

·
Registered
1979 240D
Joined
·
69 Posts
Will that kit work on a 77 300D with the filament type plugs?
I don't see why not. The kit's instructions are a bit vague, which I address in my post.

The simplest thing is to just replace the relay. No cutting or wiring needed. But the Bosch unit is a big improvement and means faster starts, especially in cold weather.
 
1 - 10 of 10 Posts
Top