Well, got the car back from a local garage yesterday for a fuel leak. Noticed when I started it to leave that the glow plug light didn't come on. Started right up though, so I assumed the light had burned out.

Try it this morning, car really does not want to start (I don't push it). Get out Haynes manual, which gives no detailed info on the glow plug system, other than testing resistance of the plugs.

I do that, and find that my #2 and #4 plugs have infinite resistance. Come to think of it, starting is not as smooth as it could have been lately. But that doesn't explain why no glow light. Then I notice that the glow light DOES come on when the connector from the relay to the plugs is disconnected. Connect it again, no light.

Does this mean a short somewhere?

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1982 240D automatic
Odometer quit at 307K...about 325K now

Yes, it does. The glow plug relay mechanism contains a mechanism which detects a burned out plug and informs you of the failure by preventing the light from illuminating.

The above-mentioned fault detection system operates by detecting an imbalance in the current to the #1 glow plug versus the current to the remaining plugs. If the connector is disconnected, there is no imbalance and, therefore, no fault is detected.

OK.


So does that mean that no glow plugs were burned out before (because the light came on normally)?
And it means that the system is still working, except with only 2 plugs instead of four? So all I should need are 2 plugs? If so, that's good to hear.

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1982 240D automatic
Odometer quit at 307K...about 325K now

Just addressed this today.

My 83 300D failed to light the glow light on the dash this week.

Megg'ed the plugs and found two plugs had failed.

Got a pair of plugs from the garage stock and installed them, and Wa-la! yellow light is on again!

Do a voltage check for the plugs. Should be 1.5 Ohm or less for a good one.

You can check at the plug or use the connector at the relay.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

The system will still give a normal indication if only ONE GP is shot. Takes two failed GPs to cause the yellow indicator to not come on.

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Ah, that makes more sense! One can fail anytime, but two at the same time seemed kind of odd. Also, I got some "bangs" in cold weather starting previously, which would make sense if one plug was out.

So much to learn.

Waited 'til about 4:30 when the car had been sitting in the sun all day (temps in the low 70s) and fired her right up (with the pedal down).

I really appreciate the help on here, guys!

Drove it to the Benz place because I have something trans related I wanted them to look at anyway and I've not yet looked into how to replace GPs but that's going to change. By the time the next one fails I'll be set up to replace them myself. It feels better to even be able to say "suspect plugs 2 and 4 bad, please replace" than "my car has trouble starting".

__________________
1982 240D automatic
Odometer quit at 307K...about 325K now

Yep, did get that from the Haynes manual. 2 plugs were infinity, 2 were some single-digit number which I just took as "good".

__________________
1982 240D automatic
Odometer quit at 307K...about 325K now

That depends on which plug fails. If #1 fails, the light will not illuminate. According to the service manual, a failure of a single plug other than #1 may or may not trigger the fault detector. Two failures should, however.

Ok, so light out means a failure of:
*Plug 1, or
*any other two (or three) plugs

For completeness' sake, is it the same on the 5-cylinder 617 engine?

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1982 240D automatic
Odometer quit at 307K...about 325K now

The exact same relay is used on both 4 and 5 cylinder engines.