1991 300d 2.5 Turbo Diesel Glow plug issue
 

I just purchased a 1991 300d 2.5 turbo diesel. Garage kept one owner, 75k miles, No rust mint interior. Yesterday I go to start it I noticed glow plug light didn't come on, I wait for 30 sec to a minute then started it. Glow plug light is now on after 30-45 seconds of idling the glow plug light is now off. Doesn't idle rough, and starts on first crank. Should I check the glow plugs and the relay?

roughly quoting the service manual here:

A fault in the preglow system is indicated by the preglow light failing to light up and when the engine starts it lights up for about a minute.

causes:

- short at the plugs/cables

- open circuit to one or more plugs

- bad 80A fuse

- bad relay

I would start by visually inspecting the fuse and pull of the harness going to the plugs at the relay and start measuring with a multimeter there

The fuse is an auto reset breaker. Just test each glow plug as mentioned.

thanks, gonna look at it when the air temp warms a bit. Had bit of a typo in the orginal post. The glow plug light shuts back off after 30 sec -2 mins of idling.

I don't know the 2.5L, but in my 1985 5 cyl (3.0L OM617), if 3 or more glow-plugs are bad, the lamp won't light (as a warning). My son said my 1984 300D wasn't starting easy on a San Jose winter morning (~40 F), but the lamp worked. I found 2 glow-plugs burned out, and it started easy once replaced even when 30 F.

It takes <1 min to check the glow-plugs in my cars, so I do it every few months when the hood is open. I always have a cheap HF multimeter nearby. Just pull the GP plug and test each of the 5 terminals to gnd. I put the black lead on the fuel filter housing. If bad, you read open. If OK you read ~2.5 ohm. Verify again with the red lead right on the glow-plug end, to assure it isn't from a broken wire.

The way this system works is that the current draw of one plug is compared to the current drawn by the other four. When they are in balance, the light works normally. Otherwise, the light stays off during preglow and comes back on after the engine is running awhile.

You can measure the resistance of the plugs to verify, the bad one will be open. but your best bet is to replace all five plugs, since you will need to remove the intake manifold to replace them.

Glow plug one and three are bad. Thanks for the input. Gonna have them put in Wednesday or Thursday.