The weather is cold now, and my '85 300TD is not starting, even though the battery seems in good shape and I've had a block heater on overnight. At first the glow light performed as usual - comes on, then extinguishes after 10 sec - but now it does not extinguish at all after a few attempts.

The problem arose quickly, as everything ran normally up until my last use of the car a few days ago.

Any suggestions? Thanks in advance for any help.

 

The Glow Plug Relay is located under the Hood the left Fender Well. Remove the plastic cover and take a really good looke at the metal Strip Fuse.
If not burned they can Crack and that crack can be hard to see.

If the Fuse is burned inspect the Wires that go to the Glow Plugs very carefully or you will end up burning up a lot of replacement Fuses.

If the Fuse is OK go to the Diesel Giant website and find his pictorial on how to check (repair) the Glow Plugs. That will give you a good idea if any Glow Plugs are bad.

If you need to repalce any Glow Plugs use a Glow Plug Reamer or find another method to clean the Carbon out of the Glow Plug Holes so that your Glow Plugs will last a long time.

 

Thanks very much, I forgot to mention that I had checked that fuse (80A), and it was solid.

I thought that if there were any problems with the glow plugs, then the light would no illuminate at all...so this not the case? In other words, can the light stay illuminated for one or more glow plugs being bad, just as it does not come on for the same condition?

Thanks.

 

This is what I could find in the Manual

If the glow current circuit is interrupted, e.g. by a defective 80 amps fuse or power relay in preglow time relay, the electronic system receives no current from terminal 30 and the preglow indicator lamp will flash for aprox. 30 seconds after the preglow system is switched on.

If one or several pencil elements glow plugs or lines are interrupted, the preglow indicator lamp will flash for aprox 30 seconds after starting.

It has no description of what is wrong if the Light does not go out.



Reguardless of what the Glow Plug Light is doing the first test should be to turn the key to the preglow position and listen for sound of the Relay Clicking. Some say the interior light will dim when the Relay is turned on.

Also if you read some of the various threads on Glow Plug Problems you find that having the light stay on or flashing indicates there is a prlbem but you still have to go through a process of elimination.
So it is easiest to test the Glow Plugs with an Ohm Meter and that will give an idea if the Glow Plugs are good or bad.
Next is to Plug the Glow Plug wire connectors back into the Relay and turn the key to the preglow position and use a Volt Meter to see if the Glow Plugs themselves are getting Voltage and how much. (This will tell you the Wires to the Glow Plugs are likely to be OK.)
If you are not getting voltage to the Glow Plugs you are back to the Relay. (I forgot to mention that there have been instances where the Screws that hold the Strip Fuse down are loose).
Again remove the Glow Plug Wire Connector at the Glow Plug Relay and probe the Pins to see if you are getting Voltage there. (No voltage at the Glow Plugs but Voltage at the Pins on the Glow Plug Regulator mean an issue with the wires going to the Glow plugs.)
With the key in the preglow position and no voltage present at the pins it could be an issue with the Relay or the ignition switch not sending the Voltage to the relay to turn it on.
There is a Hex connector on the Relay that has 4? wires going to it. One is the wire to the Ignition switch. I cannot remember which one it is.


Also probe the Strip Fuse and there is a Flat Copper Wire. They should both have Voltage all of the time. If you are not getting Voltage there you have an issue with the circuit the connects it to the Battery.