Glow plug entertainment.
 

87 190 TD has a new problem. Now the glow plug light doesn't come on when the key is activated (new key and relay , worked fine for the past 4 months). It comes on after the engine starts and then goes out after a few minutes. Didn't drive long enough to see if it cycled on and off . Started easy this am at 40 degrees. Where do I start? I didn't see this problem on the other threads.

By checking the glow plugs, they must DRAW the correct amperage or the light will work as you described.

Thanks. How best to do this? I am somewhat familiar with checking for resistance across the plugs from some of the other threads but have never done this. I do have a good multimeter, but will need to be 'cookbooked' as to how to proceed. Thanks, Harvey
P.S. When the dash light is on while engine is running, are the glow plugs still drawing current?

Checking resistance will give the same results bus slightly less accurate. Disconnect the wiring harness from the pre-glow relay & check resistance between all 6 wires & ground at the engine. The glow plugs should be around 1 ohm.

Super. Will try that first. Easier but less accurate is good for me.
PS on another issue, I found a short cut for replacing the shut off valve on this model. You can get the back bolt loose by using a slender ratchet box end which will give you just one 'click' at a time on the bolt (which takes forever). You do not have to remove the bolt completely but enough to rotate the clamp out of the way. This way you do not have to remove the ALDA or at worst the intake. Have to take the electric plug off first but that was easy after I got brave enough to try. Thought I would pass that along. Best wishes, Harvey

It was the relay and the #1 glow plug as well. The clue was that the light did not blink, but came on after start and stayed on for several minutes. Had my local pro replace the plug and attach the relay (two simple bolts left fender well). Works like charm now. Thanks for help.

Yes, the number one plug acts as the temperature sensor. The relay is basically monitoring #1 current as a thermal indication.

Have a great day,

Does that mean that the relay may have been ok?