88 diesel suburban cranks but doesn't start

I have a 1988 diesel suburban 6.2 L. It cranks but it won't start. The glow plugs don't seem to be cycling. (The glow plugs were recently replaced and were working fine until last week) Is it possible that the glow plug switch needs to be replaced? Any other suggestions as to what I should look at?

ck the glow plug timer

Where is the glow plug timer located, and how do you check it?

it is under the intake manifold. you have to take off the intake and you will see it. it looks like a little sensor

i believe its on the fire wall and there is a procedure
to check it you can find it a manual

the switch you are talking about is in the manual!