GM-GMC: Glow Plug adjustment, glow plugs, cold engine

Question
I have a 6.2 ltr GMC 2500 4 wheel drive 1993. I have been having trouble lately starting in cold weather. A mechanic from work suggested it sounded like the glow plugs were not working properly. I did both checks like he said (made sure the wire to plug worked and then tested each plug and they appear to be OK). Next, he said to time how long they came on. At the freezing point, the plugs came on for just under 10 seconds. He said that is not enough they should be on longer. They never came on as long as he said they should but the truck always started in the past. Question. How long should the plugs come on at certain temperatures and if it is too short, how can they be adjusted properly?

Answer
There is no adjustment for glow plug operation.

The glow plugs have a controller and a temperature switch that is located in the cylinder head.  If either of the devices fail, your glow plugs will not work.

Your mechanic asked the wrong question.  Your glow plugs should come on (and ten seconds is plenty of time), go off, and then cycle on and off several times when the engine is cold.

I converted my diesels to manual glow operation and ten seconds seems to work fine with a cold engine.  I only turn them on once, for about the time that the seat belt buzzer runs, than off just as I crank the engine.  They both start instantly using that method.

Glow plugs should be checked with a volt-ohmmeter set to low resistance.  They should measure 0.8 Ohms when cold and anything above twice that reading is bad.  The test is run with the glow plug wire disconnected.  It only takes two or three bad glow plugs to make a GM 6.2/6.5 diesel hard to start.