A flakey glow plug controller will cause one of two problems:

1. The glow plugs don't work at all. This is the most common failure. Usually difficulty starts when ambiant temperature is below about 70F (21 C) though this varies from engine to engine.
2. The glow pulgs are energized all the time. This generally destroys the glow plugs. This failure mode is uncommon.

Glow plug system failures:

1. Most common: the bulb is either bad or not making good contact. (I've seen this more than I'd like to have!)
2. Glow plug controller cycles but doesn't make the glow plugs go on. (Casued by burned glow plug contactor contacts.)
3. Older Hummers sometimes (due to defective glow plug controllers) have bad glow plugs. AC-9s fail easily. AC-11s are much more resistant to failure, but can fail. AC-62s don't fail.
4. There are a bunch of fusible links in the glow plug circuit. If a glow plug has failed, you must check the fusible link to ensure it is OK.
5. The glow plug controller serves as the main power connector for the Hummer's electrical system! It is connected directly to the battery, thorough a fusible link. All other power then taps off from there. Strange problems can occure if the terminal nut is loose.

If ambiant temperature is over about 22 C, or 72 F, your truck should start regardless of whether the glow plugs are working or not. At least all the trucks I know of will start.

Peter H.