To do the test, take one wire from the test light and... WAIT...
[TANGENT:] And that reminds me to make one caution... use your HF multimeter first in the 30V or higher setting - if your resistor pack is in place for 24V power supply to the glow relay, hooking up a 12V bulb to the unswitched side (or the switched side with no functioning glow plugs) will result is a fast "flash and pop" of the bulb. I did a lengthy post on how/why the cascade failure kills glow plugs in CUCVs here - if you're not timid, try and read through it. If the math doesn't make a whole lot of sense, just look at the calculated values at the end of the post of what happens with dead glow plugs by count. Also the AC60G is often recommended here on SS as a better replacement for the Wellmans, but in my personal opinion if your resistor pack is working as designed, the self-regulating type plugs are not correct for long term reliability (the supply voltage is not "guaranteed" with self regulating plugs). [/TANGENT]
Measure across chassis ground and both FAT terminals of the glow plug relay on the firewall.
With the ignition OFF:
- One terminal should roughly equal the voltage from ground to the upper 24V battery + (with no load, the glow plug ballast resistor pack doesn't drop or "consume" any voltage - it's not doing any work).
- One terminal should roughly be zero-volts, or "open".
With the ignition ON (and the wait light on for the first few seconds):
- You should hear a firm "click" from the glow plug relay as it turns on
- The terminal that measured at or above 24V (what the resting battery voltage was), should drop to approximately 12V (no more than 14V!!)
- The terminal that measured at zero-volts or open should be the same as the other fat terminal (it should not have a measurable voltage difference between the two fat terminals when ON)
- If the voltages of either terminal when the glow plug relay is above 14V without the alternators spinning (remember we haven't yet started the truck), you may have one or more dead glow plugs, or your resistor pack my be bad.
If your measured voltage is anywhere above 15V DO NOT use the test light (again: flash, pop). If the glow relay on the firewall has one terminal that never gets above 1V (stays zero, open, etc..), then we can look at either your relay or the glow plug controller card as the problem we next have to trouble shoot.
If your glow plug relay clicks and the voltage across the two fat terminals are the same, then we can assume both the controller and the relay are functioning correctly - it's either plugs or the ballast resistor pack.
With the key OFF:
- Measure from the 24V battery + to the glow plug relay with the multimeter set in the resistance or OHMS mode. We are looking for a value of 0.081Ohms or more (0.09Ohms - 10% error approximate). If the resistor pack is 0.09Ohms or more then it's probably fine or at least shouldn't kill plugs like a serial killer. Make sure the resistor pack is clean of debris - it is as hot as a household room heater when your truck is glowing the plugs.
If your resistor pack is measuring in the good range, and you still see a voltage higher that half the 24V+ battery referenced to ground, you likely have dead plugs.
With the key OFF:
- Pull the spade lugs from all of the plugs.
- With one test lead grounded, and the multimeter on resistance or OHMS mode, measure each of the glow plug connectors sticking out of the engine block, none should read a resistance higher than 1Ohm. The ones that read higher or "open" or infinity-resistance are either failing or bad. These need to be replaced.
There is also the risk of failed glow plug wires, so make sure the bare glow plug wires can't touch anything metal:
With the key ON and the wait light on, and the multimeter in the 30V+ range:
- Measure each spade lug in the glow plug harness - they should be roughly the same voltage as the fat terminal on the glow plug relay.
- If one or more is measuring zero-voltage or open, then you've found a bad wire. Replace it.
In general you should check your glow plugs as often as you change your oil or at least every 6 months. With the 24V starting system with the ballast resistor the whole set of glow plugs is sensitive to failed glow plugs. This isn't a bad design, it's just a design that needs to be paid attention to as part of PM and not just left until you're kicking your truck on the side of the road one winter morning