wait light won't come on & won't fire up

[craigrep]

Hello, I'm sure this must have been answered hundreds of times but I could not find it. My 1988 C2500 6.2l has stopped turning on the glow plugs (the wait light does not come on) and the truck won't fire up. The other day it took about 20 min. of turning the key on and off to get the light to come on. The it fired right up. Today on hour with on luck.

What controls the glow plugs? Is it just easier to put in a manual switch? If so how is that done?

Thanks for your help,
Craig

[Subzilla]

Most likely it's the controller which sits at the back/top of the engine right at the firewall. Now I'm not sure what in the ignition circuit shoots over a signal to the controller - there could be some problem there. But you can check for power at the controller to eliminate that question.

Yes, you can put in a momentary push button switch but make sure you've got good glow plugs (not 9g's). Do a search for manual glow switch. There are others here who have explained the installation quite well.

[DmaxMaverick]

No rocket science on the 88. First, verify you are sending 12V to the GP controller with the key on (when the WTS lamps doesn't come on), and all electrical connections are healthy. The GP controller is located at the rear of the engine, just inboard of the driver side valve cover. Probe the small wire (with a meter or test light). If it's getting juice, and the WTS lamp and GP's aren't heating, the controller needs replacing. Since it works "sometimes", rule out failed glow plugs (or at least enough of them work). If it isn't sending 12V to the controller, it could be the ign switch, or a wire/connector at any point in the system (sounds fun, huh?), but less likely with an intermittent problem. Very likely either the controller, or the ign switch, which are easy to isolate. Usually, if it's the ign switch, you will have other electrical gremlins in the works. If the GP system is the ONLY problem, it's likely the controller.

Controller replacement is very simple. Disconnect wires (including the battery grounds, first), remove controller, install in reverse order. And, another bonus is, they are not too expensive. Napa carries them (or used to), and it's $76 at www.gmpartsdirect.com. GM part #12496078.

[craigrep]

Thanks, it was the controller. NAPA had one for $108, a little more but got my truck fixed fast.

Should I also start thinking about replacing the glow plug also? The truck only has 75k miles on it but is 21 years old. If so what plugs should I go with?

Thanks again,
Craig

[DmaxMaverick]

If your plugs are original, now is as good a time as any to replace them. They are likely AC-9G plugs, which can swell (and get stuck in the head or break off), when they fail.

AC-11G (later OEM), or quick heats will work best with your current, unmolested controller. AC-60G plugs are better (basically the last set you'll have to buy), but heat slower, which presents starting issues during really cold starts. The controller can be modified for a longer glow cycle. Or, you can install a manual override for longer glow, when it's needed. Stay away from plugs you don't see recommended here. Most are no better, and just as much a liability, as the 9G plugs.

[craigrep]

I can only find AC-60g at the local NAPA, they said they don't make
AC-11g anymore. I do live in a cold climate (Pocono Mountains, PA).
How bad are the 60g's with stock controller?

[DmaxMaverick]

Give them a try. You may be out of the woods, this winter, but it'll come back to haunt you. Take a close look at the old controller. It's much like a starter relay. One large pole will be always hot, and the other large pole is only hot when the relay is closed (this pole goes to the plugs). The smaller pole is the relay power. If you cross the small pole with the larger, always hot pole, it will cause the plugs to heat, regardless of key position or temp. If you are familiar with it, and have a situation when the truck won't start, a 15-20 second heat cycle should get you started until you can make a more permanent fix. This is also where you can install a momentary switch, to throw 12V at it from inside the cab (the permanent fix). Or, you can install an independent relay/switch and connect it directly to the GP lug on the relay, which is a combination of both, auto and manual systems. Don't overcomplicate it. It isn't rocket science.

[craigrep]

I'm going to go with the AC-60g's and install a momentary switch as well. If I understand it correctly, I can run a wire from the always hot lead into the cab to a switch then back to the second pole that feeds the plugs. And that will heat the plugs. If correct what gauge wire to use? Do I need a fuse anywhere? Or do I have it all wrong?

[DmaxMaverick]

Almost right. Run a wire from the small pole (solenoid exciter), to a momentary switch. The 12V power can come from any 12V source (low current demand). You could run a second wire, all the way from the HOT pole at the controller (NOT the GP side), but that's 2 wires through the firewall, when only one is really necessary. You can pull 12V from the cig. lighter (or any other 10+ amp circuit), etc. All you are doing is exciting the relay in the controller, not supplying full current to the glow plugs (let the relay and heavy wires already in place do that).

Or (I mean OR, not both)......
Use a separate relay (like a Ford starter relay), and install a manual GP system independent of the existing controller. The only advantage to this would be if the relay in the controller were to fail, your manual system would still work. Essentially, you would end up with a manually operated system, parallel to the OEM controller.

Decide which way you want to go. Then, either search the forums for the best install procedure, or I can describe it in detail, in this thread (I've done it a dozen times here, one more won't hurt).

[craigrep]

OK I got it. Thanks for all your help.