Correct Voltage at glow plug connectors

I have been having some trouble with starting the truck after it has been sitting all day in the cold (below 10°). It starts very fast when plugged in, but can be very hard to start if not. I have new batteries, a new glow plug harness (oem) to the plugs, and each plug is in working condition. I seem to have a glow plug issue given the fact that the truck starts fine plugged in, so I did some diagnosis on the GP system. I checked all the wires from each glow plug back to the controller and all have continuity, which I expected because I put the harness in this summer. At the power terminal at the GP controller relay, I get 13v, which is the same voltage as the batteries. I should mention that my truck has a single large gauge wire running from the GP controller relay back to the fender solenoid with two fusible link wires connecting the cable to solenoid. This sure looks like a factory setup to me, so maybe Ford finally did away with the two overworked yellow wires through the wire harness connector in the last year of IDI production When the key is turned on, I get the full 13 volts at the other large terminal on the GP controller relay (where the metal "z bar" resistor is attached). I also checked the ground from the controller to the intake manifold bolt and that was fine, although I cleaned the loop connnector up a bit just to be sure. Resistance to the GP controller ground loop connector back to neg post on battery was 0.1 ohms. Finally, I pulled the connector of the No. 2 GP (easy to get to) and checked voltage at that connector while my Wife turned the key. I got a little over 9 volts, and I guess I expected 12v What should be the voltage at each GP connector if the system is working properly? I would guess that "z bar" resistor is stepping down the voltage? Anyway, I had a spare GP controller so I installed that just to see if that made a difference. With that controller, I got 10 volts at the No. 2 GP connector. I did notice that the "z-bar" resistor on the first controller was a bit discolored - very ligh bluish tint. Do those "z-bar" resistors go bad? Can't imagine it would as it is just flat piece of metal after all. The WTS light has been staying on about 10-12 seconds with either controller, but the spare controller does seem to be heating the plugs a little better - haven't tested it yet in temps below 10° yet. Sorry for the long explanation - just trying to give you all the details I can so you can help me diagnose this. I am not sure what else I can check now. thanks! Todd

Around 8v at the glowplugs is what ran through my mind as I read the title to your post. That's about what my dash gauge reads when I glow my truck. So 9-10v sounds about right to me. I have the old 2 yellow wire system, so your improved 1 large wire may be getting you some more power out to the glowplugs.

What are you expecting from your truck? 10 degrees is cold. Are you holding the throttle halfway or more while starting? That's what the starting procedure on the back of the sun visor calls for. I have a manual button on mine, and always count 1001, 1002, 1003, 1004, 1005 before I crank it over. At 10 degrees I would have to do this routine about 4 times before my truck would start. That's cranking it for about 3 seconds each time.

IMO, when doing a pushbutton setup, you should eliminate the factory controller/relay.
Get a better constant duty relay (such as the stancor).

That z-bar resistor does drop the voltage. If it wasn't there, the controller wouldn't run them long enough.

Pretty sure Someone changed the power wire. IIRC, even my psd still has the dual wire setup, just bigger gauge.

My Haynes book says 11v minimum.

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Thanks for the responses. I am not expecting it to be an immediate start at that temp., but I am expecting it to start! The other night (at 0°) she just refused to start. After multiple glow cycles, it still was not firing when turning over. I finally took my work truck home. I eventually got it to start next day after plugging it in for 4 hours and also charging the batteries. I know batteries, cables, starter, glow plug system all work together to get the truck warm enough and spin fast enough to build heat and compression for starting, and cold temps make everything have to work that much harder. But, it has never been this hard to start when this cold in the past, and now I have new batteries, new GP wiring and the starter was replaced last spring! I am starting to think IP/injectors are just getting worn enough that they are affecting the really cold start (82k on the clock). I do have one cyclinder that has an intermittant misfire on a really cold start and does so for a couple of minutes after start up. After that, it smooths right out and runs fine.

tbrumm said:

I do have one cyclinder that has an intermittant misfire on a really cold start and does so for a couple of minutes after start up. After that, it smooths right out and runs fine.

Click to expand...

That's a sign that for whatever reason, that glowplug is not heating in that cylinder. Mine was doing that on several cylinders with my manual switch(but it would start) and after I had time to look at it(plowing snow) I found 3 glowplugs burnt out. So it was starting on 5 cylinders.

Here is the proper tests to do with the 7.3L IDI glow plug system. First, use a test light on all your glow plugs to verify they are good. Then, reconnect your plugs, and when you connect the #2 cylinder glow plug, trap a single strand of fine wire under the cap, so that you can use it as a test point. Put your postive of your meter test lead on the single strand of wire, and your other probe to ground. Set your meter to 12 volts. On a cold engine, you should see 6 volts at the #2 cylinder glow plug. The voltage will slowly rise as the plugs heat until you get to around 11 volts at which time the controller will cut the voltage and your wait to start light goes out. IF your voltage is higher than 6 volts starting out, then some of your glow plugs are failing, or you have bad connections in the wiring. If your voltage is less than 6 volts, then you have voltage supply issues. Check the following. Put your positive voltmeter probe on the large lug on the GPC that is fed from the battery. Put your negative probe on the Z strip lug. You should see battery voltage at this point. Have an assistant turn on the key, and verify that your voltage drops to near zero. If it has over half a volt, then your contactor is worn. If this test passes, put your positive probe on the positive post of your battery, and the negative on your large GPC lug coming from the battery and measure for voltage loss there.

Thanks for the detailed test procedure, Mel. I will try that. I did check all my plugs with the test light, and all checked out fine that way. I will test them for resistance with the meter too and see if any are high.

Mel put that in the tech section so we can find it another day. Thank you for the weth of knowledge.

Tbrumm,

That dual wire you have running from the solenoid to the GPC, does it happen to be blue? I have the same set up you described, I read about the small dinky yellow wires, and decided I was gonna upgrade mine so I was getting max power, and I went out to my truck to find 2 thick blue wires with fusible links that combine into one and run to the GPC

I plan to do an article on a complete diagnostic at some point complete with pics and what not, or maybe a video, so I'll get that done and then put it in the tech articles.

Don't bother ohming out your plugs. If the test light lights, then you won't see anything with a meter.

HS108 said:

Tbrumm,

That dual wire you have running from the solenoid to the GPC, does it happen to be blue? I have the same set up you described, I read about the small dinky yellow wires, and decided I was gonna upgrade mine so I was getting max power, and I went out to my truck to find 2 thick blue wires with fusible links that combine into one and run to the GPC

Click to expand...

At the solenoid on the fender, there are two, heavy gauge fusible link wires, bur they are gray on my truck. these two fusible links are then connected to a very heavy gauge red wire that runs to the GP contoller. Sure looks factory to me. there are no yellow power wires in the engine harness or in the connector, so it is eather a factory setup or somebody did a really good job of geeting rid of the old wires. My truck came from down south so I can't imagine why the P.O. would have gone to the trouble of upgrading the power feed to the GPC, unless the connector was melted, which it is not. I also bought the truck with only 32k on it, so it hadn't been messed with. I think maybe some of the last idi's got better GP wiring.

I would say that you probably have bad connections at the gps but that is just a wag, I like to use a bore brush to clean the inside of the connection and a brass brush to clean the tip of the gps, then I dab just enough dielectric grease to cover the bore of the connector, then slide onto gp while twisting. this has never failed me so far. The gp system on these trucks will drive you crazy looking for a bad connection. I dont know if 12 sec is enough. I know that when it gets down in the teens I have seen the wts light on for 20 seconds in my truck