Hey everyone, winter finally came to Ohio. I've plugged my '97 F 250 7.3 HD in a few weeks ago and all was well. While my wife's car was iced over each morning my truck had clear windows and started on the first try. Now it just wont turn over even though its been plugged in all night. I hate to spray ether into it and will leave it as a last resort. My GPs have been week since before I bought the truck a couple of months ago but I was just gonna wait until after Christmas to work on it and rely on the block heater until then. I checked the power source and the extension cord and they both check out. I guess the block heater could have just worn out but I don't know. I drive a semi for a living so I didn't drive off while it was still plugged in or anything but I'm starting to get a little worked up about it and wanted to see if anyone on this great board had any ideas. Sorry if this post is a little long but it's my first one besides the introduction post. Anyway please let me know if you have any ideas and thanks. -Z

 

Won't crank, or cranks but won't fire? If you're considering ether, sounds like cranks-but-won't-fire, but would be good to clarify.

In either case, block heater is not the problem, or the likely solution. Heater just warms the block. Warm oil means easier cranking, warm combustion chamber helps the glow plugs get combustion chamber temperature high enough to ignite the fuel. But a warm (or at least not stone-frozen-cold) block isn't enough for a start.

If your glow plug system isn't working well, you're probably not going to start in cold temps even with the block heater working. Common failures are the relay, under-valve-cover wiring harnesses, and the plugs themselves. From experience, I can tell you that only 3 working plugs and no plug-in makes cold-start attempts in freezing weather long and iffy. And only 3 working still makes the stock voltage gage drop a lot, so that's no help to tell you how well- or not- the system is working. If you're going to do the plugs, might as well just replace the harnesses while you're in there.

It sounds like plugging in may have masked your glow plug troubles for a while, and now it's cold enough that plugging in can't compensate. There are plenty of other causes for cranks-but-won't-fire, but circumstances suggest the glow plug system. And no-crank is a whole different set of issues, but again, not related to block heater.

 

You might want to check the cord end on the block heater.(male plug) The wires have a tendicy to break just behind the plug. If this is the case a replacement end can be had from a hardware store.

 

Thanks Tom, I'll check it first thing in the morning. -Z

 

Hey Paddler, the truck cranks but wont fire. It's probably the plugs so I'll get to checking in the morning. Thanks a lot.

 

On the relay... one big terminal should always have +12V. If not, wiring problem. The other big terminal should have +12 V with key on but not cranking (ie, "wait to start"). If not, relay or signal to relay (the small wires to relay) bad.

You may also want to check the relay side of the harness to the valve covers. Should be two connecters per bank (4 total), with 5 pins to each connector. Center pin is injector ground, one of the pairs (outer or inner) is injector positive and the other GP positive. With key on but not running and GP relay engaged, two pins per connector should have +12 V. If not, with a good relay, problem is the wiring between relay and connector.

Worthwhile confirming the problem really is under the valve covers before taking them off.

If you find 8 good +12 V readings (2 for each of 4 connectors), then problem is under-valvecover harness or the GPs. Time to take off the valvecovers and replace harnesses and GPs.

You'll need a deep, thin-wall socket for the GPs (I forget the size, sorry), and a way to suck oil from the GP recess so oil doesn't get into the cylinder when you remove the GP. And there is a brand of GPs that folks urge you to avoid due to reputation of getting stuck in the head... I forget which. I think I got Bosch from NAPA.

Hopefully someone can chime in with socket size and GP brand.

 

If you find the glow plugs are bad, use Motorcraft ZD11. Other brands don't last, and can swell in the head when they burn out, especially Autolights.

 

Before removing the valve covers, TEST. Measure the RESISTANCE of the glow plugs at the connectors. Should be right around 1 ohm each.

Voltage on the "downstream" big terminal will NOT be 12V with the key in RUN. It will be less, if the glow plugs are actually working, because they'll pull the voltage down. Here's the key. Measure voltage at BOTH big terminals with the key in RUN. They'll both be something less than 12, usu. 11.5 or so. But they should be very close. The "always hot" terminal may be slightly more than the downstream terminal, but if the _difference_ is more than 0.3V, the relay is bad.

 

Thanks to you all for all the info. I'm going to keep looking back at them as I go along. I was a passenger in a wreck a while back (not in my truck) and my right side is mostly useless for the time being so I wont be moving fast. I'll update or ask more questions if I need help. Hopefully I wont. Again thanks everyone for all the advise. It is a lot more than I hoped for. Have a great day all.

 

Well, I checked the GP relay and the connections and there is no real juice on the one side with or without the key on and there is on wire missing so I'll start with the relay and go from there. If that's not it I'll put GPs and wiring harness on my Christmas list (haha). Anyway I'm hoping the relay is it. Thanks a lot for all your help. Z

 

Wait, what wire is missing? If you don't have both small terminals hooked to a wire, the relay won't work at all whether it is good or not.

 

Both the main wires are there and hooked up. the one missing is the one right next to the "always hot" terminal, but I'll go out and check again just to be sure. Thanks -Z

 

Picture of the GPR, two big terminals and two small terminals ...

 

Thanks Jim, I am embarrassed to say that my multimeter had a broken wire and there is no missing wire. it is just one of the mounting bolts with the nut loose. I thought there was a wire supposed to go there but no. Sometimes I feel like an idiot (a bit) but if it was easy everybody would be doing it right? Anyway my new multimeter is a lot different than my old one was and a lot more complicated. does anyone know where to set the dial to get an accurate reading? I feel dumb just asking but I hate making thins worse. Thanks again. -Z

 

I just about live by that statement lol. Good, Luck