my 96 7.3 is starting to have some issues starting in the morning when it gets below about 40 out. normally it starts right up but now that its starting to get down to 30 at night in the morning it wont start. i tested my old relay and it tested out to be good but i had a new one so i threw it on and nothing changed so the next morning it did the same thing so i jumped the relay with a screw driver for about 12 seconds and the truck fired right up. tonight i was checking some things out and found that when i tested for volts on the hot side of the big terminal i get battery voltage, when i check the small fused side of the relay with the key on i get battery voltage witch is good. when i check the ground/controller side of the relay with the key just turned on for about 30 seconds i get 7 volts then after that it the controller opens the ground so the meter shows 12 volts, witch means there is a high resistance in wire between the relay and the controller or in the controller itsself. i put my jumper wire between the ground of the relay and the battery ground it energizes and i get power to my glowplugs like it is suppose to. so next i checked for resistance in the wire between the relay and the pcm connector on the firewall and found .2 ohms witch is good. so whats next? do i replace the PCM because there is a high resistance internally in it? hope Iam going slow enough for people to jump on my train of thought.

 

Possibly check the engine oil temp sensor. I believe this is what tells the PCM to use the glow plugs or not. Does your "wait to start" light come on?

 

check the wiring harness back a bit and look for a burnt out wire

 

Glow plug light comes on like normal long time when its cold and shorter when its warm, it was 50 out this morning and my EOT sensor said the oil was 60 degrees, i put my meter on the glowplug side of the big relay and got no power but the truck started right up like i had just shut it off. Its not plugged in either

 

Knottyrope i have good continuity from the relay to the connector on the fire wall and there was no corrosion in the connectors. And i have good twelv volts to the other side of the small posts when the key is on.

 

PCM going to run about $150-300 unless you can find a deal out there


Lighted Toggle switch and some wire under $30

 

I know i could wire up a switch but i want to fix the problem rather than short cuting around it.

 

With the key on you need to have 12v to both sides of the glow plug relay (the two big posts) with no more than .2 to .3 difference between them. You stated above you had no power to the glow plug side of the relay this to me sounds like a faulty relay.

 

what OP said sound like a PCM issue, he jumpered the relay to ground and it worked as it should if the PCM was ok and its wiring

 

^^^^ +1; PCM appears to be not fully grounding the coil circuit. Voltage is the wrong way to test it, however. You should check continuity / resistance from the PCM terminal of the GP relay to a good ground when you first turn the key to RUN. If it reads open or high resistance, that's the bogey. I would check continuity of the PCM ground wire (actually, there may be more than one).

If it turns out this is the only op issue with the PCM, if there are no other effects of this ground issue, I would find a compatible part-out take-off PCM as a replacement, and keep this one as a spare.

 

I am thinking this may be my problem as well. I have replaced and tested everything but feel the glow plugs should be on longer.

 

How long are they staying on?

Edit - Nate, since John's question is still "open", just to avoid thread-spaghetti confusion, you might want to start a new thread.

 

I guess my guestion would be why my pcm is not giving a good ground to complete my glowplug control circut? Where does the pcm ground at?

 

I'll have to check the EVTM, but there's at least one ground wire on the PCM connector (AFAIK, it does NOT ground through the case/mount). There may be multiples. If that ground is good, then the ground path WITHIN the PCM between the GPR wire and that ground is somehow incomplete. Probably inoperable. If that's the only thing wrong with the PCM, then as I said, it'd make a good spare (spare PCM and IDM are good things to have on hand anyway).

 

Well there is good continuity between the GPR and the PCM bulkhead connector on the fire wall there was like .2 ohms of resistance, ive also checked the air temp sensor and the EOT sensor with my scan tool and their fine.