With OBDLink MX plugged in- truck will start, unplugged it won't
 

Hi guys,

My 2001 7.3L Diesel has 222,000 miles on it. Yesterday I shut it off at a drive through window, went to start it and it just cranked, no start. The engine light and the wait to start light did not come on. I plugged in a code reader this morning and when I turned the key I heard a buzz and a click, I noticed the engine light and the wait to start light came on. I cranked it over and it started right up, when I unplugged the code reader, it died. Plug it back in and it runs, dies every time I unplug it. Any ideas?

All the fuses check out fine. Now, even with the code reader plugged in, it won't start. The batteries seem a bit low, 12 volts and 10.4 when cranking. We jumpered I believe it was pin 5 to ground and the engine light and the wait to start light come on but won't start.

I "think" the truck needs at least 10.5 when cranking. Not sure how precise it needs to be, but if you are close I think that may be your problem.

I'll charge the batteries overnight and see what happens. I may swap out the CPS tomorrow, see if that does anything. If not, is the PCM next?

My experience is if it cranks the engine with good RPM, there's enough voltage to start. If it does the slow "whr-whr-whr", then it won't start.

This does sound like a 5 volt issue.

5 volt issue?

Charged the batteries overnight, replaced the CPS to no avail.

VRef is 5 volt power supplied from the PCM to each sensor. Lose that and truck won't start. Easiest place to measure it is at the MAP sensor with key ON. One of the three wires will show ~ 5 volts. If you got it there everything probably is OK regarding VRef. If not one of the sensors can cause it to go short out, or a bad wire somewhere in the harness. Easiest way I've found is to unplug each sensor, one at a time while measuring VRef. The PCM will protect itself and VRef will come back up when the issue is resolved. Uncommon, but not unheard of.

I measured minimum 8.5 volts on all wires of each sensor from the MAP on down. What does that tell me?

That sounds like a very bad ground. Every sensor has a ground and a 5 volt reference, then at least one signal line out. 8 volts might make sense if you touch on a 5 volt supply while using battery plus as a reference with a charger plugged in.

I pulled the PCM out and need to have it tested. Any suggestions as to where?

Pin #5 to what connection?

Did you use battery ground when measuring with your volt meter?

^^^ I agree ^^^

Pin#5 where the OBD scanner plugs in.
Yes I used battery ground.
I sent the PCM out for testing and even though I looked at all the grounds, I'll be looking even closer after the PCM is checked out and back in the truck.

I sent the PCM to Swamp Diesel for testing, it checked out flawless. I pulled the GEM and it looked good, I pulled the fuse block and some of the contacts were a little corroded, water has been dripping down off the driver side windshield pillar and being delivered to the back of the fuse block via the interior pillar plastic cover. Poor design, perfect delivery of water. I cleaned up the fuse block, checked every ground I could get my hands on (all good), tightened up connector connections on the firewall and put it all back together. It runs good now. Can't really explain exactly what it works now.