2008 4.2L crank no start PCM?
#1
10-02-2017, 05:17 PM
Join Date: Dec 2008
Location: Houston
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2008 4.2L crank no start PCM?
My 2008 was running fine, Saturday ran an errand, came home shut it off and it wont start. Can hear fuel pump priming and can smell raw gas at exhaust pipe.
Connected scanner and monitored RPM as I suspected crank sensor but am reading 170 +/- while cranking.
Checked for spark and no spark. Checked fuse 33 and fuse was blown, replaced and blew again. Replaced coil pack and now fuse does not blow but still no start.
So I pulled the connector from the coil pack to see if getting signals from PCM. Using test light connected to B+ I checked pin 4 on harness side with Key On and got solid light as expected.
Next connected Test light to B- and probed pin 1 while son cranked. Light pulses dimly, Probe pin 2 light pulses dimly, probe pin 3 light is on bright solid with key on and not cranking, no change when cranking, still on bright solid indicating a persistent ground.
I also have a 2002 4.2L 150, did same test on it to compare. Pins 1,2 and 3 all pulse dimly as I would expect.
Am going to get the schematic out and check the wire from pin 3 back to PCM to make sure it is okay but assuming it is any ideas before I order a replacement PCM? Thanks..
Connected scanner and monitored RPM as I suspected crank sensor but am reading 170 +/- while cranking.
Checked for spark and no spark. Checked fuse 33 and fuse was blown, replaced and blew again. Replaced coil pack and now fuse does not blow but still no start.
So I pulled the connector from the coil pack to see if getting signals from PCM. Using test light connected to B+ I checked pin 4 on harness side with Key On and got solid light as expected.
Next connected Test light to B- and probed pin 1 while son cranked. Light pulses dimly, Probe pin 2 light pulses dimly, probe pin 3 light is on bright solid with key on and not cranking, no change when cranking, still on bright solid indicating a persistent ground.
I also have a 2002 4.2L 150, did same test on it to compare. Pins 1,2 and 3 all pulse dimly as I would expect.
Am going to get the schematic out and check the wire from pin 3 back to PCM to make sure it is okay but assuming it is any ideas before I order a replacement PCM? Thanks..
#2
10-03-2017, 06:03 PM
Join Date: Dec 2008
Location: Houston
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It is the PCM
So it is the PCM, PIN 12 on C175E which feeds Pin 3 on the coil pack is shorted to ground. Pins 16 and 17 which feed pins 1 and 2 on the coil pack read open in reference to ground which is expected.
Anyone ever open one of these up? It has to be a shorted switching transistor. Seems like a pretty easy fix as the switching transistor is almost certainly a standard part (if I can determine what it crosses to)
Anyone ever open one of these up? It has to be a shorted switching transistor. Seems like a pretty easy fix as the switching transistor is almost certainly a standard part (if I can determine what it crosses to)
#3
10-03-2017, 08:11 PM
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Location: Houston
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Got it open
Got it open, take out the torx on both sides and pry open slowly. Pry each side approx 30 degrees and you can pull the frame out leaving the shell with the circuit boards attached open enough to get to the transistors.
There is an RTV bead around the edge.
Could smell the burnt electronics smell. Sure enough there are three power mosfets and one is shorted. It is marked is 8206N but that crosses to a dual channel 8 pin package device.
Anyway since this mosfet is between the load (the coil) and ground it has to be an N channel. Am going to stick a 30v/60a N channel power automotive mosfet in it and see what happens...
There is an RTV bead around the edge.
Could smell the burnt electronics smell. Sure enough there are three power mosfets and one is shorted. It is marked is 8206N but that crosses to a dual channel 8 pin package device.
Anyway since this mosfet is between the load (the coil) and ground it has to be an N channel. Am going to stick a 30v/60a N channel power automotive mosfet in it and see what happens...