QuestionQUESTION: i got a 2000 dodge 1/2 ton with a 5.9. It always cranks but doesn't always start. It doesn't make a difference if it's hot or cold from what I can tell. It has died going down the highway at 70 it has died idling in the drive. Sometimes when it won't start the fuel pump kicks on and sometimes it doesn't. When the fuel pump doesn't kick on I have checked power at the relay and have nothing. I have checked the 5v wire at the tcm and it has always checked out fine. Sometimes when it starts it'll be a little choppy but then smooth out. Sometimes after it dies it'll start back but be extremely rough and eventually die and just crank with nothing.
ANSWER: Hi Will,
The fuel pump relay has two fuses involved:
fuse 9 in the fuse box under the dash and fuse 3 in the power box under the hood.
Fuse 9 shows 12v only in the start-run position of the key and that should show up on pin 86 of the fuel pump relay which is the actuation coil of the relay on a light green/black wire. The other end of the coil is controlled by the PCM on pin 85 of the relay which is a wire from the PCM pin 19 of plug 3 on a brown/white wire. That wire is grounded (which closes the relay) by the PCM when it gets info that the engine is indeed cranking over. It will also ground the wire for about 1 second when you first turn the key to run position.
Once the relay is actuated, the 12v on pin 30 of the relay, which is always present because it come from fuse 3 which is connected to the battery, is passed on from pin 87 of the relay on a dark green/black wire to the fuel pump module at pin 4 of the module.
So the next time it dies and won't start you could check out the voltage situation on the fuel pump relay.
The other thing to try is for a fault code readout:
turn the key "on-off-on-off-on and leave on: doing that in 5 seconds or less elapsed time. Then watch the odometer window to see if the mileage reading is replaced by a 4-digit number preceded by a P. If so, tell me the number.
Roland
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QUESTION: Yes I forgot to add the code. The odometer says p720. There was one instance that I noticed the odometer said no bus. I don't no if it had happened previously but I have been watching for it since and it hasn't happened since which has been probably ten times. I was leaning toward something causing the fuel pump to not kick on until today when I realized that sometimes the fuel pump is kicking on and it still doesn't fire.
ANSWER: Hi Will,
Are you sure about the 0720 code as that is about a problem with the transmission's output speed sensor and its circuit?
If it were an 0320 instead then that would say there is a problem with the crankshaft position sensor signal which in fact were true then that would indeed caused the PCM to think that the engine wasn't running and thus turn of the fuel pump by opening the pump relay.
So keep checking for fault codes until we get new ones. You can erase old ones by disconnecting the battery briefly.
Roland
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QUESTION: It is code p720 and it's the only code I can make show up. If I erase it it comes back in a few seconds after starting without even moving the truck. The check engine light and this code showed up 4-6 months before I had this no start issue. Of the numerous times it doesn't start I have only noticed once when it gave me the no bus on the odometer. Most of the time the fuel pump does not get power but sometimes it does and I can hear it kicking in but it still just cranks without firing. All of this just seems very random and I don't know how to start eliminating possibilities.
AnswerHi Will,
The odometer readout should be "P0720" not just "720"; if it just shows "720" then I would be inclined to get a fault code readout at a shop using a plug-in code reader. A nationwide auto parts store (e.g. Autozone) will often do that for free. I would also not expect that code should show up immediately after you erased it as the speed sensor code is saying that it is changing its reading too abruptly, so the vehicle has to have been moving.
The fuel pump should always run for about 1 second when you first turn the key to 'run'. If not then verify the fuel pump circuit voltages (that I listed earlier) when you first turn the key to run, and also verify them when you are cranking it over (with a helper operating the ignition switch). I would focus on that circuit until you find why it is not always coming "on" for about one second when you turn the key to the "run" position.
Thanks for you earlier rating and nomination of me. If you would do that again I would be appreciative.
Roland
