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Q: Truck cranks but won't start.

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Truck will crank but not start. Checked the fuel pump in the gas tank. Not getting power. Checked the 15a EFI no 1 fuse, good. Replaced the fuel pump relay but getting power to it.The EFI is getting power but the C/OPN relay is not getting power. How do I check to see if power is getting to the engine control module? How do I check the engine control module?

My car has 390760 miles.
My car has an automatic transmission.

A: Hi and thanks for contacting YourMechanic. ...

Hi and thanks for contacting YourMechanic. The EFI supplies power to the C/OPN relay. Check the wiring from the EFI and the C/OPN relay for any breaks. Use a digital ohm meter and check for a complete circuit in the wiring from the EFI and the C/OPN relay. If the meter reads high resistance or OL, then the wiring is burned and needs repaired. If the meter reads good with .05 ohms or resistance, then check the pin connectors and make sure that they are touching the relay. If you need further assistance troubleshooting your no start situation, then seek out a technician, such as one from YourMechanic, that can assist you.

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A: First, ECM failure is very rare. I highly s...

First, ECM failure is very rare. I highly suggest you don't suspect it just yet. This system uses three relays in a complicated array. In order to properly test it electrically, you need an accurate wiring diagram. Before you do this, I recommend testing the fuel pressure. The first thing you need to know is the fuel pump is only powered for a few seconds after turning the key on. In addition, the fuel pump may not power until the key has been off for a period of time. Usually about a minute. You can listen for the fuel pump by having someone turn the key on while another person listens as close to the fuel tank as possible. If you are simply turning the key on and waiting more than a few seconds, there would not be any power at the fuel pump.

In addition, there are a few other fuses involved with this system. I recommend checking all fuses. This will rule out the possibility that one fuse powers another. I learned this the hard way by spending a lot of time chasing my tail as a rookie mechanic.

If you find you still don't have fuel pressure, it is important to know, each relay should have power at two pins. If you have power at only one, then you will need to find out why, taking into consideration the timed cycling of the fuel pump power circuit. It is unlikely you will be able to make sense of this without a wiring diagram. Also, the C/OPN relay powers the fuel pump relay at both pins. So if you have power at the fuel pump relay, the C/OPN relay is working.

At this point I have some concerns with a few of your conclusions. With any crank no start, you need to consider fuel pressure, spark, injector pulse and camshaft timing. Once all these things are tested for, then you can begin an accurate diagnostic path. If your injector pulse is missing, then the motor will not get fuel even if it has fuel pressure. The usual cause of a lack of injector pulse is a failed crank sensor. A scanner can be used to reveal this.

If you need help with these checks, I recommend booking an appointment with one of our experienced technicians to help you with you car's starting issue.

Good luck!

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