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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8421
Experience:  I am ASE Master Truck Certified.
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C-12 cat...cranks but will not start..ecm, power to three

Customer Question

C-12 cat...cranks but will not start..ecm, power to three red wires on the ecm...changed crank and cam...fuel pressure still primed and will not run on ether
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 1 year ago.

Thanks for asking your question. I will try and help you out.

Does the yellow check engine light come on when the key is turned to the on position?

When you say it will not run on ether, do you mean it runs for a few seconds on the ether and then shuts off? Or it will not even fire and run at all on ether?

Customer: replied 1 year ago.

will not fire..check engine on when key on

Expert:  Cat Man replied 1 year ago.

Thanks for the information.

Since the yellow check engine light comes on, this indicates the engine ECM is receiving power and functioning. There are several things that can cause a crank no start on this engine.

The first is a low or no fuel pressure condition. The engine needs at least 35 psi to start. If it is low, the injectors will not get filled and it will not start. If the engine has a hand priming pump, and it gets stiff when operating it, the engine has enough pressure to start. If there is no hand priming pump, I would connect a psi gauge and measure the pressure. If the pressure is low, I would change the fuel filters first if you haven’t already. If it is still low, I would suspect a failed fuel pressure regulator in the secondary fuel filter housing, a weak fuel transfer pump, or restricted fuel inlet line. The pump is located on the front of the engine.

Second is a failed engine speed timing sensor. Some engine have one, some have two. It depends on your engine serial number. If the engine speed timing sensor has failed, the ECM gets an RPM/TDC signal and will not send AC to the injectors.

Next is low battery voltage to the ECM while cranking. The ECM must have at least 7.5 volts DC to operate. I have seen weak batteries or a poor connection cause low voltage. You could have a poor connection at the batteries or starter.

Next is a faulty engine ECM. The ECM has an injector driver module inside. This is what powers the injectors. If it fails, the ECM will log injector codes for all the injectors.

Lastly are worn injectors. Over time the injectors can wear if they have a lot of hours on them. To verify, I like to check AC to the injectors while cranking. You should have between 20-40 AC while cranking. If you do, and you have good fuel pressure, the injectors are worn. If you do not have 20-40 AC while cranking, you have either a failed engine speed timing signal, low power to the ECM while cranking, ot a failed ECM.

Most often when these things are happening there will be diagnostic codes in the ECM. If your engine has cruise control switches, you can pull the codes. If you have never done this, let me know. I can get you the procedure.

Please reply back if you need more help or have more questions. I am happy to assist.

Customer: replied 1 year ago.

how do I get the codes from the ecm

Expert:  Cat Man replied 1 year ago.

Use the following procedure to retrieve the flash codes from the check engine light:
Turn the ignition key switch to the On position/ Engine OFF.
Turn the cruise control ON/OFF switch to the OFF position.
Move the SET/RESUME switch to either position. Hold that position until the yellow lamp begins to flash.
The yellow lamp will flash in order to indicate a two digit flash code and the SET/RESUME switch may be released. The sequence of flashes represents the system diagnostic message. Count the first sequence of flashes in order to determine the first digit of the flash code. After a two second pause, the second sequence of flashes will identify the second digit of the flash code.
Any additional flash codes will follow after a pause. These codes will be displayed in the same manner. Flash Code 55 indicates that No Detected Faults have occurred since the ignition key switch has been turned to the ON position.

Let me know what two digit code you get and I can tell you what the problem is.

Customer: replied 1 year ago.

will not flash yellow........tried my other identical truck..flashed 31 & 27

Expert:  Cat Man replied 1 year ago.

Thanks for the information. So the yellow engine light never comes on? Or just will not flash codes?

Customer: replied 1 year ago.

never comes on

Expert:  Cat Man replied 1 year ago.

Thanks for the information.

If the yellow check engine light does not come on when the key is turned to the on position and the engine will not start, there are only 2 things that can cause this.

The first and most common is the engine ECM is not getting the correct power and ground to function. The ECM should have power on pins 52, 53, and 70 of the rear most ECM connector. This is the truck side harness connector. 70 is the key switch wire. The grounds are on pins 65 and 67. You can remove the connector and check for power on these pins. If you do not have power on these pins, you most likely have a blown fuse in the electrical system. I would check all fuses first.

If you do have correct power and ground to the ECM, you most likely have a failed ECM. I have seen this many times over the years. To confirm the ECM is bad, you would need to install a known good ECM on the engine or you can take the ECM to the dealer to be tested. Unfortunately the ECM cannot be repaired. Replacement is the only option. I am sorry. I wish I had better news.

Please reply back if you need more help or have more questions. I am happy to assist.

Expert:  Cat Man replied 1 year ago.

Hey. I just wanted to see how things were going. Please let me know if you need more help. I am happy to assist. If my information was helpful, please leave a good or higher rating so that I receive credit. Thanks.

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