Is my ECU bad?

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svdsinner

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Ram Year
1998
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5.9L V8
'98 2500, 5.8L gas.

My truck went suddenly from running to cranking without firing.

The ECU isn't sending signals either to activate the fuel pump relay, or activating the spark plugs. The ignition coil and the relay have both tested as good.

From what I've seen, the ECU waits until it gets signals from the crank position sensor and the cam shaft sensor before turning on the fuel pump and the ignition coil. I replaced the crank position sensor since I heard this is a common cause. No improvement.

I think the issue has to be either the ECU or the camshaft sensor, neither seem like high probabilities to me. Am I missing any other likely culprits?

I've got plenty of electrical test equipment, so I can take oscilloscope reading on things if it will help.

How can I confirm the ECU as either good or bad?

If it is the ECU, is it easily replaceable?



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dudeman2009

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The fuel pump should prime as soon and the truck is turned to run. Check the ASD relay in the underhood fuse block. Sometimes those go bad and cause this issue.
 
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svdsinner

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The fuel pump should prime as soon and the truck is turned to run. Check the ASD relay in the underhood fuse block. Sometimes those go bad and cause this issue.
What's the logic relationship between the relays? Firstly, is the ASD relay supposed to energize when the key moves to On? (Or does it energize/de-energize on some other event?) If it does, the fuel pump relay turns on, and if not it won't? (Or do the two relays output work in series? IOW, if the ASD is bad, will the Fuel Pump relay always be de-energized?)

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dudeman2009

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What's the logic relationship between the relays? Firstly, is the ASD relay supposed to energize when the key moves to On? (Or does it energize/de-energize on some other event?) If it does, the fuel pump relay turns on, and if not it won't? (Or do the two relays output work in series? IOW, if the ASD is bad, will the Fuel Pump relay always be de-energized?)

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The ASD relay and fuel pump relay should be energized at key on by the PCM separately. They do not come into contact in anyway outside of both being wired into separate circuit of the PCM. The ASD relay is energized by the PCM when it senses a signal from the crank sensor while cranking, in the run mode the relay shuts off after 1.8 seconds if no input is detected from the crank sensor. The fuel pump relay is energized at key on at all times during cranking, however it shuts off after 3 seconds if the engine is not running.
 

Brandon pixley

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I have a 99 Dodge that will start run for about 10 minutes die but will start right back up and act like it's out of time I have replace the crankshaft sensor and the camshaft sensor is the computer bad
 

SportRam00

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With key on/engine off, do you see you something that says NO BUS in the odometer read out? No start plus that NO BUS would be a tell tale sign of an ECU issue.

Also, have you tried to reseat the plugs on the ECU itself? When mine was bad, I would reseat and press on the plug closest to the engine and the truck would start.
 
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