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3126 cat will only start on ether. 8yl54721 in a 2000

Customer Question
3126 cat will only...

3126 cat will only start on ether

Mechanic's Assistant: What is the complete model and serial number of your machine?

8yl54721 in a 2000 freightliner fl70

Mechanic's Assistant: Are you fixing this yourself? What have you tried so far?

Heui pump replaced, cam sensors replaced, checked cam sensor gap, installed 2 different test ecm's, checked for injector oil leaks, have 1300 psi oil at block, good fuel pressure, good power and grounds at ecm, checked ecm harness and load tested wires, after starting with ether it runs smooth all day.

Submitted: 4 years ago.Category: Medium and Heavy Trucks
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Answered in 31 minutes by:
3/27/2018
Technician: Cat Man, Cat Engine Expert replied 4 years ago
Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 10,344
Experience: I am ASE Master Truck Certified.
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Thanks for asking your question. I will try and help you out.
So the engine will not start on it’s own? But when using ether it will start and run all day?
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Technician: Cat Man, Cat Engine Expert replied 4 years ago
Thanks for the information.
Have you checked Power to the injectors while cranking?
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Customer reply replied 4 years ago
2 volts then tapers down to nothing
Technician: Cat Man, Cat Engine Expert replied 4 years ago
Thanks for the information.
You should have between 20-40 ac while cranking. Since you do not. I assume your ecm is not getting a tdc signal.
Yes it’s possible to get an rpm and not get a tdc signal. When this happens. The ecm will not send Power to the injectors.
Whenever I have one not start but start on ether and run all day with no codes. I often find one of three things
One is bad timing sensors. Which you have replaced.
Second is a problem in the wiring harness to the timing sensors.
Third is a gap problem between the timing sensors and cam gear. You may have had the cam gear slide forward on the camshaft.The only other thing I have seen is low voltage to the ecm while cranking. If the voltage drops below 7.5 to the ecm. The ecm can’t function. But when you spray ether and release the starter. The ecm wakes up and starts the engine. To test. I connect the Cat ecm Power bypass harness. If it starts. You have a low power the ecm while cranking.
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Customer reply replied 4 years ago
the sensor gap is .026, ecm voltage doesn't drop below 10 volts while cranking
Technician: Cat Man, Cat Engine Expert replied 4 years ago
Thanks for the information.
Then I must suspect a problem in the harness. Form some reason the ecm can not get a tdc at cranks speed from the primary start sensor. But when you use ether and the secondary high speed sensor picks up tdc. The engine starts.
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Customer reply replied 4 years ago
it takes half a can of ether, volts at the ecm from the cam sensors are .5 volts and 1.5 volts. ohms reading 0 from connector to ecm
Technician: Cat Man, Cat Engine Expert replied 4 years ago
If you have actuation pressure. You have rpm.
If you do not have ac to the injectors. Your not getting a tdc.Have you had the ecm read for codes?
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Customer reply replied 4 years ago
no codes
Technician: Cat Man, Cat Engine Expert replied 4 years ago
Thanks for the information. Then I would definitely suspect a no tdc condition.
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Customer reply replied 4 years ago
what should we be getting for a reading at ecm when cranking for cam sensors
Technician: Cat Man, Cat Engine Expert replied 4 years ago
Unfortunately Cat does not give a spec for that. Sorry.
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Customer reply replied 4 years ago
when we test injectors for power, we unplug injector harness, would ecm turn injectors off or keep trying to fire them.
Technician: Cat Man, Cat Engine Expert replied 4 years ago
It will not turn them off. It will keep firing them.
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Customer reply replied 4 years ago
basically were at changing harness?
Technician: Cat Man, Cat Engine Expert replied 4 years ago
If the gap is good. The sensors and ecm have been changed. Yes. Next step would be harness. It sounds like you have done a good job testing and troubleshooting everything else.
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Customer reply replied 4 years ago
we will try a harness and get back to u thanks.
Technician: Cat Man, Cat Engine Expert replied 4 years ago
You are very welcome my friend. I am happy I could help.
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Customer reply replied 4 years ago
Replaced harness same thing
Technician: Cat Man, Cat Engine Expert replied 4 years ago
Then I must suspect a gap problem between the sensors and cam gear.
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