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Cat Man
Cat Man, Caterpillar Engine Expert
Category: Heavy Equipment
Satisfied Customers: 8865
Experience:  23 Years Caterpillar Engine experience.
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where is the speed sensor on a 3126 cat engine only starts

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where is the speed sensor on a 3126 cat engine only starts with ether will run good but will not restart
Hi. Thanks for using JA! I will try and help you out.

The 3126 engine has two engine speed timing sensors. It looks like you may have already replaced them. The are on the drivers side of the engine on the back of the front cover. Under the high pressure pump.

Have you already replaced them?
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Customer: replied 5 years ago.
yes any idea why it won t start
I see. Thanks for the information.

Most often when an engine has a hard starting issue, it is the timing sensors. But since you have replaced then, you problem most likely somewhere else.

The next most common thing I have seen cause your problem is low actuation pressure. Your engine needs at least 870 psi to start. If it is low, it will causenhard starting.

The main item that causes low actuation pressure is a faulty injection actuation pressure control valve.

Plese reply back if you need more help.
Customer: replied 5 years ago.
where is that and how hard to fix
Its easy to fix. It is located on the high pressure pump on the drivers side of the engine. The pump is bolted to the bqck of the front cover. You will see two wires going to the valve. First remove the solenoid, then the valve. It screws out of the pump.
Customer: replied 5 years ago.
does the sensor need to be replaced at the same time
No. I would not replace the Injection Pressure sensor. Most often the IAPCV or Injection Actuation Pressure Control Valve is the problem.
Customer: replied 5 years ago.
thanks will check it out
You are very welcome. Let me know if you need more help.