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Dan, Technician
Category: Construction and Road Equipment
Satisfied Customers: 5564
Experience:  17+ yrs. shop and field experience with all major brands of heavy equipment and trucks - 1995 WyoTech graduate.
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I have a cat 3126 7as motor that will rev up to 1500 rpm by

Customer Question

I have a cat 3126 7as motor that will rev up to 1500 rpm by its self. It does it at idle and driving. No codes from ecm. If u depress clutch or brake pedal it goes to normal idle for a minute but will jump back to 1500rpm. Also water light on but no code. It is in a peterbilt 330.
Submitted: 1 year ago.
Category: Construction and Road Equipment
Expert:  Dan replied 1 year ago.

Hi Matt, My name is***** will be glad to try to help. Check the TPS sensor and harness for problems. If okay then the likely cause is an ECM problem. Try a test ECM or have yours bench tested. Let me know

Customer: replied 1 year ago.
Tps readings are good. I already tried a different ecm. Still does it. When at an idle it will sound like pto is trying to engage and then it goes straight to 1500 rpm. Pto is air operated not electric. And like I said when it does this if you hit the clutch to trip the switch it goes back to idle for a minute and then goes right back to 1500.
Expert:  Dan replied 1 year ago.

If this is PTO related your will need to check the switches and harness for the remote throttle and monitor and check the switch status and parameters with Cat ET connected. Could be a remote switch or harness problem or a software issue but since it does it with two ECMS it will likely be a remote switch or harness problem.

Expert:  Dan replied 1 year ago.

Click on this link for your diagram and ECM pinouts to help


Expert:  Dan replied 1 year ago.

Hi Did you get this resolved?