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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8625
Experience:  I am ASE Master Truck Certified.
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I have a 1999truck 3126 cat engine. cant get it to go over

Customer Question

I have a 1999truck 3126 cat engine. cant get it to go over 20 mph. is there problem with throttle position sensor?
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 1 year ago.
Thanks for asking your question and requesting me. I will try and help you out.It's very possible there is a problem with the TPS.Does the yellow or red engine light come on while driving?
Customer: replied 1 year ago.


Customer: replied 1 year ago.

No. can't get out of second gear. and speedometer doesn't work.

it's a 1999 8500GMC Allison automatic MT 653

very urgent. need help now

Expert:  Cat Man replied 1 year ago.
Thanks for the information Since the light does not come on I would first suspect a low fuel pressure problem. I have seen this many times. The engine light will not come on when pressure is low and the engine will have low power. There are three things that can cause this. The first is clogged filters. Inwould change them first Second is a failed fuel pressure regulator on the back of the head Lasty is a weak fuel pump. Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 1 year ago.

wont get out of 2nd gear and speedometer wont work.

It seems like an electrical problem.

when speedometer works everything works and shift

Customer: replied 1 year ago.

it seems electrical problem. Is there a sensor that would prevent the transmission from shifting over 2nd gear.

urgent/ broke down on highway

Expert:  Cat Man replied 1 year ago.
I see. Thanks for the information.I the speedometer does not work when the truck is doing this, you may have a faulty vehicle speed sensor in the transmission, or faulty wiring that connects to the vehicle speed sensor. When the vehicle speed signal is lost, the trans will not shift correctly as the engine RPM may be limited. When the engine ECM does not know the speed the truck is going, it will limit the engine RPM and speed.
Customer: replied 1 year ago.

where is location of transmission speed sensor?

Expert:  Cat Man replied 1 year ago.
The vehicle speed sensor is located on the tail end of the transmission. It screws into the transmission. It has 2 wires on it.
Customer: replied 1 year ago.

will tapping on it help?

wires look ok.

Expert:  Cat Man replied 1 year ago.
Its worth a try. Normally when they are failing they will need to be replaced.
Customer: replied 1 year ago.

hard to find replacement stuck on highway, middle of no where, and no tools.

how long does a senor usually last? is this a come problem?

Expert:  Cat Man replied 1 year ago.
I hate you are stuck on the highway. I know how frustrating it can be. Sensors do fail from time to time. Normally they last for 5-7 years.
Customer: replied 1 year ago.

at one point it came out of it and shifted fine. Transmission sounds great. But when we stopped for fueling, stopped engine. We started it up and again the same thing.... cant get out of 2nd gear.

Does this sound like sensor problem?

Expert:  Cat Man replied 1 year ago.
I see. Thanks for the information. If the sensors fails like that the trans will not shift correctly. I wish you had a way to get a sensor to put in.
Customer: replied 1 year ago.

any chance you know any parts store I can get one on a Sunday outside Toledo?

Need a special dealer right?

Expert:  Cat Man replied 1 year ago.
Unfortunately I do not. Sorry. But you will need to get it from a GM dealer. Nornally they are not open on sundays.
Customer: replied 1 year ago.
Thought as much
Expert:  Cat Man replied 1 year ago.
Yeah sorry. Most all parts places are closed on Sunday's.
Expert:  Cat Man replied 1 year ago.


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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