How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Cat Man Your Own Question
Cat Man
Cat Man, Caterpillar Engine Expert
Category: Heavy Equipment
Satisfied Customers: 8857
Experience:  23 Years Caterpillar Engine experience.
Type Your Heavy Equipment Question Here...
Cat Man is online now
A new question is answered every 9 seconds

Freightliner..slow down..this occurs the temp gauge reads zero..tps

This answer was rated:

i have a freightliner columbia with a c-12 cat in my shop it has an intermitent problem of only reving to 1200 rpm it also will speed up and slow down by itself when the problem is occuring. also when this occurs the temp gauge reads zero. i have replaced coolant temp sensor, tps. sensor and ohmed the wires between the coolant temp. sensor and the ecm i have also checked connections at the ecm, bulkhead and instrument cluster. what else is most likly wrong?

Hi, Welcome to Just Answer, I am Clayton. I can help you with this.


When this occurs, does the tach and the speedo also fall to zero, also what is the voltage reading on the dash during the occurance?



Customer: replied 6 years ago.
no the tach and the spedo. both read fine all of the time, its just the temp. gauge that acts weird. the voltage is about 14.0v all the time.
Can you flash the codes, lets see if t here are any historic codes. Turn the cruise switch to on, and use the set/resume switch to flash the codes, tell me what you come up with
Customer: replied 6 years ago.

i have a heavy duty otc code reader but it says there is no code. if i flash the codes withe the cruise switch how do i do it? it will take me 15 minutes or so to get to the shop and back


If there is no code with your code reader, then there most likely is no code. I am going to OPT-OUT and let CatMan handle this, I have never heard of this before.
Customer: replied 6 years ago.
thanks for trying

Hi. Sorry for the delay. I will try and help you out.


First I would like to ask you a few questions if that is OK.


Could you get me your engine serial number?


Under what conditions does this happen?


When it is first started? When the truck is parked? Driving down the road?


Do you have any PTO switches or a remote throttle on the truck?

Customer: replied 6 years ago.
im not at the shop but i can go get th ser.# XXXXX you would like. the customer claims it has happend twice going over railroad tracks but it has happend to me several times sitting still. the truck does have a pto lever on the dash

Thanks for the information.


That sound likely a very likely item that can cause your problem.


If the truck has a remote throttle, while hitting a bump or even when it was sitting in your shop could cause this problem. It sounds like the engine is going in and out of what is called PTO mode. Since it is going to and from 1200 RPM's, 1200 RPM's may be a programmed limit.


Do you have a tool at your shop that can read the "Status" of the parameters?


If so, you will be able to see if this is the problem.

Customer: replied 6 years ago.
i do not have a tool that will read parameters but the pto with cruise is set at 1200 rpm max is there anyway that i can disconnect this from the truck

There are 2 options.


The first would be to have someone with Cat ET software to go into the ECM and re program the PTO parameter.


The other way would be to locate the wires and switch that activate the PTO. You can trace these wires back to the ECM and remove them. This way the ECM will not get a signal to activate the PTO.



Cat Man, Caterpillar Engine Expert
Category: Heavy Equipment
Satisfied Customers: 8857
Experience: 23 Years Caterpillar Engine experience.
Cat Man and 2 other Heavy Equipment Specialists are ready to help you