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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3382
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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2005 saf-liner bus. transmission temp light comes on as soon

Customer Question

2005 thomas saf-liner bus. transmission temp light comes on as soon as you crank vehicle. I have serviced the trans. with both filters and fluid. I have called thomas and they say there is no temp. sensor on trans. and allison said they did not install one either. Sometimes when you crank it up and run it the temp light does not come on and the trans. works perfect and other times it goes to limp mode and it want change gears and the light is on then. I'm thinking this is an electrical issue but don't have an electrical schmetics.
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Derek Neustaeter replied 1 year ago.

Good Morning,

My name is ***** ***** qualified Heavy Equipment Mechanic.

In this case I would be a little skeptical of what the dealer told you. Most Allison transmissions in your application do have a transmission temperature sensor. In your case it is likely a bad sensor or a bad wire going to it. Have a look under the bus and look at the tranmission, looking for a sensor on the transmission and send pictures if necessary. Do you know what model of allison transmission is in the bus? If it is an Allison 545, they commonly use a transmission temperature sensor.

Let me know what you find and I would be happy to help further if needed.



Customer: replied 1 year ago.
It has a AD 2000.IT has Three sensor on Transmission.One crank,input, and output sensor, with two wires on each. See attached pics. Thanks
Expert:  Derek Neustaeter replied 1 year ago.

Is the bus equipped with just a warning light, or a temp gauge as well? Disconnect each 2 wire sensor separetly and check the gauge to see if the trans temp changes at all.

Let me know what you find

Customer: replied 1 year ago.
It just has a temp light. We unplugged all the sensors and the light still stays on.
Customer: replied 1 year ago.
my cell # *****
Expert:  Derek Neustaeter replied 1 year ago.

Is my phone number not provided to you by just answer? On my end it says the offer of live phone call has expired...

Customer: replied 1 year ago.
still have not solved problem
Expert:  Derek Neustaeter replied 1 year ago.

If I could troubleshoot the issue myself, I would be able to find the issue. But it is just too difficult to narrow down the problem from behind the computer. I'll see if any other expert has any ideas...

Expert:  Jeremy replied 1 year ago.

My name is ***** ***** I will assist you.

Your transmission will have a temp sensor on the top tide of the valve bosy. I highly suggest taht if you are not Allison trained or never took one apart, don;t attempt to do the swap of the sensor yourself. I would have a Allison shop do it.

Here is why, to get to that sensor, you have to pull the transmission and roll it upside down. Then you need to remove the valve body. On the valve body, there are 3 bearing balls in there. If you drop one or misalign them, the transmission will not shift and you will burn out a clutch in minutes. I know for I made this exact mistake myself. Hard lesson to learn. You can drop the valve body while installed, but there is a $5,000 tool that is needed to get it to align back up properly. Most people don't have that tool for they don't do Allison's every day to need one.

With that light being on, even on a cold engine, most likely that sensor is bad. The sensor reads through the main harness that goes into the transmission. The signal is sent from the senor to the TCM (transmission module) and then to the vehicle chassis module on the bus in which it will light up that light.

I know you were hoping for a simple fix for the light, but there just isn't one.

At least now that you know what you need to do, you can move forward as needed.

I don't know why Allison said there isn't a temp sensor for there is one. All of them have it. There may not be an external gauge sensor, which is what they are thinking of, but it has an internal temps sensor for that is how the transmission controller knows how to do the shift point for smooth shifting of the clutches and how much engagement is needed on the solenoids as well at what RPM to shift at due to the temp of the transmission fluid.

CLICK HERE for a troubleshooting manual, if needed.

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Expert:  Jeremy replied 1 year ago.

Did you need anymore assistance with this? I'm following up since the question is still showing as open/unresolved. Thanks.