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I HAVE A 2008 T300 WITH ONLY A LITTLE OVER 7,000 MILES AND…

I HAVE A 2008 T300...
I HAVE A 2008 T300 WITH ONLY A LITTLE OVER 7,000 MILES AND THE SPEEDOMETER SUDDENLY STARTED TO BE VERY ERRATIC AND THEN JUST SETTLE DOWN TO ABOUT 10 MPH REGARDLESS OF HOW FAST THE UNIT IS MOVING, BUT GOES DOWN WHEN ENGINE IS TURNED OFF. I HAVE HAD THE SPEEDOMETER BENCH CHECKED AND CHECKED OUT FINE AFTER A 25 MILE TEST. THE CRUISE CONTROL IS WORKING FINE, SO I ASSUME THE SPEED SENSOR IS OK IF THEY BOTH READ FROM THE SAME ONE.
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Answered in 12 minutes by:
8/22/2013
Technician: Toby, Owner/Operator replied 9 years ago
Toby
Toby, Owner/Operator
Category: Medium and Heavy Trucks
Satisfied Customers: 370
Experience: Owned Operated Maintained MCI buses for over 25 years. Very knowledgable on Series A thru D
Verified

Hello
My name is XXXXX XXXXX I am pleased to try and answer your problem today. I will do my best to research and solve your issue.

 

On some units the cruise control actually read from the transmission so if cruise is working then check speed sensor

If the speed senor has been jolted to far from the wheel then the magnetic pick up will not read correctly or if there is a break in wire or sensor is just bad

 

Failing that you should know that the speedo also gets its info from the ECM. Signal goes from speed sensor into ECM then back out to Speedo
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Customer reply replied 9 years ago

ARE YOU FAMILIAR WITH THIS SPECIFIC CONFIGURATION?


 

Technician: Toby, Owner/Operator replied 9 years ago

Not with the specific configuration but we are talking about a KW T300. Unless they have done some dramatic change in the way they route the wiring it should be the same as other trucks for the same model year

The speed sensor is located on the transmission not as previously mentioned

Therefore everything is feed back to ECM

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Customer reply replied 9 years ago

I HAVE LOCATED THE SPEED SENSOR ON THE TAIL SHAFT OF THE TRANSMISSION (ALLISON 5 SPEED AUTOMATIC) AND REMOVED IT AND REINSTALLED. IT APPEARED TO BE TIGHT, IN GOOD, SHAPE AND THE CONNECTION SEEMED TO BE GOOD AND TIGHT. I ALSO CHECKED ALL WIRING HARNESSES AND THEY SEEMED TO BE IN GOOD SHAPE. WHEN I DISCONNECT THE WIRE TO THE AFOREMENTIONED SPEED SENSOR, THE TRANSMISSION DISENGAGES AT ABOUT 5 MPH INDICATING THAT IT IS AT LEAST SENDING SOME SORT OF A SIGNAL. TRANSMISSION AND CRUISE CONTROL WORKED FINE WHEN RECONNECTED.

Customer reply replied 9 years ago

IF YOU DON'T HAVE ANY OTHER IDEAS, IS IT POSSIBLE TO REFER MY ISSUE TO ANOTHER TECHNICIAN THAT MIGHT HAVE SPECIFIC KNOWLEDGE OF A KW T300 WITH ALLISON AUTOMATIC TRANSMISSION?


 

Technician: Toby, Owner/Operator replied 9 years ago
I have forwarded your problem to another tech as I have to leave for doctor appointment
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Customer reply replied 9 years ago

THANK YOUR FOR YOUR ASSISTANCE.


 

Technician: Cat Man, Cat Engine Expert replied 9 years ago
Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 10,344
Experience: I am ASE Master Truck Certified.
Verified
Hi. Thanks for using JA! I will try and help you out.

Does the yellow check engine light or ABS light come on when the speedometer is acting up?
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Customer reply replied 9 years ago

NO OTHER PROBLEMS OR ISSUES WITH TRUCK. JUST THE SPEEDOMETER.


DOES THE SPEEDOMETER OR CRUISE CONTROL POSSIBLY READ FROM ONE OR MORE OF THE ABS SENSORS AT THE WHEELS?

Technician: Cat Man, Cat Engine Expert replied 9 years ago
Thanks for the information.

The cruise control switches are connected to the engine ECM. Since no lights are coming on, I'm sure the ABS computer and the engine ECM are receiving a speed signal. If they were not, a light would be on.

Since you have tested the speedometer, I assume it is OK. So the only other thing I would suspect is a wiring problem between the speedometer and the computer or ECM it gets its signal. Most likely the engine ECM. The reason I say this is because if the wiring is the problem, there will be no check engine or abs light on and the speedometer will not work.

Please reply back if you need more help or have more questions. I am happy to assist.
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Customer reply replied 9 years ago

WHAT IS MY NEXT BEST STEP?


PLEASE DON'T SAY TAKE IT TO KENWORTH SO I CAN EMPTY MY POCKETS!

Technician: Cat Man, Cat Engine Expert replied 9 years ago
The next thing I would want to do is verify where the speedometer is getting its signal. I suspect it gets it from the engine ECM. Most often it does.

Do you have any way of reading the engine ECM?
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Customer reply replied 9 years ago

THE CLOSEST THING I HAVE IS A DIGITAL VOLTAGE/OHM METER. IF THAT WILL DO ANYTHING.


 

Technician: Cat Man, Cat Engine Expert replied 9 years ago
Unfortunely not. I am Sorry.

A tool like Cat ET software will be needed. With this you can verify the ECM drives the speedometer and run a test to make it move. If it does you can verify a problem in the wiring. I am sorry. I wish there was a way without a tool.

I hate going to the dealer too. Let me know if you need more help.
Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 10,344
Experience: I am ASE Master Truck Certified.
Verified
Cat Man and 87 other Medium and Heavy Trucks Specialists are ready to help you
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Customer reply replied 9 years ago

THANKS FOR ALL YOUR HELP!!! Laughing


YOUR RESPONSE TIME WAS EXCELLENT AND APPRECIATED!

Technician: Cat Man, Cat Engine Expert replied 9 years ago
You are very welcome. I am happy to assist.
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