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Hal, Chevy Master Tech
Category: Chevy
Satisfied Customers: 1031
Experience:  ASE and Chevy Master , 31yr Dealership exp,
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1992 chevy: 6.5 diesel with a turbo but not HD...overdrive..solenoids

Customer Question

We have a 1992 Chevy KL3500 with a 6.5 diesel with a turbo but not HD. It would not shift onto overdrive. We replaced all the solenoids still wouldn't shift. We bought a new tranny and still have the same problem. Is it an electrical problem?
Submitted: 4 years ago.
Category: Chevy
Expert:  Hal replied 4 years ago.

Hi my name is XXXXX XXXXX Your complete satisfaction is my goal. Please provide as much information about your concern as you can.

Have you checked the throttle switch on the side of the injector pump? If not then you will need to scan the computer for diagnostic codes to see if the electrical is actually keeping it from shifting. Most likely it is. The throttle switch gives input as to the throttle opening to shift the transmission. If it is stuck high, the transmission will stay in third, and not shift up. Please check for diagnostic codes first, and let me know if any are set.

Customer: replied 4 years ago.
We were not sure where to start looking after changing everything we did. We thought maybe it was a relay or something. If it is the throttle switch is that easy to change?
Expert:  Hal replied 4 years ago.

Is it fairly easy to change, bu just difficult to get at. It does have to be set to a specific voltage. I would check the trouble codes first and make sure what is at fault.

Do you know how to do this?

Expert:  Hal replied 4 years ago.

To check trouble codes without a scanner. look at the picture below and you will see the diagnostic connector in the lower left. You will need to ground the ( A ) to ( B ) terminal to get the check engine of transmission light to flash codes. If a 21 is set then the tps is at fault. See descriptions below.




Customer: replied 4 years ago.
We have codes 68 and 85. What does it mean when it says set pressure to maximum
Expert:  Hal replied 4 years ago.

68 deals with the transmission not shifting into over drive.The 85 deals with the ratio expected is not correct( undefined ratio)..

What ( set pressure to max ) means , is that the pressure regulator inside the transmission will be commanded to maximum fluid pressure since there is a fault indicated. It is just a default to try and keep the transmission from failing completely.

Here are the diagnostics for these two codes. I would start with the 68 first .



Circuit Description:
The PCM/TCM monitors the difference in engine rpm and input shaft rpm. With transmission in "drive," the "Scan" tool reading should show engine speed closely matching input speed.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. Code 68 will set if:

^ Engine speed is 200 rpm higher than input speed.
^ 4th gear is indicated.
^ TCC is enabled.
^ All conditions are met for 2 seconds.
^ Not in park or neutral.

2. This test monitors signal sent to the PCM/TCM from the transmission.

3. This test confirms engine rpm signal to PCM/TCM.

Diagnostic Aids:
^ Check for deformed connectors at pass-thru connector.
^ Code 68 will set when going to default (2nd gear).
^ Refer to PCM/TCM intermittant codes.




Circuit Description:
The PCM/TCM calculates the actual gear ratio from input and output speed readings while in each gear, then compares these to what the gear ratio should be, taking into consideration the selected gear range. This monitor includes reverse but does not include overdrive.

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This test verifies the proper function of the input sensor.

2. Test two verifies the correct functioning of the output sensor.

Diagnostic Aids:
The PCM/TCM relies on the PSM to indicate what gear range has been selected, so the PSM must be functioning properly or Code 85 may be set.

Check all connections, especially those at the transmission pass-thru connector.

Compare "Scan" tool gear ratio reading to chart below

( PSM , is pressure switch manifold)

Make sure you clear the codes and drive truck to verify these codes are current, or you may miss diagnose the concern.

Let me know if you need help going through these diagnostic charts. Make sure you print them off .

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