Attention: TDR Forum Junkies 
Hi all,
My '99 4wd auto 47 RE (80,000 miles) had been working great until I shifted to 4 low. I have a 4 low kit that allows use of 2wd in low range.
I noticed that the truck would not upshift properly in 4 low or 2 low.
A call to Tony Garcin at Dunrite yielded the suggestion that I check the governor pressure curves.
I connected a guage to the governor pressure port and went for a drive in normal 2wd.
I noticed nominal performance with the pressure rising about 1 psi per MPH up to about 30, where it jumped to 60 psi. These are normal readings.
I shifted to low range. I noticed that the truck would not upshift to second until 15 MPH and would never upshift to 3rd or 4th (OD). The governor pressure readings were the same, never going over 15 psi at 15 MPH.
Nothing wrong here...
According to the service manual, the PCM uses 4 different governor pressure curves to achieve different shift points.
Curve 1 is for transmission fluid temps under 30 degrees F.
Curve 2 is for temps above 50F and is called the 'normal' curve.
Curve 3 is for wide open throttle.
Curve 4 is for 4wd low range.
Eureka! the PCM must have a way of knowing that the driver has shifted to
low range so it can lower the shift point speeds. But how? The manual was vague but said that the PCM uses both the Vehicle Speed Signal (VSS) and a signal derived from the transmission output shaft speed sensor(TSS).
Since any fault in the VSS system sets a DTC, there could be no problems with VSS. VSS is derived from the wheel speed sensors on each wheel and is used for many things like the ABS system, cruise control, and the speedometer. All these were working properly so I concluded that the problem was either a defective PCM, a faulty Transmission output shaft speed sensor, or bad wiring between the two.
I unplugged the TSS plug at the TSS (on the transmission overdrive extension housing. ) and went for a drive. No change. Good shifts in 2wd and no shifts in low range. I also noted that no DTC's were set.
A review of the emissions book revealed that no code was defined for a missing TSS signal, TSK TSK Chrysler... .
Apparently a missing TSS signal merely caused the PCM to default to curve 2.
I checked the wiring first with an ohmeter and it was good. Next I ran off and bought a $22 TSS and replaced it. (Be quick because it leaks transmission fluid all over you. ) Test drive
roblem solved.
The TSS has a small magnetic pickup on the tip that senses shaft rotation via the park rod gear. You know, that thing that grinds when you slam it into Park in low range? It sheds small iron particles that find a new home on the TSS magnet. The old sensor is probably good but I haven't tried it after a clean-up of the iron paste. The buildup of iron gunk was rather minor; I change my trans fluid frequently. It doesn't take much iron to cause the signal to fail... .
My '99 4wd auto 47 RE (80,000 miles) had been working great until I shifted to 4 low. I have a 4 low kit that allows use of 2wd in low range.
I noticed that the truck would not upshift properly in 4 low or 2 low.
A call to Tony Garcin at Dunrite yielded the suggestion that I check the governor pressure curves.
I connected a guage to the governor pressure port and went for a drive in normal 2wd.
I noticed nominal performance with the pressure rising about 1 psi per MPH up to about 30, where it jumped to 60 psi. These are normal readings.
I shifted to low range. I noticed that the truck would not upshift to second until 15 MPH and would never upshift to 3rd or 4th (OD). The governor pressure readings were the same, never going over 15 psi at 15 MPH.
Nothing wrong here...
According to the service manual, the PCM uses 4 different governor pressure curves to achieve different shift points.
Curve 1 is for transmission fluid temps under 30 degrees F.
Curve 2 is for temps above 50F and is called the 'normal' curve.
Curve 3 is for wide open throttle.
Curve 4 is for 4wd low range.
Eureka! the PCM must have a way of knowing that the driver has shifted to
low range so it can lower the shift point speeds. But how? The manual was vague but said that the PCM uses both the Vehicle Speed Signal (VSS) and a signal derived from the transmission output shaft speed sensor(TSS).
Since any fault in the VSS system sets a DTC, there could be no problems with VSS. VSS is derived from the wheel speed sensors on each wheel and is used for many things like the ABS system, cruise control, and the speedometer. All these were working properly so I concluded that the problem was either a defective PCM, a faulty Transmission output shaft speed sensor, or bad wiring between the two.
I unplugged the TSS plug at the TSS (on the transmission overdrive extension housing. ) and went for a drive. No change. Good shifts in 2wd and no shifts in low range. I also noted that no DTC's were set.
A review of the emissions book revealed that no code was defined for a missing TSS signal, TSK TSK Chrysler... .
Apparently a missing TSS signal merely caused the PCM to default to curve 2.
I checked the wiring first with an ohmeter and it was good. Next I ran off and bought a $22 TSS and replaced it. (Be quick because it leaks transmission fluid all over you. ) Test drive
roblem solved. The TSS has a small magnetic pickup on the tip that senses shaft rotation via the park rod gear. You know, that thing that grinds when you slam it into Park in low range? It sheds small iron particles that find a new home on the TSS magnet. The old sensor is probably good but I haven't tried it after a clean-up of the iron paste. The buildup of iron gunk was rather minor; I change my trans fluid frequently. It doesn't take much iron to cause the signal to fail... .
