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Saturn: shifting..second gear when slowing to 40 MPH

Customer Question
why does my 02 saturn...
why does my 02 saturn sc1 with 93k miles suddenly start down shifting into second gear when slowing to 40 MPH and then on continued slowing to 30MPH it shift up to third gear?
Submitted: 9 years ago.Category: Car
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Answered in 2 hours by:
4/1/2008
Auto Mechanic: John, Master Automotive Technician replied 9 years ago
John
John, Master Automotive Technician
Category: Car
Satisfied Customers: 9,176
Experience: ASE A1-A8 & L1 Master GM Tech 40 years experience
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HelloCustomer

Have you serviced the transmission in the last 20-30,000 miles(fluid & filter).If not this is a good place to start.Inspect the pan for any metal debris.Look at the color of the fluid & smell of it,it should be bright red & NOT smell burned.Scan the powertrain control system to check for any failure codes.This is an electronically controlled transmission.

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Customer reply replied 9 years ago
Reply to John's Post: I said that I had done the reverse-slam fix. This involues changing the fluid, filter, adding a tranny additive, and running at idle in reverse for 30 min. Fluid was good and clean, not burnt. No debris in pan. I have a DTC p0733 and sometimes a DTC 1404. I have had upshift and down shift problems ever since reverse-slam fix. I have changed the value body out with a used one. I got the same problems after the change. So I then took my original value body apart. I cleaned and inspected all parts. No problems found. I ohm meter checked all solinodes (4.5 ohms each all the way to the plug in top of tranny pan). I put the original value body back in the tranny after clean and inspection. All shifts(up and down) remain the same. I also have noticed that the shift lever will not control the tranny. If in top gear overdrive, putting the lever in 3 will not cause down shift at all. If when you take off the lever is in 3 the tranny will still go into top gear overdrive. During this time there is no passing gear except it will drop to 2nd even at high speed 70 and above.
What are the two(2) sensors on the front side of the tranny? How do you check them? Do they effect the tranny shifting?
Auto Mechanic: John, Master Automotive Technician replied 9 years ago

I am sending a couple of repair flow charts to help.Maybe these will help you.I believe you probably have some internal problems(clutches slipping,pump fault).Your p1404 is the EGR.

DTC P0733

Circuit Description

The powertrain control module (PCM) uses the crankshaft position (CKP) sensor, input speed sensor (ISS) and output speed sensor (OSS) to determine engine speed, input shaft speed and output shaft speed. These sensor readings are used to determine the current gear ratio and RPM slip across the torque converter. The PCM uses this information from the sensors to determine if the commanded gear matches the actual gear. During normal driving conditions, the PCM will adapt main line pressure via the pressure control (PC) solenoid to prevent slipping and harsh shifts. Shift solenoid 3 is used to apply or release third gear clutch. The PCM controls 3rd gear by controlling the ground for shift solenoid 3. The PCM will turn the solenoid OFF, open the driver circuit, when 3rd gear is commanded On to apply 3rd clutch. Shift solenoid 3 is supplied ignition voltage from the fuel pump relay whenever the PCM commands the fuel pump relay ON. DTC P0733 sets when 3rd clutch still slips after the PCM has commanded high main line pressure.

DTC Parameters

DTC P0733 will set if the PCM detects an incorrect actual 3rd gear ratio when 3rd gear is commanded On when:

The condition exists for longer than 3 seconds (2001)*
The condition exists for longer than 3 seconds, 1 minute is allowed to elapse, then condition exists again for longer than 3 seconds (2000)
The high main line pressure has been commanded
The transaxle is in a forward gear
The input speed is greater than 224 RPM
The vehicle speed is greater than 5 km/h (3 mph)
The speed noise parameter = NO
The steady state ratio fail counter equals 255 for an in-gear failure, or the shift fail counter is greater than 100 on a downshift or upshift to 3rd gear (128=0 or normal operation)

DTC P0733 diagnostic runs continuously once the above conditions have been met.

DTC P0733 is a type A DTC for 2000, type B for 2001.*

*Applicable to vehicles built after June 2000.

Diagnostic Aids

Important: If DTCs P0745, P0746, P0756, P0758, P0761, P0763, P0766, P0768 or P0789 is set, diagnose that DTC first. A diagnosed PC solenoid, shift solenoid 2, 3 or 4 low/high circuit fault DTC may have caused DTC P0733 to set.

Important: If a shift solenoid performance DTC P0783, 4th gear stuck ON, is set, diagnose that DTC first. A 4th gear stuck ON may cause no 3rd gear, no 2nd gear, or TCC stuck Off.

Refer to Pressure Ports - Air Check and Parts Description for pressure apply points and for component/location description.

Possible causes of no 3rd gear are listed in the flowchart on the succeeding page.

Verify TRANSMISSION ISS and TRANSMISSION OSS read correctly on Scan tool.

Make sure the pressure control, shift 2, shift 3 and shift 4 solenoids are not intermittently sticking. A sticking shift solenoid 4 may cause 4th gear to be applied from an initial start resulting in no 3rd gear. Audible sound should be similar for all solenoids. A muffled tone indicates the need for replacement.


Object Number: 875316  Size: FP

DTC P0745

DTC P0745 - PC Solenoid Pressure Low


Object Number: 870972  Size: MF
Click here for detailed picture of above imageidth=

Circuit Description

The pressure control (PC) solenoid is used to control main line pressure to the transaxle hydraulic system. The powertrain control module (PCM) pulse width modulates (PWM) the PC solenoid whenever the engine is running. The PCM controls the PC solenoid by pulse width modulating an internal driver that pulls the solenoid circuit to ground. When the PC solenoid is commanded Off, main line pressure is highest. During normal shift operation, the PCM will only turn ON or OFF the 2, 3 or 4 shift solenoids and PWM the PC solenoid to prevent clutch slip and harsh shifts. The PC solenoid circuit is monitored for low and high voltage faults. The PCM also monitors main line pressure for low pressure. The PCM calculates main line pressure based on shift time and clutch slip. DTC P0745 sets when the PCM has set all forward no gear DTCs indicating all clutch packs are slipping resulting from low main line pressure.

DTC Parameters

DTC P0745 will set if the PCM has set DTC P0731, P0732, P0733 and P0734 indicating low main line pressure.

Since DTCs P0731, P0732, P0733, P0734 are set, no forward gear ratios are correct and most likely no forward gears are available.

DTC P0745 diagnostic runs continuously when the above conditions have been met.

DTC P0745 is a type D DTC.

Diagnostic Aids

Important: If DTC P0746 or P0747 is set, diagnose that DTC first. A diagnosed PC solenoid circuit DTC may have caused DTC P0745 to set.

Refer to Part Description and Pressure Ports Air Check for component location/description and main line pressure specifications and test procedures.

Ensure that the PC solenoid is not intermittently sticking. Audible sound should be similar for all solenoids. A muffled tone indicates the need for replacement.

An intermittent short to ground on circuit 1530 will cause low main line pressure. Use scan tool to command TRANS. SOLENOID POWER ON, while monitoring the PC SOLENOID - LOW VOLT or PC SOLENOID - OPEN test. Wiggling wires while monitoring the test may locate a problem.

PC solenoid resistance: 3.8-7.2 ohms

Nominal value: 4.5 ohms at 68°F


Object Number: 875479  Size: FP

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Customer reply replied 9 years ago
Thanks for the info, but you have not addressed either of my questions. What are the two(2) sensors on the front side of the tranny? What do they do? How do you check them? Why does the tranny while do 50MPH are faster when slowing downshift at 40MPH to 2nd and then on continued slowing upshift to 3rd?
Auto Mechanic: John, Master Automotive Technician replied 9 years ago

OK, 1 switch is the VSS it sends a square wave signal to the PCM at a speed modulated frequency.You will need a good Fluke meter to check this.If you can describe the other switch I will try & help you.

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Customer reply replied 9 years ago
I guess the last answer of "OK" is just a note and futher info will be comming soon.??
Auto Mechanic: John, Master Automotive Technician replied 9 years ago
One of your switches is a VSS(vehicle speed sensor) that produces a square wave signal modulated by speed.You will need a good Fluke meter to read the signal.If you will describe the other switch I will try & help you.
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Customer reply replied 9 years ago
The top sensor is just to the right of the tranny filer. It is small in diameter about 1/4" and the other one is lower down on the tranny a a little futher to the right of the first on. It is a little larger in diameter about 3/4" Both have two wires.
Auto Mechanic: John, Master Automotive Technician replied 9 years ago

The smaller switch is a TFT(transmission fluid temperature sensor).

The transaxle fluid temperature (TFT) sensor is a thermistor that varies resistance according to changes in transaxle fluid temperature. The PCM supplies a five volt reference through a pull-up resistor to the sensor, which is connected to ground. When the sensor is cold it has high resistance (high signal voltage at PCM). As the sensor temperature increases, its resistance decreases (lower signal voltage at PCM). The PCM uses the signal voltage to determine transaxle fluid temperature.

The other is the input speed sensor(ISS)

The input speed sensor (ISS) is threaded into the transaxle case located near the transaxle fluid filter. The ISS produces an AC voltage of different amplitude and frequency depending on the velocity of the input shaft. The ISS uses 8 total machined notches off the 2nd gear clutch to produce different AC voltage signals. This ISS signal is sent to the PCM, which is used to determine the velocity of the input shaft. A readable input occurs when the input shaft speed is greater than 250 RPM.

The PCM uses the ISS to monitor the speed of the input shaft. In conjunction with the crankshaft position (CKP) sensor, the PCM can determine the slip across the torque converter. In conjunction with the output speed sensor (OSS), the PCM can determine the slip across the commanded gear clutch pack. The PCM can use this information to determine the torque converter slippage, clutch gear slippage, actual gear ratio or lack of main line hydraulic pressure.

The input speed sensor (ISS) is threaded into the transaxle case located near the transaxle fluid filter. The ISS produces an AC voltage of different amplitude and frequency depending on the velocity of the input shaft. The ISS uses 8 total machined notches off the 2nd gear clutch to produce different AC voltage signals. This ISS signal is sent to the PCM, which is used to determine the velocity of the input shaft. A readable input occurs when the input shaft speed is greater than 250 RPM.

The PCM uses the ISS to monitor the speed of the input shaft. In conjunction with the crankshaft position (CKP) sensor, the PCM can determine the slip across the torque converter. In conjunction with the output speed sensor (OSS), the PCM can determine the slip across the commanded gear clutch pack. The PCM can use this information to determine the torque converter slippage, clutch gear slippage, actual gear ratio or lack of main line hydraulic pressure.

 

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Customer reply replied 9 years ago
Thank you again for all the good info. The only question we have not answered for me is the original question.

why does my 02 saturn sc1 with 93k miles down shift into second gear when slowing to 40 MPH and then on continued slowing to 30MPH it shift up to third gear?
Auto Mechanic: John, Master Automotive Technician replied 9 years ago
You have an internal transmission slip problem that the PCM is detecting.Remember the P0733,well the PCM has not forgot & until you address this problem you will not fix your problem.
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Customer reply replied 9 years ago
I do not have any slipping in any gear that I know of. All shifts both down and up are solid. Why would slippage cause a tranny to downshift to 2nd from the top gear at 40MPH? Will slippage cause a tranny to upshift at 30MPH?? If I have a slippage problem why does the tranny shift both up and down correctly sometimes?
Auto Mechanic: John, Master Automotive Technician replied 9 years ago
The slippage you cannot feel the PCM judges this.
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Customer reply replied 9 years ago
thank you again for all the help and info, but dropping out of overdrive into 2nd gear is not a slippage problem. Then upshifting into 3rd is not a slippage problems either. I agree and know that the 733 is in correct gear ratio, but couldn't this be caused by one of the sensors(VSS or CRANKSHAFT) reporting the wrong info?
Auto Mechanic: John, Master Automotive Technician replied 9 years ago
Sure.
John
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