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1997 Passport, P1870 and TC doesn't seem to lockup. How to tell if it's TCC solenoid or TCC friction material gone?

1191 Views 42 Replies 4 Participants Last post by  pizzaman5000

Recently got my Passport running after sitting for a year. Did a transmission fluid/filter change, some clutch material in pan, not terrible. While driving on the highway at 60mph, the dummy lights came on, and the code was P1870. I've done some research, and I think it's either(in order of hopefulness):

1. TCC solenoid gummed up or failed
2. Vehicle output speed sensor failed/failing
3. TCC friction material gone
4. Pump not producing enough pressure to lock up TCC

Can the 4L30E throw a P1870 if the TCC solenoid is bad? Wouldn't it throw a P1860 instead? Can a failing vehicle speed sensor cause a mismatch between the input and output speed readings and cause a P1870?

I am going to test the solenoid from the connector on the outside of the trans, I believe it has to have a resistance between 17.5 ohms and 18.5 ohms to pass.

Thank you in advance, I'm going to go crawl under my truck in the rain and see if I can get a resistance on the TCC solenoid.
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The Powertrain Control Module (PCM) in your 97 Passport is comparing the engine rpm with the transmission tail shaft rpm, the tail shaft rpm is being counted by the transmission "speed sensor" mounted to the transmission extension housing. DTC P1870 is set when the PCM determines that the difference between input and output rpm with the TCC applied is more than expected during the slip test. It may be that the TCC is slipping or the 3rd clutch or 4th clutch is slipping or the TCC solenoid is not working.

Step 1. Measure the TCC solenoid resistance between the signal wire at the PCM end and the negative battery terminal. I think the TCC solenoid connection at the PCM is on connector C-2 Pin D2 and is a Red/Yellow wire. Make the resistance measurement with the connector disconnected from the PCM. The TCC signal wire goes to pin 4 of the overdrive section 5 Pin connector.

NOTE that one side of the TCC solenoid coil is connected to the case inside the transmission ("grounded"), and if the transmission case is not grounded properly to the engine block the TCC solenoid won't operate.
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Pull out the 5 pin connector on the overdrive case to be sure your reading the correct pin on the PCM , it the reading goes from 16.6 to zero your on the correct pin . 16.6 is close to expected so likely not a solenoid problem.
Reverse Locked Out
When the vehicle is moving forward above approximately 7 mph and Reverse (R) gear range is selected, the PCM/TCM energizes the TCC solenoid. With the TCC solenoid energized, solenoid feed fluid flows through the solenoid and fills the solenoid signal fluid circuit. Solenoid signal fluid is routed to the spring end of the reverse lockout valve, thereby assisting spring force to keep the valve closed against reverse fluid pressure. This blocks reverse fluid from entering the reverse clutch fluid circuit and prevents the transmission from shifting into Reverse.

Therefore if you applied +12 Vdc to the solenoid causing it to energize with the engine running in Park it should prevent Reverse from being selected when the shifter is moved to the R position.
You could also connect a 12 LED between PCM connector C-2 D2 and ground to see if the PCM is actually sending On/Off commands to the TCC solenoid. You should be able to stick the LED wire into the wire side of the connector and make electrical contact. The positive side of the LED goes to the connector.
Btw, not all 4L30e transmissions have the Reverse Lockout feature but I am reasonably sure your 97 does.
The TCC will unlock when the foot brake switch operates, if the switch is out of adjustment road conditions could cause it to randomly operate causing erratic TCC operation.
I don't know if a specific throttle position percentage causes the TCC to unlock but any upshift or downshift of the transmission will.
The two Shift Solenoids & Brake Band Solenoid share a common +12 Vdc reference from C3 E-14

The Control signal for two Shift Solenoids & Brake Band Solenoid is a chassis ground level signal.

The TCC solenoid control signal is +12 Vdc. The TCC solenoid control return is chassis ground.

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Connect the shift solenoid Led or brake band solenoid Led in parallel with the solenoid, making sure the Led polarity is correct. The plus side of the shift solenoid Led's and brake band Led are all connected to the same point. When the Led illuminates the solenoid is energized as well. With different colored Led's you can better monitor the state of each solenoid.

For the TCC solenoid, ground the negative side of the Led.
Use small low current Led's
Vehicle speed at 3000 rpm is is about 10mph lower than expected for 4th gear with 28.5 diameter tires and 4.3 rear diff.
If you keep the shift selector in the 3rd gear position what is the vehicle speed at 2000 rpm?
Was the TCC solenoid energized when you observed the speedo?
35 mph in 3rd at 2000 rpm with the TCC disengaged seems about right.
At what vehicle speed does the transmission typically shift from 3rd to 4th according to the Led's?
According to this chart the hydraulic pressure is lower than expected, but probably not the cause of the P1870 DTC

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