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Transmision Fluid Temperature (TFT) Sensor Trouble

If any of you Trany guys have some information for me on this one I would appreciate it.

I have a 1997 K1500 with the C50 4 speed automatic transmission.

The other day,my check engine light came on as I was driving home from work. I own a Autotap OBDII diagnostics system, so I connected up my laptop and found a DTC of P0712. My shop manual and the Autotap called this a "Fluid Temperature Sensor CKT Low input". I put up a graph of the sensor with the engine stopped, and the voltage was right around 3 volts corresponding to a temperature of about 165 deg F. The voltage graph was a flat line and all seemed normal. I cleared the DTC and started up the engine. Again all seemed normal until I revved up the engine. As soon as I revved the engine, the graph for the TFT dropped to zero for a couple of seconds, then came back up to 3 volts. I repeated this many times with the same results. I looked in the shop manual and found this circuit to be a sourcing circuit that uses a themister to pull the voltage to ground. I assumed at this point that I had a wiring harness problem and a wire must be touching to ground periodically. But I went through the entire wiring harness, pulling, pushing, etc and could not make the voltage drop. I then restarted the engine and got the same results, the voltage dropped to zero above idle speed.

I then started messing with the transmission and found that the voltage was also dependent on what gear the transmission was in. In reverse, the signal was zero all the time. In drive, the signal was 3 volts, but was not very stable (spiked down to zero every once in a while).

I am convinced that the problem is in the transmission, but I am not sure what it might be. I did find that the thermister is mounted on the same circuit board as the Transmission Fluid Pressure Switch (TFP).

So my questions:

  1. Has anyone had a similar failure with this sensor?
  2. Has anyone had troubles with the internal wiring harness in the transmission?
  3. Any other suggestions would be great.

I plan to drop the pan in the next couple of days (about ready for a fluid / filter change anyway) and I will take a look at the wiring harness and the sensor board. If I do not see anything strange, I may just replace the circuit board (I have not checked on the price yet).

Thanks for any help.

Wayne

Reply to
Wayne R. Roller
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Correction: the transmission is a 4L60-E

Reply to
Wayne R. Roller

. . =3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D Transmision Fluid Temperature (TFT) Sensor Trouble Group: alt.autos.4x4.chevy-trucks Date: Mon, Aug 9, 2004, 8:34pm (CDT+5) From: snipped-for-privacy@comcast.net (Wayne=A0R.=A0Roller)

If any of you Trany guys have some information for me on this one I would appreciate it. I have a 1997 K1500 with the C50 4 speed automatic transmission.

The other day,my check engine light came on as I was driving home from work. I own a Autotap OBDII diagnostics system, so I connected up my laptop and found a DTC of P0712. My shop manual and the Autotap called this a "Fluid Temperature Sensor CKT Low input".

I put up a graph of the sensor with the engine stopped, and the voltage was right around 3 volts corresponding to a temperature of about 165 deg F. The voltage graph was a flat line and all seemed normal. I cleared the DTC and started up the engine. Again all seemed normal until I revved up the engine. As soon as I revved the engine, the graph for the TFT dropped to zero for a couple of seconds, then came back up to 3 volts. I repeated this many times with the same results. I looked in the shop manual and found this circuit to be a sourcing circuit that uses a themister to pull the voltage to ground. I assumed at this point that I had a wiring harness problem and a wire must be touching to ground periodically. But I went through the entire wiring harness, pulling, pushing, etc and could not make the voltage drop. I then restarted the engine and got the same results, the voltage dropped to zero above idle speed.

I then started messing with the transmission and found that the voltage was also dependent on what gear the transmission was in. In reverse, the signal was zero all the time. In drive, the signal was 3 volts, but was not very stable (spiked down to zero every once in a while).

I am convinced that the problem is in the transmission, but I am not sure what it might be. I did find that the thermister is mounted on the same circuit board as the Transmission Fluid Pressure Switch (TFP). So my questions:

  1. Has anyone had a similar failure with this sensor?

  1. Has anyone had troubles with the internal wiring harness in the transmission?

  2. Any other suggestions would be great. I plan to drop the pan in the next couple of days (about ready for a fluid / filter change anyway) and I will take a look at the wiring harness and the sensor board.

If I do not see anything strange, I may just replace the circuit board (I have not checked on the price yet).

Thanks for any help. Wayne =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Wayne, you've got several choices as I see it.

1) Do a battery of electrical tests on the transmissions Preasure Switch Manifold at the transmission and at the ECM to pinpoint where the problem is.

2) Replace the Preasure Switch Manifold and hope the problem goes away.

NOTES 4-U:

The Trans Fluid Temp sensor shares a ground with the preasure switches incorporated in the manifold.

(this "MAY" be why the readings change while running through the gear pattern)

Also, You are correct in how the TFT sensor works by letting voltage pull to ground. Just keep in mind that the preasure switches are on the same ground circuit.

MarshMonster ~:~

Reply to
Marsh Monster

Marshy scores a free Autotap!

LOL

Refinish King

Transmision Fluid Temperature (TFT) Sensor Trouble

Group: alt.autos.4x4.chevy-trucks Date: Mon, Aug 9, 2004, 8:34pm (CDT+5) From: snipped-for-privacy@comcast.net (Wayne R. Roller)

If any of you Trany guys have some information for me on this one I would appreciate it. I have a 1997 K1500 with the C50 4 speed automatic transmission.

The other day,my check engine light came on as I was driving home from work. I own a Autotap OBDII diagnostics system, so I connected up my laptop and found a DTC of P0712. My shop manual and the Autotap called this a "Fluid Temperature Sensor CKT Low input".

I put up a graph of the sensor with the engine stopped, and the voltage was right around 3 volts corresponding to a temperature of about 165 deg F. The voltage graph was a flat line and all seemed normal. I cleared the DTC and started up the engine. Again all seemed normal until I revved up the engine. As soon as I revved the engine, the graph for the TFT dropped to zero for a couple of seconds, then came back up to 3 volts. I repeated this many times with the same results. I looked in the shop manual and found this circuit to be a sourcing circuit that uses a themister to pull the voltage to ground. I assumed at this point that I had a wiring harness problem and a wire must be touching to ground periodically. But I went through the entire wiring harness, pulling, pushing, etc and could not make the voltage drop. I then restarted the engine and got the same results, the voltage dropped to zero above idle speed.

I then started messing with the transmission and found that the voltage was also dependent on what gear the transmission was in. In reverse, the signal was zero all the time. In drive, the signal was 3 volts, but was not very stable (spiked down to zero every once in a while).

I am convinced that the problem is in the transmission, but I am not sure what it might be. I did find that the thermister is mounted on the same circuit board as the Transmission Fluid Pressure Switch (TFP). So my questions:

  1. Has anyone had a similar failure with this sensor?

  1. Has anyone had troubles with the internal wiring harness in the transmission?

  2. Any other suggestions would be great. I plan to drop the pan in the next couple of days (about ready for a fluid / filter change anyway) and I will take a look at the wiring harness and the sensor board.

If I do not see anything strange, I may just replace the circuit board (I have not checked on the price yet).

Thanks for any help. Wayne =============== ===============

Wayne, you've got several choices as I see it.

1) Do a battery of electrical tests on the transmissions Preasure Switch Manifold at the transmission and at the ECM to pinpoint where the problem is.

2) Replace the Preasure Switch Manifold and hope the problem goes away.

NOTES 4-U:

The Trans Fluid Temp sensor shares a ground with the preasure switches incorporated in the manifold.

(this "MAY" be why the readings change while running through the gear pattern)

Also, You are correct in how the TFT sensor works by letting voltage pull to ground. Just keep in mind that the preasure switches are on the same ground circuit.

MarshMonster ~:~

Reply to
Refinish King

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