TPS Bypass Switch

[DmaxMaverick]

It's not exactly that simple. You need a diagram and verify which end of the circuit needs ground, and which needs Batt+. There are also pressure switches inside the tranny (valve body), that complete the ground to the solenoid (bypass them). Yours should have 3, maybe 4. I have a diagram scan for my 85 I can send you (email), if you want to give that a try. It just happens to have the TCC circuit highlighted.

[AugustDiesel]

It's not exactly that simple. You need a diagram and verify which end of the circuit needs ground, and which needs Batt+. There are also pressure switches inside the tranny (valve body), that complete the ground to the solenoid (bypass them). Yours should have 3, maybe 4. I have a diagram scan for my 85 I can send you (email), if you want to give that a try. It just happens to have the TCC circuit highlighted.

If only things were that simple ... I'll pm you, thanks!

However I may just try replacing the tps first. But then again, I do have to tear into the valve body anyways to install the transgo 700-2&3 kit ...

[convert2diesel]

It's not exactly that simple. You need a diagram and verify which end of the circuit needs ground, and which needs Batt+. There are also pressure switches inside the tranny (valve body), that complete the ground to the solenoid (bypass them). Yours should have 3, maybe 4. I have a diagram scan for my 85 I can send you (email), if you want to give that a try. It just happens to have the TCC circuit highlighted.

I beg to differ. It is that simple. Have used this method on three vehicles.

Mechanically, the convertor gets its pressure from the second gear circuit but lock-up depends on either the 3rd gear pressure solenoid or the 4-3 pulse solenoid giving it a path to ground through the computer. The lock-up solenoid has a two second delay built in to momentarily unlock the convertor during shifts. The TPS also has to be below 33% to 59% for the computer to command a lock-up.

Grounding out the upper left (TCC) data terminal (sorry had to recheck my old records to be sure), through a switch, by passes the whole works actually giving you lock up in the top 3 gears including second (very disconcerting when you forget to turn the switch off). You can also bridge the upper left to the upper right (ground) to achieve the same thing.

You're right. The TPS is expensive. Used to be less than 50 bucks. If you can get access to it, loosen off the two mounting screws and turn it counterclockwise all the way then take it out for a spin and see if that corrects your problem.

This is a "hall effect" sensor and before you condemn it, you should check the output voltage through the entire throttle range. There should be 3 wires coming out of it:

5 volts
ground (typically the black wire)
output.

With the plug connected, you should have access for a thin probe to the back of the terminals. With the key on, not running, use a voltage tester between the ground and one of the other two terminals. Find the one with 5 volts. That is the power. The other one is the one you want as it is the output voltage. With the key still on, open the throttle slowly. You want to see a smooth transition from just under 1 volt to close to 5 volts from the output terminal. If you find it jumps suddenly from one voltage reading to the next (i.e. 1.5 volts to 3 volts) then the sensor is toast. If the voltage rise is linear, then your sensor is good.

The TPS signals an unlock condition below 1 volt (coasting throttle off) and above 2.5 to 3 volts if memory serves.

Another thing you probably won't want to hear is the possibility that your IP is going south, needing more throttle angle to achieve the same power. That would mean the throttle is to far ahead and the TPS is commanding a convertor un-lock.

Hope this helps

Bill

[john8662]

1988 - 1991 6.2's with the 700R4/4L60 did have a Throttle Position Sensor, not a Switch. The TCC lock on these truck is controlled by the PCM.

Adjustment is critical, ensure you adjust the sensor and roll the throttle through full throttle and observe voltage. You should see a smooth increase in voltage.

If not smooth, the sensor is bad, I have seen many of them defective and would cause the PCM to disable lockup.

Search the forum for adjusting the TPS on a 6.5 MFI truck (92-93) to get the procedure. Requires a digital volt meter.

The TPS from the 92-93 6.5 is the same, just has a different connector on the end of the sensor. I've swapped these in a pinch.

[AugustDiesel]

Just wanted to provide an update on this:

Installed the toggle switch as prescribed by convert2diesel. Ran like that for a while, able to lock the TC with the toggle switch. However, TC continued to sporadically engage/disengage unpredictably when the toggle switch was off, so I pulled the TPS out completely. Took it for a ride and seemed to operate fine. I now can engage/disengage the TC manually when I want it to be. Not a permanent fix likely but seems like it will suffice for now.