TPS Bypass Switch

I've done a lot of reading up on bypassing the TPS sensor, but haven't really found anything concrete on exactly how to do it. I'd like to install a bypass toggle switch so that I can have more control on when the TC locks and unlocks. I've seen where some install jumper wires and I think where Dmax installed a push/pull switch. I'd like to find out the following:

1) where the tps is located? On the side of the injection pump?
2) where exactly will the bypass wires plug in at?
3) what gauge wiring is best?

Thanks in advance!

If you want manual control of the TCC, the TPS is the LAST piece you'll mess with, if you mess with it at all. All the pressure switches inside the tranny, the brake switch and the ECM/ALDL lead need bypassed before the TPS ever comes into play. I've done this on my rigs, and a bunch of others. The TPS bypass was never needed, once the rest was done correctly. I suggest leaving it alone unless and until you have a very specific need to mess with it. If you still feel the need, try it without messing with it, first. You can always "bypass" it with 1% throttle pressure.

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Originally Posted by DmaxMaverick

If you want manual control of the TCC, the TPS is the LAST piece you'll mess with, if you mess with it at all. All the pressure switches inside the tranny, the brake switch and the ECM/ALDL lead need bypassed before the TPS ever comes into play. I've done this on my rigs, and a bunch of others. The TPS bypass was never needed, once the rest was done correctly. I suggest leaving it alone unless and until you have a very specific need to mess with it. If you still feel the need, try it without messing with it, first. You can always "bypass" it with 1% throttle pressure.

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Thanks I will give that a try. I only asked about bypassing the tps bc there are a few posts on here that recommend bypassing the tps when the tc isn't locking/unlocking when it should. But I must have misunderstood what was being suggested. I'll give the throttle pressure suggestion a try! Thanks!

Bypassing the TPS may be helpful for diagnosing a problem, but little else. It's a switch, not a sensor/sender. It only closes when your foot is on the skinny pedal, which allows for lockup if all other conditions are met.

It also has another purpose, in addition to TCC. It will also disengage the EGR, if so equipped.

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Originally Posted by DmaxMaverick

Bypassing the TPS may be helpful for diagnosing a problem, but little else. It's a switch, not a sensor/sender. It only closes when your foot is on the skinny pedal, which allows for lockup if all other conditions are met.

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My apologies, I meant to say the switch, not the sensor. :)

No worries. It can be a little confusing. Your 6.2L and early 6.5L (mechanical injection) both have a TPS. But, they are not the same. The 6.2L with an auto tranny (700R4) has a Throttle Position Switch. The MFI 6.5L with an auto (4L60E, 4L80E) has a Throttle Position Sensor. They are not interchangeable. Add to that, the 6.2L with a TH400 has no switch/sensor, but a vacuum valve in the same place. Clear as mud!

You may also find that by replacing said TPS, your lock-up situation may be corrected. Additionally, if you run a wire from the far right (IIRC) terminal of the OBD data port through a switch on the dash to ground, you can lock up your convertor at any time after going into 2nd gear. Just remember to turn it off when you move away from stop. After the tranny hits 2nd, your convertor stays locked. Makes for some interesting shifts.

Bill

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Originally Posted by DmaxMaverick

No worries. It can be a little confusing. Your 6.2L and early 6.5L (mechanical injection) both have a TPS. But, they are not the same. The 6.2L with an auto tranny (700R4) has a Throttle Position Switch. The MFI 6.5L with an auto (4L60E, 4L80E) has a Throttle Position Sensor. They are not interchangeable. Add to that, the 6.2L with a TH400 has no switch/sensor, but a vacuum valve in the same place. Clear as mud!

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So, to add to the confusion, what I have is an NA 6.5 (506) mated to a 700r4. The 6.5, as far as I can tell, has all the 6.2 accessories since the 6.5 was a warranty block. So, I'm assuming I have the throttle position Switch since the truck originally had a 6.2.

From what I understand, once the tc locks, it's suppose to stay locked until I apply the brakes (correct me if I'm wrong). Well, I gave the truck a good road test today and that thing was all over the place. Constantly locking and unlocking in both Drive and Overdrive.

I'll look into a new tps, but I'd still like to consider a way to manually control the tc if possible.

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Originally Posted by AugustDiesel

So, to add to the confusion, what I have is an NA 6.5 (506) mated to a 700r4. The 6.5, as far as I can tell, has all the 6.2 accessories since the 6.5 was a warranty block. So, I'm assuming I have the throttle position Switch since the truck originally had a 6.2.

From what I understand, once the tc locks, it's suppose to stay locked until I apply the brakes (correct me if I'm wrong). Well, I gave the truck a good road test today and that thing was all over the place. Constantly locking and unlocking in both Drive and Overdrive.

I'll look into a new tps, but I'd still like to consider a way to manually control the tc if possible.

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I have to assume you are using the old (88) computer. The tps had only two functions when mated to the 700R4. Controlled the EGR (if installed) and commanded the lock up. Your shift patterns are controlled with a throttle valve in the tranny connected via a cable to the throttle quadrant.

The lockup is activated any time you go into 2nd gear or higher and the throttle position is less than 1/3 to 1/2 throttle position (33% to 50%). If you want to go to full manual control of the lockup, you're going to have to disconnect the tps altogether and or the computer and install a switch in the transmission ground position of your data port (I think it is the terminal on the far right of the OBD connector under the dash). The switch will override any command from the computer.

As I said in my previous post you are going to have to be vigilant as you don't want to have the tranny going through the gears with the convertor locked up. Your brake switch will still unlock the convertor as before. Ran the Buick with this setup for close to 100K without issue, after I got used to unlocking it when appropriate. You're also going to need a piece of electrical tape to stick over the check engine light :rolleyes:

I would try a new TPS first. You may find your issues disappear. Try the cheap fix first.

Hope this helps.

Bill

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Originally Posted by convert2diesel

I have to assume you are using the old (88) computer. The tps had only two functions when mated to the 700R4. Controlled the EGR (if installed) and commanded the lock up. Your shift patterns are controlled with a throttle valve in the tranny connected via a cable to the throttle quadrant.

The lockup is activated any time you go into 2nd gear or higher and the throttle position is less than 1/3 to 1/2 throttle position (33% to 50%). If you want to go to full manual control of the lockup, you're going to have to disconnect the tps altogether and or the computer and install a switch in the transmission ground position of your data port (I think it is the terminal on the far right of the OBD connector under the dash). The switch will override any command from the computer.

As I said in my previous post you are going to have to be vigilant as you don't want to have the tranny going through the gears with the convertor locked up. Your brake switch will still unlock the convertor as before. Ran the Buick with this setup for close to 100K without issue, after I got used to unlocking it when appropriate. You're also going to need a piece of electrical tape to stick over the check engine light :rolleyes:

I would try a new TPS first. You may find your issues disappear. Try the cheap fix first.

Hope this helps.

Bill

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Those TPS switches sure aren't cheap. I'm seeing prices of $100-$150+. Does that sound right?

By cheap fix, are you referring to the Bypass Switch? Unplug tps, connect wire to far right terminal of obd data port then to toggle switch on dash then to ground?

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.

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Originally Posted by 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.

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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 ...

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Originally Posted by 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.

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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

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.

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.