Eliminate the TPS clamp?

[beatle]

There's an interesting thread (http://forum.miata.net/vb/showthread.php?t=347647) over on m.net regarding the PC Pro and a Moss MP62 kit. Granted it's not one of your kits, but it is about the PC Pro. Ben mentioned a few things about the TPS clamp function that have me scratching my head. I know that the O2 clamp helps low rpm drivability in the 94-97 cars now :) but what if the TPS clamp were eliminated? Is this something you guys tried earlier on in the development of the kit and found it to be worthwhile? If so, why?

[FormerDatsun510Man]

The TPS clamp and the O2 clamp serve two different purposes.

The TPS clamp is the method by which we ensure that the stock ECU will go Open Loop as soon as boost is encountered. This is the preferable way of getting the engine to go rich because it occurs very quickly and then we can add any amount of fuel we wish. Think of the TPS clamp as a proactive method of causing the stock ECU to allow the engine to run rich as soon as boost is made. Remember, when making boost on a stock ECU, Open Loop is really what you want. Basically, that makes what fuel adjustments you make like that of a standalone since there is no funny business from the ECU.

The O2 clamp is the method by which we prevent the stock ECU from maintaining a 14.7:1 A/F when it is still in Closed Loop, yet the engine is making boost (however little). This is not the preferable of a way because it is reactive and basically entails shifting the O2 voltage, when in boost, so that the stock ECU "thinks" 12:1 = 14.7 (approximate). In other words, it takes the stock ECU a few cycles of feedback operation with the O2 sensor to actually pull the AFR from 14.7:1 down to 12:1. The question is then, why would we want this at all? Well, the answer is we don't unless we have to :). That situation is only when the TPS clamp is not enough to cause the stock ECU to go Open Loop. I have found that this annoyance only occurs on the 1st gen Miata when the rpm are below approximately 3500rpm.

I have tried testing on my '00 Miata running without the TPS clamp and instead using the O2 clamp. I have also tried running it without either. I found that without the TPS clamp, even when using the O2 clamp, that the throttle response got less responsive and also that the Accel Pump function didn't seem to have much if any effect anymore. I could see this with the response time of the fuel going rich with my wideband AF gauge. Makes sense, because the stock ECU would still be in Closed Loop during this transition point into boost. Even with the O2 clamp it takes up to a second to enrichen the fuel, whereas with the TPS clamp it is very nearly instantaneous.

You are certainly welcome to try bypassing the TPS clamp if you wish to try. Just hook up the in and out wires for "B" on the Main Card on the stock ECU side. Perhaps I could make a few test files for those of you with PC Pro programmers that wish to experiment without having to cut any wires.

I will also add that on our program used on the PC Pro we have not seen that condition of the car going way rich at part throttle. As you guys have seen, we have them pretty well tuned now. The only issue is that with the TPS clamp hanging on the '01+ models on certain cars. That is something I am investigating.

Bill

[Race Grandpa I]

Bill,

One thing about the TPS on my car is that in the past the CEL came on indicating that the TPS was not functioning properly. The CEL came on and off at irregular intervals. So I put in a new TPS and later on the same thing began to occur. Took it to the dealer and they made an adjustment to the cable pulley, which took care of it.

I will never fool around with the TPS. That was too expensive.

Now my purge selenoid occassionaly starts to click when the clutch is disengaged at stop lights. When I put it in neutral and engage the clutch the clicking stops. Now figure that one out. :biggrin:

[FormerDatsun510Man]

It sounds to me like the Black Hornet is suffering from a case of indigestion!

[Race Grandpa I]

It sounds to me like the Black Hornet is suffering from a case of indigestion!

I think I bought a haunted car. Sort of like that 56 Chevy in the movie where the car takes control by locking the doors and windows, turning on the radio and driving away with the driver in the car.

Or maybe an :alien: mole is beginning to slowly take control of my car.

[Mark]

On my 95, I occasionally get an ECU trouble code 15. Research says this is "Heated oxygen sensor (inactivation)". I believe this means that the ECU is not seeing enough voltage from the sensor. Would the O2 clamp affect this?


And for the record, Christine was a 58 Plymouth Fury! Great book!!! :)

[beatle]

Mark, it sounds like the O2 sensor heater circuit is not working. This helps get the O2 sensor up to temperature faster than just letting the exhaust gasses do the job. It also helps maintain a steady temperature in case it's not warm enough. How old is your sensor?

I suppose it's possible that the O2 clamp could affect this since it sends a lower voltage signal to the ECU, indicating a lean condition which would cause the ECU to add fuel.

Bill will probably be here in a second to confirm/deny this hypothesis. :)

[FormerDatsun510Man]

The heater circuit is a separate pair of wires so my answer is pretty simple there... if the code is telling you the O2 sensor is not heating up as quickly as expected this means those heater wires (or more likely in the O2 sensor itself) are going south and it is time to replace the sensor.