Hi y'all New to the forum but been a lurker for a while now. Well I purchased a 98 firebird a couple months ago and it's been a good ride so far(except for some things here and there that needed attending), however a couple days ago it started acting funny. It seems when I'm cruising in 4th at about 70-80 if I were to go wot it won't downshift to 3rd, instead it feels like it stays in 4th and is just gradually gaining speed.

If I decrease speed to about 40-50 and floor it, it will downshift normally and business as usual. Basically all the other gears are working in order but if I get to those highway speeds and punch it, it won't punch lol I've disconnected the maf and ran it to see if it was the cause, although it seems like it ran a little better the same problem occurred on the highway.

Anyone had any experience with this? I've searched but haven't really found a solution for it, I'm going to take it to the shop but I want to see if y'all had any ideas or suggestions, I've seen it may be the tps but I haven't tried testing it yet. From the previous owner I was told it has a 4l60e with a transgo shift kit, sonnax servos, sonnax .500 boost valve, and a New converter. Thanks for y'all time.

 

Bump?

 

I would want to get scanner on it and command that downshift, as well as other up and downshifts, and converter locking and unlocking to get hard verification of what's going on. Also log TPS, VSS, MAP, MAF and make sure those are all within spec. I agree a bad TPS is a simple and sensible explanation for the issue. But the cause may certainly lie elsewhere.

I'm moving this to the tranny section where you will get more input than the Firebird forum.

 

thanks for that havent logged in been a lil busy but anyways i went to a shop just to see what they would say but they were kinda busy, wanted me to leave the car and drop the trans. I took it home figured i would take a crack at it with the sensors. I first said screw it if i had to buy new ones to make sure they worked then its worth a shot but i had almost forgotten about cleaning the sensor. I went to autozone got the maf cleaner, took the maf out and cleaned it up, at the time i thought if it didnt work id replace both the maf and tps and if that didnt work it was back to the shop. I put the maf back on, drove it around town then the freeway, and it worked! Its been about 4 days now and i havent had the problem. Also to note the day before i did this i drove the car and when i would stop the car would start surging and within 30 seconds it would stall. Happened twice within 2 stop lights, now its cleared up. Thanks

 

Glad to fixed it yourself! Congrats.
As you have probably figured out now, the PCM/TCM completely controls the shifting and the shift points are determined by calculating the load from various engine sensors and the VSS.

 

Good for you! Why didn't I think to suggest that?! Simplest trick in the book for odd tranny behavior - clean your MAF!

 

This is all good info and the MAF cleaning was not obvious to me.
For one, I often run SD (no MAF) and my trans shifts the same with or without the MAF connected.

According to the trans shifting table in HP Tuners, the shift points are based on TPS. But perhaps it is strictly TPS % only in SD mode; else it is load % based on TPS and MAF.

In the future I will suggest not only checking the TPS, but also cleaning the MAF when the trans shifts erratically.

 

So guys been driving the car off and on since its been kind of rainy here in socal and i dont like driving it in the rain. Anyway it ran well the first 5 runs but today i decided to drive it to work and while on the freeway at about 65 70mph i noticed kind of a hiccup from the car, like if i hit the brake softly. As i kept going i hit traffic and slowed down to about maybe 5mph, bumper to bumper when i braked the idle dropped and almost stalled. I was about a mile from work so i pulled up and did my shift.

Coming home i tried to repeat it by going about 65mph but nothing this time, as i get off the offramp my SES light comes on and i figure its the evap p0442 code i been chasing down. I pull up to the house plug the scanner in and 1 code for the p0442 and low and behold, it pulled a p0121.... the damn TPS sensor lol i have a feeling it was it the whole time. I read somewhere that i can check with the scanner the volt that its producing if its bad but im still a lil new to the scanner and specefications of it all. Either way im hoping this is it

edit: ived looked a lil more into it, wouls this be just a replacement of the sensor? Im reading now that the sensor have to be calibrated or tuned after it is installed. Also i found a post on a camaro forum that kinda threw me off stating that this code refers to the predicted throttle angle doesnt match the actual. Here it is, any help appreciated. Thanks

P0121 sets when the predicted throttleangle does not match the actual throttle angle.
P0122 sets when the TP sensor signal voltage is less than 0.2 volts.

It sounds like you have a wiring problem ( broken wire somewhere). You can count on this low voltage being lower than normal closed throttle voltage. A bad ground would increase sensor output so rule that out. It is possible that your 5 volt reference voltage took a dive so carefully inspect the wiring to the sensor. To save weight, the wiring is smaller and is more prone to breaking inside the insulation. The telltale signs of internal breakage would be a bubble in the wiring insulation. You could also have had a loss of 5 volt reference from the PCM. If so, you would expect DTCs from other sensors using the same 5 volt reference. Usually this is an EGR DTC.