IAT affecting trans?? [Archive] - GrandAmGT.com Forum

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sbart385
05-05-2009, 02:11 AM
So I have been searching around and noticed that some people have made comments about the IAT affecting shifting from 1st to 2nd. Just curious if anyone knows why or how the IAT actually affects the tranny. I just put in a tranny from the junkyard from an 05 GA into my 99 GAGT and now it is hesitating shifting from 1st into 2nd. Would that be the fluid level or the IAT? Any input would be greatly appreciated.

AaronGTR
05-05-2009, 03:18 PM
The IAT does not affect shifting at all as far as I know. I've never read anything or heard anything about it having anything to do with it. The computer uses the MAF sensor as part of it's calculation for determining engine load and adjusting the line pressure for shifts.

OneFstGt
05-05-2009, 08:21 PM
Yes it can affect shifting, cause my car was shifting weird and the wire was broken into.
Replaced the harness and its back to normal.

PhantomLover007
05-05-2009, 08:36 PM
Yes it can affect shifting, cause my car was shifting weird and the wire was broken into.
Replaced the harness and its back to normal.



I hope you find out who broke into it... lol lol



Hey Shane. What's w/ the tape in your av?

juanvan
05-05-2009, 08:58 PM
damn copper must be going up - breaking into wires now

sbart385
05-05-2009, 09:53 PM
Just went and looked and one of the wires is broken off my connector for my IAT sensor... guess thats another thing getting replaced..

The tape is cuz I busted the clips that hold the ram air duct on so I busted out the trusty dusty Duct Tape lol... as the saying goes... "If you cant Duck it.... **** it!"

ga2001gt532
05-05-2009, 10:18 PM
i'm pretty sure the computer uses the iat to tell the tranny what temp it is outside or tries to anyways. once it gets dirty or broken wire it can't do that no more then i'm pretty sure the comp tells the tranny to up the pressure so the tranny shifts harder.

AaronGTR
05-05-2009, 10:55 PM
i'm pretty sure the computer uses the iat to tell the tranny what temp it is outside or tries to anyways. once it gets dirty or broken wire it can't do that no more then i'm pretty sure the comp tells the tranny to up the pressure so the tranny shifts harder.

Why would the tranny need to know what temp it is outside?


Look people, I double checked in my manual and I was right. The PCM does NOT use the IAT signal to adjust shift pressures. A lost IAT signal may seem to affect how the tranny shifts, but only as a by product of lost torque from timing reduction. Not from any direct calculation of shift pressures.

The PCM only uses the IAT to determine ignition timing. It also uses the signal from the ignition module that the coils sit on (which receives signals from the cam position sensor, and both crank sensors), the coolant temp sensor, MAF, throttle position sensor, vehicle speed sensor, and trans range inputs from the transaxle range switch. The last one it uses because the pcm wants to know how close to a shift you are, because it has built in software for torque reduction (less timing advance) during shifts.

Now the pcm has two modes. IC mode is when it is receiving all the above signals and the pcm is controlling timing advance. If it loses one of those signals, it goes into bypass mode, where it will still allow the fuel injectors to fire and the engine to run, but on a base ignition advance always at 10 degrees BTDC. This would result in poor engine performance and great loss of power, making for "soft" feeling shifts because the engine isn't pulling hard. ;)


If you're thoroughly confused now, all you really need to know is this.... if your IAT sensor is damaged or the harness is cut so that you aren't getting a signal, performance is going to be lost and of course you need to fix the sensor. It's not affecting your trans though.

OneFstGt
05-05-2009, 11:03 PM
I hope you find out who broke into it... lol lol



Hey Shane. What's w/ the tape in your av?

Thats messed up...lol

sbart385
05-06-2009, 01:36 AM
Thanks for the info on the IAT Aaron. Now does anyone know what could be causing the delay or hesitation when shifting from 1st to 2nd? It almost feels like I am taking my foot off the gas then putting it back on only I am not moving my foot from the gas. I am stumped on this one.

skifreak122
05-06-2009, 07:44 AM
Just went and looked and one of the wires is broken off my connector for my IAT sensor...


Fix this first, then do more diagnostics.

ga2001gt532
05-06-2009, 03:48 PM
opps i was totally wrong but it could indeed be that sensor still or its just your tranny slipping. mine kind of did that so after i hammered on it for a while it stopped i think the clutch got glazed up for some reason.

sbart385
05-06-2009, 05:52 PM
I fixed the IAT connector and it is still hesitating when shifting. Could if have something to do with the fact that the transmission is out of a 05 GA and is in a 99 GAGT? It doesnt really bother me that much I just dont want to have to drop the damn tranny again to fix it or replace it.

Nick M
05-06-2009, 10:02 PM
When the IAT or its wiring harness fails the temp. will default to a very low temperature (somewhere around -40). That will cause more power to be delivered to the engine via rpm, gas, etc. Shifting will be very hard from gear to gear, you will feel it in the seat. Your problem sounds different. If you can get a scanner that reads sensor temperatures you can verify that the correct temp is being read by the IAT and eliminate that as a possible cause of your problem.

rocketfast123
05-07-2009, 02:55 PM
Any cheack engine lights? Vacuum leaks?

sbart385
05-07-2009, 04:29 PM
yeah just started getting the SES light. Pulled the code and its a P1810.

AaronGTR
05-07-2009, 05:21 PM
That might have something to do with your transmission problems. P1810 is a code for the TFP (transmission fluid pressure) Valve Position Switch Circuit.

Here's the circuit description from the manual.
"The automatic transmission fluid pressure (TFP) manual valve position switch consists of six pressure switches. Five of the switches are normally open, and determine gear range selection. The sixth switch is normally closed, and detects torque converter clutch (TCC) release fluid pressure. The TFP manual valve position switch also contains the automatic transmission fluid temperature (TFT) sensor. These components are combined into one unit, mounted on the valve body.

The PCM provides battery voltage on the three range signal circuits. By grounding one or more of the switches with fluid pressure from the manual shift valve, the PCM detects the selected gear range. When you disconnect the transmission electrical connector and the ignition is ON, the ground potential for three range signals from the PCM will be removed, and an invalid gear will be indicated.

When the PCM detects and of the following conditions, DTC P1810 sets.
-an invalid gear state
-a drive range after start up
-an incorrect gear ratio"

So basically is sounds like a problem of the pcm not receiving the correct signal for the gear it's supposed to be in. That would definitely cause some screwy shifting. You say you are using a newer transmission in a '99? GM made a bunch of improvements to that trans over the years up until about the beginning of 2004. How long have you been using that trans? It sounds to me like either there is some type of compatibility issue (no clue what it could be), or there is a faulty component inside the trans.

sbart385
05-07-2009, 05:54 PM
I have been using that trans for less than a week now. As far as I know there was nothing wrong with this trans when I got it from the junkyard. The car that it came out of was in a front end collision but neither the engine or trans were damaged. Other than the weird shift between 1st and 2nd and the SES light there have been no issues with it.

sbart385
05-11-2009, 01:39 PM
So new things keep popping up... Lately after I have been driving for a while it starts to shift really hard through all gears. Usually if I catch it early enough and can stop at a light I can shift it to reverse for a couple seconds and it goes back to being fine. If not then if I shift it through the gears it slams into each gear.

Any ideas what could be causing the new issues?

sbart385
05-12-2009, 10:59 PM
Anything from anyone? I need some ideas here cuz I am stumped and dont know what else to look at.

AleroB888
05-13-2009, 12:31 AM
Anything from anyone? I need some ideas here cuz I am stumped and dont know what else to look at.

Like Aaron GTR said, could be some kind of compatibility issue with the PCM. For example, I bought an extra PCM at a salvage yard out of a GrandAm GT that caused similar problems to what you have (after running awhile), but it worked fine when I reflashed it with the stock Alero calibration.
Make sure the wiring diagram for an '05 model is exactly the same as your '99, to start with. Being able to scan it with a DHP or HPTuner seems necessary at this point. Coud be a high resistance in one of the electrical circuits connecting the tranny to the PCM.
Try disconnecting the battery for an hour or so, and see if it shifts normally afterward, and if it changes as time goes on.