Air Pressure Sensor Drift?

MacShee98 · 04-06-02, 10:30 AM

Question: Anybody know if the TPS monitors (in wheel) can be calibrated, or are they NON-adjustable. I realize what a pain it would be to do so (I assume it would be a wheel-off, tire-off rim thing, but having had way too much coffee this morning, I just thought I'd ask.)

After 5000miles, the TPS sensors seem to be drifiting a bit from where they were when new. New, they all read the same, 30psi cold, 33psi warm.

After careful adjustment of the cold inflation pressures (all 4 tires set to 30psi at 60 deg Fahrenheit), my WARM readings are now typically:
Front L=32
Front R=33
Rear L=31
Rear R=32

Rechecking the pressures with an Accu-Tire digital and a pencil-type TP gage when WARM, they are all about 33psi, despite the above dash readings.

My money is on c4c5Specialist for the answer to this one.

Thanks everybody! And drive safely!

c4c5specialist · 04-06-02, 05:56 PM

Hithere,
Ok, well, lets start with the sensor functions, and then proceed.
First, the tire pressure monitoring system, or TPM, consists of the following.
The receiver, the 4 sensors, and the DIC.
Now, the sensors will always transmit when the vehicle is rolling more than 10 mph. However, the key to accuracy falls on the PCM, which transmits a barometric pressure reading, at key on, to the TPM receiver.
Now, GM gives us a tolerance of + or - 2 psi, as a standard check. That, with the sometimes interrupted data signal to the TPM receiver, may sometimes show a slightly inaccurate tire pressure.
The sensors will not and cannot recalibrate the TPM signal for pressures. This is strictly done by the barometric pressure from the PCM at key up.
So, with that said, I would say that maybe you might have a TPM receiver that might not be compensating just quite right, or a slight deviation in signals from the sensors to the given pressure.
Honestly, you are still within spec, and I would consider this a normal characteristic of your system, if you were to deviate more than 3 psi, then I would attempt a diagnostic check of the system.
However, in this case, this is simply c5 traits, and I would leave it alone.
Hope this helps, and you got your money back, besttoyou, c4c5

danl72 · 04-06-02, 09:12 PM

c4c5specialist is correct as usual. My sensors will say between 31-33 psi when cold, then when they are warm they will say between 33-36. It will vary, but I am not worried because I know there is + or - tolerance. I still think they are pretty acurate and I would rather have them than not have them.

Dad · 04-06-02, 11:04 PM

Here's what I have done to re-calibrate (actually) it's "program the sensors" in the service manual lingo) the sensors is a magnet to engage sensors during the training sequence. The GM tool is J41760, but maybe a decent powered magnet with a horseshoe shape or hole in it would suffice???

Make sure vehicle has been stationary for at least 2 minutes.
1 - Ignition on
2 - Press RESET to clear all IPC messages from DIC
3 - Press and release OPTIONS, scroll til IPC is blank
4 - Press and hold DIC RESET for 3 seconds
5 - Press OPTIONS again until TIRE TRAINING message is displayed
6 - Press RESET until IPC LEARN L FRONT TIRE to begin programming
7 - Install J 41760 over left front valve stem (I guess you could try a hefty magnet now)
8 - The horn will sound indicating LF sensor is programmed
Notice - if after 15 seconds the horn doesn't sound, remove and reinstall tool on stem. This may have to be done up to three times in some instances.
9 - IPC will direct you to LEARN R FRONT TIRE, R REAR TIRE, then L REAR TIRE.

Programming Cancellation:
1. Programming mode will cancel if any of the following conditions are met:
- program mode exited through DIC
- ignition turned off
- all four sensors programmed
- TPM program mode for 2 minutes with no sensors learned
2. If cancelled with less than 4 codes stored, the receiver will only accept the codes programmed up to that point.

I've had alot less variation since I reset them last year. Just setting in the sun will throw them off 3 or 4 degrees from one side to the other. Starting out from a warm garage to road tempeture will change 6 degrees in short order.

Sensor numbers
1997 - 2000 10438853
2001 - 2002 25648293

There you have it, good luck.

MacShee98 · 04-07-02, 08:04 AM

Thanks! If I can find a suitable magnet, I might just try that. But what will be suitable do you think? How powerful does the magnet have to be?

Has anyone out there done this without the J41760 tool, using just a simple magnet?

Thanks c4c5Specialist, thanks Dad. You guys are great!

c4c5specialist · 04-07-02, 08:23 AM

Hi there,
Sure, you can purchase a simple magnetic pickup tool from the local auto parts store.
A large one will be strong enough.
Besttoyou, c4c5

Dad · 04-07-02, 09:21 AM

I had 2 stick magnets that I held in a "V" and set them over the valve stem. I was so sure it wouldn't work that the horn scared the devil out of me when it sounded. By the way, it has been almost a year and a half since I have had to add any air to these tires. Good luck.

HallenTi · 06-09-06, 08:11 PM

Here is my story... before I begin, while I would like the tire pressure sensors to be totally accurate, I understand that they just can't be 100% accurate, and they don't really need to be. Just "close" is fine.

Anyway. I have a 1999 coupe. After about 45 minutes of driving, I compared the readings on the DIC for each tire to the actual reading from a known-accurate gauge. Here is what I found: (shown below as DIC/actual)

LF: 38/33
RF: 38/32
LR: 40/36
RR: 42/37

The actual cold pressures of the tires were 30 PSI. About 45 days ago, the LR sensor went out (battery died) and I purchased a brand new one from GMPARTSDIRECT and had it installed, along with a new set of four Goodyear EMT tires (OEM type). Afterward, I trained the sensors and they all seemed to train properly.

I took the old sensor apart, purchased the proper battery for it and installed it into the old sensor. This rebuilt sensor was just installed into the RR tire this week. I retrained them as I had before.

The brand new GM sensor (LR) is inaccurate by about 4 PSI, and the rebuilt one is inaccurate by about 5 PSI. The two front units are innacurate by 5 and 6 PSI as well. I wouldn't mind it really, but often times I get a "high pressure" warning for the RR tire. I do check the tires manually pretty often, so know they are not what the readings would indicate.

C4CSpecialist indicated that the PCM is responsible for the accuracy of the reading(s), so I am wondering if removing power from the system for a period of time, then retraining the pressure sensors would bring them closer to the actual pressure?

As a side note: the black plastic casting on the sensors has a molded-in "clock" type dial into the plastic, which indicates the manufacturing year and month of manufacture. On the one I just removed from the RR, the "arrow" points to "7" which indicates July, and around the "arrow" is "98" which indicates 1998. This means that the battery lasted almost 8 years. The other one I replaced indicated 9/1998.

Tim

c4c5specialist · 06-10-06, 07:31 AM

Quote:

Originally Posted by HallenTi

Here is my story... before I begin, while I would like the tire pressure sensors to be totally accurate, I understand that they just can't be 100% accurate, and they don't really need to be. Just "close" is fine.

LF: 38/33
RF: 38/32
LR: 40/36
RR: 42/37

C4CSpecialist indicated that the PCM is responsible for the accuracy of the reading(s), so I am wondering if removing power from the system for a period of time, then retraining the pressure sensors would bring them closer to the actual pressure?

Tim

HI there,
From what you are saying here, I would suspect your receiver is NOT working correctly to process the information from the PCM to give correct barometric pressure. The sensors are very accurate, +-4 is too much.
This would give you the inaccuracy that you are experiencing.
If there was a problem with the engine MAP sensor, you would experience PCM codes and a check engine light.
Allthebest, c4c5

Mr. Lucky · 06-10-06, 08:46 AM

For those of you with connections in the IT or computer repair industries, a free substitute for the GM tool may be had by using the "emergency retract" magnet from a crashed hard drive. The one I obtained is ~ 1.5" wide and shaped like a miniature brake pad. Let me tell you, these suckers are strong! I had one cupped in the palm of my hand and then happened to brush against a steel file cabinet with the back of my hand - it pinned my hand to the cabinet drawer.

I was able to reprogram all four wheels in less than a minute (once I freed my hand!).

HTH!

HallenTi · 06-10-06, 01:04 PM

No check engine light, and no codes (I checked the diagnostic mode), so sounds like the receiver. So I can see what I might have to pay for a new receiver, what is the receiver actually called (could it be as simple as "tire pressure sensor receiver")? :-) Also, while I am asking, where is it located (I can check my ALLData account for he location once I knwo the actual name actually). Thanks!