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Old 06-09-2012, 06:10 PM
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Flat Tire Monitor Light keeps coming on. What gives?

Checked all the tire pressures and they are dead nuts on. The light only comes on after I have been driving on the highway for an hour or so. Is there that much expansion due to heat? Could it be a wheel speed sensor? I'm baffled. TIA.
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Old 06-09-2012, 07:26 PM
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I had the same once. 4.8is too. At one point I did the left right steering trick, and it went away. Go figure. Could have been a fault.
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Old 06-09-2012, 08:50 PM
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Quote:
Originally Posted by dinan e39 View Post
Checked all the tire pressures and they are dead nuts on. The light only comes on after I have been driving on the highway for an hour or so. Is there that much expansion due to heat? Could it be a wheel speed sensor? I'm baffled. TIA.
Yes, tires can gain considerable real psi at speed, but unless you have an 'over filled pressure alert', the usual/normal psi gain shouldn't trigger a 'low pressure' warning. Could be wheel sensors or, Slick's trick or...
GL, mD
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Old 06-10-2012, 01:21 AM
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Thanks guys, I'll try and check the DME for any stored codes.
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Old 06-10-2012, 07:36 AM
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Interestingly, I can drive by a business going home and within 30 sec to a minute of passing near that intersection the yellow tire alert would light up. Anytime I didn't drive around that area, nothing.

Would a certain RF signal emitted from that business match the tire sensor RF and trigger the yellow alert?
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Old 06-10-2012, 09:46 AM
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try to reset the TPM and see if it comes back?
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Old 06-10-2012, 04:27 PM
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Quote:
Originally Posted by dinan e39 View Post
Checked all the tire pressures and they are dead nuts on. The light only comes on after I have been driving on the highway for an hour or so. Is there that much expansion due to heat? Could it be a wheel speed sensor? I'm baffled. TIA.

We need to go back to how the "low pressure alert" works. It is measuring a difference between the average rotational speed, and the fastest rotational speed, at each wheel. Fastest rotation speed = lowest pressure according to the logic in the system.

When tires warm up, they will still have the same relative speed, as long as the pressure changes in each tire are relatively similar. That's the trick. Air expands at the same rate as each constituent gas (including nitrogen), but water vapour doesn't. If there is a higher percentage of water vapour or moisture in any one tire, it will expand more with heat (ie driving for an hour). Then, the other tires will appear to have low pressure.

Try releasing the air from each tire and then reinflating it, not with the weight of the car on the tire so as to avoid damaging the sidewall, and then see if it still happens. If so, it could be a bad wheel speed sensor, but it is easy to check the air first. Another way to check would be to set the tire pressures cold, and reset the alert system. Then drive for an hour, and check the pressures. See if they are all the same (higher, but the same). Don't bleed them down, you don't want to have them underinflated when cold, just look at the relative pressures, as a difference there is what sets off the alarm.
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Old 06-10-2012, 11:17 PM
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JCL, you continue to be the voice of reason on this site. I'll drain and inflate the tires to spec and report back. Thanks again to all for the advice.
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Old 06-11-2012, 03:24 AM
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Quote:
Originally Posted by dinan e39 View Post
JCL, you continue to be the voice of reason on this site. I'll drain and inflate the tires to spec and report back. Thanks again to all for the advice.
Remeber to reset the TPM after doing that.
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Old 06-11-2012, 04:59 AM
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JCL, thanks for another "I never thought o' that" moment..
EDIT: site does have 'helpful' buttons, I owe ya a hunnert..
cut: wish the site had
(thanx mD for the heads up, it's a wonder what old eyes can miss)
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