CODE 25 - 1990 CORVETTE (not ZR1)
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Instructor
Joined: Jun 2006
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From: Arlington Texas
CODE 25 - 1990 CORVETTE (not ZR1)
I have DTC 25 set. Someone told me that this is MAT (Manifold Air Temperature circuit. I changed the MAT sensor and disconnected the battery to clear the code, drove car to work and when I got home it has DTC 25 again. Any ideas? TIA
Melting Slicks
Joined: Aug 1999
Posts: 2,240
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From: Baltimore, MD USA
DTC 25 is Manifold Air Temperature sensor circuit High temperature indicated.
The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies about 5 volts to the sensor on the Tan wire. the Blk/Pink wire is sensor ground via the ECM.
When manifold air is cold, the sensor resistance is high, therefore, the ECM will see a high signal voltage. If the manifold air is warm, the sensor resistance is low therefore, the ECM will see a low signal voltage.
Code 25 sets if,
Signal voltage indicates a manifold air temp. greater than 302 degrees F for 12 seconds.
Engine has been running longer than 4 minutes or longer and vehicle speed is greater than 5 mph.
Here's a chart showing what the temp. vs resistance of the sensor should be.
210 degrees F =185 ohms
160 degrees F =450 ohms
100 degrees F =1.8K ohms
70 degrees F=3.4K ohms
40 degrees F =7.5K ohms
20 degrees F =13.5K ohms
0 degrees F = 25K ohms
-40 degrees F =100.7K ohms
If you were to unplug the two pin connector from the MAT sensor you should read zero ohms to ground on the Black/Pink wire.
With the ignition on and the MAT plug disconnected, you should read close to 5 volts from the Tan wire to the Black/Pink wire.
The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies about 5 volts to the sensor on the Tan wire. the Blk/Pink wire is sensor ground via the ECM.
When manifold air is cold, the sensor resistance is high, therefore, the ECM will see a high signal voltage. If the manifold air is warm, the sensor resistance is low therefore, the ECM will see a low signal voltage.
Code 25 sets if,
Signal voltage indicates a manifold air temp. greater than 302 degrees F for 12 seconds.
Engine has been running longer than 4 minutes or longer and vehicle speed is greater than 5 mph.
Here's a chart showing what the temp. vs resistance of the sensor should be.
210 degrees F =185 ohms
160 degrees F =450 ohms
100 degrees F =1.8K ohms
70 degrees F=3.4K ohms
40 degrees F =7.5K ohms
20 degrees F =13.5K ohms
0 degrees F = 25K ohms
-40 degrees F =100.7K ohms
If you were to unplug the two pin connector from the MAT sensor you should read zero ohms to ground on the Black/Pink wire.
With the ignition on and the MAT plug disconnected, you should read close to 5 volts from the Tan wire to the Black/Pink wire.
Thread Starter
Instructor
Joined: Jun 2006
Posts: 124
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From: Arlington Texas
Well I have always thought my car did run a little warmer than it should. I think the temp gauge can hit 240-250 degrees in this Texas heat (103 degrees as I am typing right now) if I get stuck in traffic with A/C on. But do you think my manifold is getting too hot? What would cause this? I have a 190 degree thermostat in my water neck. TIA
Melting Slicks
Joined: Aug 1999
Posts: 2,240
Likes: 42
From: Baltimore, MD USA
Are you positive you're getting code 25?
Did you pull the code by shorting A to B or
by shorting pin A to G.
If you used the A to B method it's easy to count the light flashes wrong.
Any ways the ideal method would be to use scanner software such as Diacom to look at the MAT sensor temperature. There are free scanner software packages available. You would need to make a cable to connect a laptop computer to the Diag. connector located above the drive side knee area.
I don't believe the temp. inside the Plenum is over 302 degrees F.
You've replaced the sensor so the only thing left is the wiring which I explained how to test in my post above or the ECM is bad.
Did you pull the code by shorting A to B or
by shorting pin A to G.
If you used the A to B method it's easy to count the light flashes wrong.
Any ways the ideal method would be to use scanner software such as Diacom to look at the MAT sensor temperature. There are free scanner software packages available. You would need to make a cable to connect a laptop computer to the Diag. connector located above the drive side knee area.
I don't believe the temp. inside the Plenum is over 302 degrees F.
You've replaced the sensor so the only thing left is the wiring which I explained how to test in my post above or the ECM is bad.
Last edited by Hooked on Vettes; Aug 18, 2006 at 07:43 PM.
Thread Starter
Instructor
Joined: Jun 2006
Posts: 124
Likes: 0
From: Arlington Texas
Unplugged MAT sensor connector.
Black wire to ground (Negative battery post) measures .3 ohms with ignition off.
Measured from Black wire to Tan wire inside connector with ignition on and I read 5.02 volts.
Black wire to ground (Negative battery post) measures .3 ohms with ignition off.
Measured from Black wire to Tan wire inside connector with ignition on and I read 5.02 volts.
