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2001 3.8 ls common no temp gage reading problem code p0128
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speedymouseman
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PostPosted: Sun Jan 03, 2010 3:11 pm    Post subject: Grrrr ! 2001 3.8 ls common no temp gage reading problem code p0128 Reply with quote

Ok, this problem is all over the web,

my 2001 3.8 LS temp gauge does not move . i am not over heating. i do have heat and A.C.
this is where im at so far. does anyone have a concrete fix for this? is it a gauge driver pcm, or gauge? (fans wont run in default mode from broken gauge or will they?

1- i just got the engine light.
2- code p0128
3- reset the fuse and battery for 12 hours. no luck, gauge at bottom.
4- re-seated the grounds above headlight for metal to metal contact.
5- tested the sensor 1.3 k ohms cold and 44 ohms hot. it looks good so far.
6- installed a new temp sensor for giggles on harness (instead of taking it out of the block) and stuck it in a cup of boiling water to trick the computer. no gage reading. by doing this i eliminated the thermostat being a problem. it does open and close by feeling the hose when engine reached tepmerature.
7- 5 volts at connector going to the pcm.
8- fans run while engine is running an about a minute when engine turs off----fail safe.
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Bill58
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PostPosted: Sun Jan 03, 2010 6:25 pm    Post subject: Grrrr ! Re: 2001 3.8 ls common no temp gage reading problem code p0 Reply with quote

speedymouseman wrote:

6- installed a new temp sensor for giggles on harness (instead of taking it out of the block) and stuck it in a cup of boiling water to trick the computer. no gage reading. by doing this i eliminated the thermostat being a problem. it does open and close by feeling the hose when engine reached tepmerature.

Does the sensor get ground via the block or by the harness/connector? I don't know, I'm just asking.
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98ss
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PostPosted: Sun Jan 03, 2010 6:45 pm    Post subject:   Reply with quote

I got the same code...replaced the t-stat, and it solved my problem. I dont remember if the 3.8L has a sensor and a switch. Other GM vehicles have a coolant temp sensor like the one you tested by the T-stat housing that goes to the pcm, and a temp switch which is often screwed into the side of one of the heads that goes to your guage on the dash.

If it were me I would just replace the t-stat and see what happens.
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speedymouseman
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PostPosted: Sun Jan 03, 2010 7:02 pm    Post subject:  ground for sensor Reply with quote

the sensor at the stat is 2 wire black and yellow.
the sensor resistance is the same for the old and new sensors. thats why im assuming its not the sensor.
i first did this with the sensor ungrounded in a hot cup of water.
then i grounded the sensor with alligator clips to make sure anyway.
does anyone know if there is even a sensor for the gauge or does it get it from the pcm/bcm? if so, where? i dont see it glancing in a repair manual.
people are saying "change the stat" but wasn't i by-passing the stat with a hot cup of water simulating a hot engine?
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98ss
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PostPosted: Sun Jan 03, 2010 7:13 pm    Post subject:   Reply with quote

Either way...that sensor definetely goes to the PCM, the code it threw told you your t-stat is on its way out. It will take you like 30 minutes if you pause and have beer breaks.

Yes, just checked, there is a temp sending unit...that goes to your guage.

So to fix your 2 problems, get a t-stat and a temp sending unit.
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speedymouseman
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PostPosted: Sun Jan 03, 2010 7:19 pm    Post subject:  temp sending unit? Reply with quote

i know of the coolant temp sensor under the t stat. (goes to the pcm)?
but there is another, going to the gauge??? and where????? Confused
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xnick
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PostPosted: Sun Jan 03, 2010 7:21 pm    Post subject:  Re: temp sending unit? Reply with quote

speedymouseman wrote:
i know of the coolant temp sensor under the t stat. (goes to the pcm)?
but there is another, going to the gauge??? and where????? Confused


I would assume the PCM sends that data from the sensor.

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speedymouseman
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PostPosted: Sun Jan 03, 2010 7:29 pm    Post subject:  sensor Reply with quote

ok, maybe i didnt clear the code by doing the hot water trick, but would it still not let the gauge move up if the code is still stored ????????? i seriously dont think its the sensor or the stat but my cheapest bet is to change them both.
this way i can bring it to the dealer if that does not work after clearing the code manually..
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98ss
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PostPosted: Sun Jan 03, 2010 7:39 pm    Post subject:   Reply with quote

You have to go thru a a couple drive cycles to clear the code. But its not going to clear if your T-stat is not getting up to temp fast enough.

Look for a sensor that attaches to the head via screw in connection...It will have 1 wire, a green wire. That sensor controlls your guage.
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speedymouseman
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PostPosted: Sun Jan 03, 2010 7:48 pm    Post subject:   Reply with quote

i cant find a one wire sensor coming from the engine block.
does anyone know where it is on a 3.8 engine?
i cant find a sensor with a green wire in the repair manual for a 3.8 engine, only for a 3.4..
IF there is ONLY ONE coolant sensor, why is the gauge not registering? its at zero. (in the hot water?)
shouldnt the gauge register regardless of a stored code? im thinking maybe its a way to alert the driver if there is a code stored as well as activating the engine light.


Last edited by speedymouseman on Sun Jan 03, 2010 7:55 pm; edited 1 time in total
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98ss
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PostPosted: Sun Jan 03, 2010 7:50 pm    Post subject:   Reply with quote

There are 2 sensors, independant of each other...one controlls the gauge and one controls the PCM.

Just pulled out he wiring diagrams, yellow and black go to the sensor to the pcm, green goes to the temp switch/sender and goes to the guage.
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speedymouseman
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PostPosted: Sun Jan 03, 2010 7:57 pm    Post subject:   Reply with quote

well thank you!
but now i need to look harder for this green wire.
i cant wait to freeze in this weather to look for it.
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98ss
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PostPosted: Sun Jan 03, 2010 8:00 pm    Post subject:   Reply with quote

I think its on the rear head, sounds like a pull it in the garage and pull the space heater out, have a beer and come back out.
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PostPosted: Sun Jan 03, 2010 8:26 pm    Post subject:   Reply with quote

98ss wrote:
There are 2 sensors, independant of each other...one controlls the gauge and one controls the PCM.

Just pulled out he wiring diagrams, yellow and black go to the sensor to the pcm, green goes to the temp switch/sender and goes to the guage.


Incorrect. I've torn that engine down and there is no separate temp sensor. The temp gauge is fed by information sent over the OBD2 bus by the PCM. The PCM does measurements by the temp sensor installed in the LIM just under the thermostat.

If anything the temp sensor itself is a dual sensor. I've seen that before, but regardless there is only one.

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98ss
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PostPosted: Sun Jan 03, 2010 8:32 pm    Post subject:   Reply with quote

I hope he's not out there right now freezing, my wiring diagrams are wrong then. stupid haynes.
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