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Old 10-16-2023, 03:59 PM
 
Location: NE Mississippi
25,552 posts, read 17,256,908 times
Reputation: 37264

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On my 1990 K1500 with its 5.7 liter engine:
Is the coolant temperature sensor that feeds the ECM the same one that feeds my dash indicator? Or are there two different sensors?
I am assuming the one beside the thermostat provides the input into ECM.


For all who may not know: The 90 GMC uses OBD I, which gives very little information. It gives so little in fact, that it hardly gives any at all.


Why I am asking: I have a high idle problem. Not bad, just too high. 1100 or so. I will replace and reset the Idle Air Control (it's the original), but always like to plan step #2 in advance. So it could be that the computer never gets the message that the truck is all warmed up... maybe?


Thanks
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Old 10-16-2023, 05:06 PM
 
Location: on the wind
23,250 posts, read 18,764,714 times
Reputation: 75145
https://www.city-data.com/forum/chev...lac-buick-gmc/
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Old 10-16-2023, 06:28 PM
 
15,793 posts, read 20,472,889 times
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No separate sender/sensor.

The temp switch for your cluster is located between cylinder 1&3 on driver's side, and the ECU temp sensor is next to the tstat housing.


If you yank the IAC plug with the engine running, does the engine idle drop? Or does it still stay high? If the latter, it may be a sign of a vacuum leak, or the idle set screw is set too high. If this is a recent change, likely not the idle screw.

I admit, I know Ford OBD1/EEC4 like the back of my hand, so I try to cross-reference what I know what that EFI system over to the contemporary GM counterpart. So i'm troubleshooting by way of what I'd do on a Ford 302.
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Old 10-16-2023, 09:01 PM
 
Location: Western PA
10,809 posts, read 4,506,581 times
Reputation: 6662
Quote:
Originally Posted by Listener2307 View Post
On my 1990 K1500 with its 5.7 liter engine:
Is the coolant temperature sensor that feeds the ECM the same one that feeds my dash indicator? Or are there two different sensors?
I am assuming the one beside the thermostat provides the input into ECM.


For all who may not know: The 90 GMC uses OBD I, which gives very little information. It gives so little in fact, that it hardly gives any at all.


Why I am asking: I have a high idle problem. Not bad, just too high. 1100 or so. I will replace and reset the Idle Air Control (it's the original), but always like to plan step #2 in advance. So it could be that the computer never gets the message that the truck is all warmed up... maybe?


Thanks

they are different things. the gauge sender is 1 wire, ECT is 2. have a heart to heart talk with the C3 system using an old scan tool find like the actron cp9110? (get one off ebay and the atari like cart for cheap. I keep mine around cuz I still have something that speaks to it... no vacuum leaks?
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Old 10-16-2023, 09:02 PM
 
Location: Western PA
10,809 posts, read 4,506,581 times
Reputation: 6662
Quote:
Originally Posted by BostonMike7 View Post
I admit, I know Ford OBD1/EEC4 like the back of my hand,

a lost art. nothing better pre-2.
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Old 10-18-2023, 03:32 PM
 
Location: NE Mississippi
25,552 posts, read 17,256,908 times
Reputation: 37264
Quote:
Originally Posted by RetireinPA View Post
they are different things. the gauge sender is 1 wire, ECT is 2. have a heart to heart talk with the C3 system using an old scan tool find like the actron cp9110? (get one off ebay and the atari like cart for cheap. I keep mine around cuz I still have something that speaks to it... no vacuum leaks?
OK. I'm straight. Almost.
I see the 2-wire and I found the 1-wire. The 1-wire sensor obviously works because I can see the engine heat on the dashboard. The 2-wire sensor is old and cheap so I'll replace it as a matter of course.


But I don't know what the C3 system is... And I don't know what sort of information I might get from Actron cp9110 - I found several of them for sale, I just don't know why I would buy one.
So what am I missing..


Vacuum leaks: Always a possibility in a 33 year old truck. There aren't all that many vacuum hoses so I will just replace them all as I go along. They are all original. I have a hand pump that I'll use to test diaphragms; there aren't very many of those, either.
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Old 10-18-2023, 04:36 PM
 
Location: Not far from Fairbanks, AK
20,292 posts, read 37,157,521 times
Reputation: 16397
Quote:
Originally Posted by Listener2307 View Post
On my 1990 K1500 with its 5.7 liter engine:
Is the coolant temperature sensor that feeds the ECM the same one that feeds my dash indicator? Or are there two different sensors?
I am assuming the one beside the thermostat provides the input into ECM.


For all who may not know: The 90 GMC uses OBD I, which gives very little information. It gives so little in fact, that it hardly gives any at all.


Why I am asking: I have a high idle problem. Not bad, just too high. 1100 or so. I will replace and reset the Idle Air Control (it's the original), but always like to plan step #2 in advance. So it could be that the computer never gets the message that the truck is all warmed up... maybe?


Thanks
There are some very large GM/Chevy forums that may be of help. I sent you a PM.
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Old 10-18-2023, 04:57 PM
 
Location: Western PA
10,809 posts, read 4,506,581 times
Reputation: 6662
Quote:
Originally Posted by RayinAK View Post
There are some very large GM/Chevy forums that may be of help. I sent you a PM.

full size chevy tend to be jerks. trust me I know many of them from carlisle lol
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Old 10-18-2023, 05:02 PM
 
Location: Western PA
10,809 posts, read 4,506,581 times
Reputation: 6662
Quote:
Originally Posted by Listener2307 View Post
OK. I'm straight. Almost.
I see the 2-wire and I found the 1-wire. The 1-wire sensor obviously works because I can see the engine heat on the dashboard. The 2-wire sensor is old and cheap so I'll replace it as a matter of course.


But I don't know what the C3 system is... And I don't know what sort of information I might get from Actron cp9110 - I found several of them for sale, I just don't know why I would buy one.
So what am I missing..


Vacuum leaks: Always a possibility in a 33 year old truck. There aren't all that many vacuum hoses so I will just replace them all as I go along. They are all original. I have a hand pump that I'll use to test diaphragms; there aren't very many of those, either.

C3 = computer command control (3 C's get it? lol) is the OBd1 ECM that was in constant revs EVERY engine family


the CP9110 box and the GM 'atari cartridge' are kinda stupid, you have to tell them some vin stuff and year stuff as the ECM didnt push this info, but it will read all the code and read live data when running. It will tell you where the TPS, MAP, BARO, RPM, timing advance, O2S etc are at for any minute


test for a vacuum leak FIRST....put a gauge on an intake manfold port and look for at least 17 inches at idle at least.


then use the underhood VECI label and trace all the lines - make sure they are a) there and b) not rotted split.


If the vacuum is low - intake leak or a REAL BAD TBI unit leak.


btw - the IACM tries to get that idle down, if it stays high too long, it does set a code....
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Old 10-18-2023, 05:05 PM
 
Location: Eastern Washington
17,208 posts, read 57,041,396 times
Reputation: 18559
Only tangentially on topic, but I do know that BMW and Toyota OBD-I systems are pretty informative *if* you have the "decoder sheet" for that model. A factory service manual might have that for your Chevy.

OBD-I systems are put into a "read" mode by a jumper wire (Toy) or by turning on ignition, but not starting, and "patting" the throttle 5 times (BMW). At that point the Check Engine Light will flash out "dot and dash" like Morse code, you use the decoder sheet to figure out what's going on.
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