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Discussion Starter · #1 ·
1991 s10 5 spd manual 2.5
changed vss, drac, cluster (used off ebay, said it worked)
checked power, ground to cluster, all good
Question: could a bad PCM cause this issue?
Truck also has a rough idle that I cant diagnose
I have thrown almost every part at these issues with no luck.

please help
thank you.
 

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I had previously thought that all S10s through '92 had mechanical speedometers. Anyway, if you were able to find a way to connect to an OBD1 interface, you could see if the calculated vehicle speed is accurate. You would also be able to see if your air and temperature sensors are reading correctly. I often recommend that people do some tests on the EGR valve and the fuel pressure.
 

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Discussion Starter · #3 ·
I had previously thought that all S10s through '92 had mechanical speedometers. Anyway, if you were able to find a way to connect to an OBD1 interface, you could see if the calculated vehicle speed is accurate. You would also be able to see if your air and temperature sensors are reading correctly. I often recommend that people do some tests on the EGR valve and the fuel pressure.
I had previously thought that all S10s through '92 had mechanical speedometers. Anyway, if you were able to find a way to connect to an OBD1 interface, you could see if the calculated vehicle speed is accurate. You would also be able to see if your air and temperature sensors are reading correctly. I often recommend that people do some tests on the EGR valve and the fuel pressure.
Fuel pump and filter are new, 13 psi. EGR has been eliminated. Runs the same as it did before replacing / deleting these items. I suspect the 2nd instrument cluster may be bad as well, but also curious if the PCM plays a factor in the speedo/odo working.

Thanks
 

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In 1991 the only wire connected to the ECM related to the vehicles speed is brown wire connecting the DRAC to the ECM. It's only purpose is output from the DRAC to the ECM.
The wire that operates the speedometer is the lt.blu/blk from the DRAC to A11 on a digital dash or pin 14 with an analog gauge dash. In reality the digital dash receives an analog signal also. It's just the display that is digital.
Since you have replaced everything, try a close inspection of those wires, including the ones from the VSS to the DRAC for any damage, chaffing, or burned places. You should also test them for continuity because they can break inside the insulation. Just need an ohm meter and an extra 6-8' length of good wire.
 

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Discussion Starter · #5 ·
In 1991 the only wire connected to the ECM related to the vehicles speed is brown wire connecting the DRAC to the ECM. It's only purpose is output from the DRAC to the ECM.
The wire that operates the speedometer is the lt.blu/blk from the DRAC to A11 on a digital dash or pin 14 with an analog gauge dash. In reality the digital dash receives an analog signal also. It's just the display that is digital.
Since you have replaced everything, try a close inspection of those wires, including the ones from the VSS to the DRAC for any damage, chaffing, or burned places. You should also test them for continuity because they can break inside the insulation. Just need an ohm meter and an extra 6-8' length of good wire.
In 1991 the only wire connected to the ECM related to the vehicles speed is brown wire connecting the DRAC to the ECM. It's only purpose is output from the DRAC to the ECM.
The wire that operates the speedometer is the lt.blu/blk from the DRAC to A11 on a digital dash or pin 14 with an analog gauge dash. In reality the digital dash receives an analog signal also. It's just the display that is digital.
Since you have replaced everything, try a close inspection of those wires, including the ones from the VSS to the DRAC for any damage, chaffing, or burned places. You should also test them for continuity because they can break inside the insulation. Just need an ohm meter and an extra 6-8' length of good wire.
Thanks for the advice.
Both VSS wires have 0.5 ohms from vss plug to DRAC plug.
Blu/black has 0 ohms from drac to cluster plug.
VSS has 1.4K ohms approx.
Getting rising voltage from VSS when moving faster (up and down driveway)
Not sure how to test voltage signal at cluster plug.
Tested the upper and lower brass contacts on plug while driving down driveway ..0-150 mv. AC?
What else should I do?
Thanks.
 

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Discussion Starter · #6 ·
Thanks for the advice.
Both VSS wires have 0.5 ohms from vss plug to DRAC plug.
Blu/black has 0 ohms from drac to cluster plug.
VSS has 1.4K ohms approx.
Getting rising voltage from VSS when moving faster (up and down driveway)
Not sure how to test voltage signal at cluster plug.
Tested the upper and lower brass contacts on plug while driving down driveway ..0-150 mv. AC?
What else should I do?
Thanks.
tested the upper and lower contacts where blue/black wire goes, while moving
 

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It sounds like the wiring is doing it's job. Almost has to be a faulty component. I'm not current on 1st gen clusters, but I know on later years the clusters were different every year. Is the replacement cluster you bought the same part number? It's usually on them somewhere. But could be a tag that's gone. Is the DRAC the same code as the original? The DRAC is the component most prone to failure in the chain.
 

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Discussion Starter · #8 ·
It sounds like the wiring is doing it's job. Almost has to be a faulty component. I'm not current on 1st gen clusters, but I know on later years the clusters were different every year. Is the replacement cluster you bought the same part number? It's usually on them somewhere. But could be a tag that's gone. Is the DRAC the same code as the original? The DRAC is the component most prone to failure in the chain.
I have 2 of everything (original and replacement) original speedo/odo works for about 5 minutes about once a week. Replacement didn’t solve problem and oil pressure gauge reads 80 all the time and other gauges off a bit. I put the original cluster back on for now. I am guessing I have two bad speedos/cluster units, but would like to absolutely verify that.
 

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Discussion Starter · #9 ·
I have 2 of everything (original and replacement) original speedo/odo works for about 5 minutes about once a week. Replacement didn’t solve problem and oil pressure gauge reads 80 all the time and other gauges off a bit. I put the original cluster back on for now. I am guessing I have two bad speedos/cluster units, but would like to absolutely verify that.
Sorry forgot to answer a couple questions. DRAC is same part , cluster I will have to double check.
 
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