Obd2 rpm discrepancy

[slowstart]

Hopfully i am posting this correctly. I did all the google searches i could think of and also serched this site and found nothing. Maybe i am just using the wrong words in my serch. Anyway, i have a 2000 k3500 with a 454. (I just got it and its the reason i joind this site) the problem (of many) i am trying to figure out is when i connect my obd2 scanner (innova 5610) and look at the live data the rpm's are way off. It reads over 3,000 rpm at idle. The tachometer is reading correctly and the obd2 reading does respond when i speed up the engine but it is constantly like 2,500 rpm higher than what the engine really is. I am worried that the incorrect rpm reading is being fed to the other systems and that might be causing some of my other problems (transmission, etc.) Or is it just a glitch with the live data feature / obd2 reader. Any thoughts or suggestions would be appreciated. Thank you!

 

[slowstart]

Bump

 

[smokymtn65]

Do you have access to any other scanner for comparison?

 

[1998_K1500_Sub]

Scan tools can often be borrowed/ “rented” from the retail stores, AutoZone and the like. Give one a try, as @smokymtn65 suggested.

 

[BeXtreme]

Hopfully i am posting this correctly. I did all the google searches i could think of and also serched this site and found nothing. Maybe i am just using the wrong words in my serch. Anyway, i have a 2000 k3500 with a 454. (I just got it and its the reason i joind this site) the problem (of many) i am trying to figure out is when i connect my obd2 scanner (innova 5610) and look at the live data the rpm's are way off. It reads over 3,000 rpm at idle. The tachometer is reading correctly and the obd2 reading does respond when i speed up the engine but it is constantly like 2,500 rpm higher than what the engine really is. I am worried that the incorrect rpm reading is being fed to the other systems and that might be causing some of my other problems (transmission, etc.) Or is it just a glitch with the live data feature / obd2 reader. Any thoughts or suggestions would be appreciated. Thank you!

My guess is that the scanner is just looking at the wrong PID. The way that the wiring is set up means that other systems aren't getting the wrong RPM if the Tach is reading correctly. The computer is calculating RPM from the crank/cam sensor signals rather than the coil firing like it did on older HEI distributors, so that really isn't a thing.

What issues are you having with your transmission and other systems that would lead you to be looking at the RPM signal?

 

[slowstart]

Ok, thank you. I will try that. If the obd2 reader is correct (works perfectly fine on all my other vehicles) where do readers get the rpm data from? And what other systems get rpm data from that same sorce? And what sorce does the tachometer get its data. I have been trying to find schematic for my truck to answer all these questions but am having a hard time finding anything for it. I have found everything for all the other OBS years but not 2000 (1 tons were still obs that year).

Just a little background. I bought the truck from a vehicle flipper and now i have to go through and fix all of his "fixes" but its hard not knowing what problems he created (already found several things wired wrong) or problems that are actuall problems and will need parts replaced. Not knowing anything about its history and not having access to any documents has made this quite the puzzle solving process. I had to trailer it home. The guy i got it from got halfway through an engine swap (not sure why) and then decided to just put it back together but didn't do so correctly. This is the only 7.4 i have ever worked on so its been tricky to say the least haha.

 

[slowstart]

@BeXtreme
Thank you. After i posted my last reply i refreshed my page and saw your reply.
The transmission just shifts sporadically. Somtimes it shifts fine but othertimes it just wont. The only consistency i have found is at wide open throttle. It seems to always shift when and where it should if my foot is on the floor. Otherwise somtime it just wont shift or tries to shift to early and bogs down. But usually it will just shift between 3,500 and 4,000 rpm (not accelerating and no load) and shifts hard when it does. I am assuming its an electrical issue or sensor of some kind causing this. Maybe i am barking up the wrong tree and its just a bad transmission.

 

[1998_K1500_Sub]

Factory manuals for our trucks can be found here in .pdf form.

88-98 Service Manuals​

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/}

 

[smokymtn65]

Was this truck a factory 7.4? When you mentioned flipper made me wonder. I am not the electrical specialist by any means. I think in stickies here the manual can be found or another member sent me a link a long time ago

 

[smokymtn65]

Factory manuals for our trucks can be found here in .pdf form.

88-98 Service Manuals​

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

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