Tachometer off ls conversion 1971 c10

[SReddy]

Hey guys,

This is my first post. I just bought a 1971 Chevy c10 with a 5.3l ls motor and 4l60e transmission. The wiring harness and PCM were purchased from PSI (performance systems integration). The truck was initially a v6 truck with 3:73 rear end. The tachometer is from a V8 truck. The guy I bought the vehicle from had told PSI to change the tach variable from 6 to 3. He thought everything was functioning fine so I asked him to tell me how many rpms the truck was pulling at 70mph and he said it was 2600 so I told him that sounded high and if he could a scan tool to the OBD plug and sure enough the truck was actually at 2200 rpms. At idle the tach was showing 700 rpm when the truck was actually running at 560 rpm. By the way the truck has 275/40/20 tires. Now my question is how do I get the factory v8 tach to register correctly. The guy who I bought the truck from called PSI and it sounded like the guy from PSI was going to charge him and wasn't sure if changing the variable to 4 would work or not. The PSI guy uses HP tuner and so I guess I would have to find somebody that uses HP tuner so I wouldn't have to buy a new PCM to get this fixed. Anybody have any suggestions on what the fix would be? I am kind of in a time crunch because the guy who I bought the vehicle from is willing to send in the PCM and reinstall it. The vehicle won't be picked up for another week. So I would like to get this done as quick and as cheap as possible. I would think this would be a simple fix. Sorry for being longwinded but I just wanted to be thorough.

Thanks
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[chevy72blu]

The only thing that worked with mine was to use my local tuner to "guess and check" using different variables in EFI live/HP Tuners, mentioning that it wasnt always an exact science depending on the tach, year of PCM, etc. Im sure you could find someone locally on craigslist or this board.

What tach are you running? Autometer has switches for 4/6/8 modes. It may be worth trying them

[SReddy]

Thanks, I am to have a shop with an HP tuner do what you suggested. Unfortunately, they are charging me $200 to do this.
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