Tachometer question?

[jatatkupak]

I have a 97 F350 SRW/ 7.3 PS/E4OD tranny all stock. My tach. in the dash or instrument cluster is reading about 500 to 700 rpms more than it should ( compared to diagnostic tool hooked into diagnostic port). The tach once read properly but went flaky intermittantly reading high. Now it reads high all the time. I have an AIC (fast idle box or sometimes called "acpm") it has never read rpms, but will go into high idle mode when I ask it to. I just had my cam position sensor changed per a factory recall, and was hoping this would fix the tach and the AIC too. But everything is still the same no read-out for rpms on AIC and tachometer in dash still reading high.

I thought that the transmission was having trouble due to the elevated rpms but the guys at the tranny shop said it was fine, no codes registered, and shifted as expected while being monitored by diagnostic tool.

Does any one have any insight as to what the problem may be?

thanks

[alberta 7.3]

Your tach may be crapping out. If you can find another from a diesel at a wrecking yard, it just plugs in. Some PCMs didn't have programming for the APCM, or something wasn't right, a dealer may be able to re-flash it for you. My APCM in my 96 didn't show RPMs or voltage in charge protect mode when I installed it. When I bought my custom chip from Tony Wildman, I mentioned this problem, and he must have added something to the programming, because it worked properly after installing the chip.