What operates the tach???

1996 GMC Jimmy. Factory tachometer in IPC. Original engine was 4.3L, unsure of original fuel-system. Planning to install a 1990 TBI 5.7L. Is there a way to keep the factory tach??? Would it need recalibration because of the addition of 2 cylinders??? What exactly needs to be done to somehow connect the factory tach to the ignition-system of a '90 TBI engine???

Also, the truck is quipped with a VCM. Can I totally do away with it??? I will be swapping in the older TBI computer, and will definitely not be using the original VCM for engine management. The EVAP system will also be changed, to a simple vented fuel-hose. The tranny will be a non-electric 700-R4, so the VCM will play no part in transmission shifting. ABS is controlled though a separate module. Air-bag operation is through the SIR module. The air-conditioning will be discarded. The truck is NOT equipped with power-locks, cruise, or power-windows. The headlights are still the old sealed-beams. So can I ditch the VCM??? Do any of the dash gauges go through the VCM??? What about fuel-pump???

The info that your current IPC uses to run the gauges is primarily sent over the Class 2 data bus, though there are a few direct-wired inputs, depending on the specific model/year. This IPC will use a Class 2 bus, whereas the older engine will run a few outputs on the ALDL bus, like the speedometer, with everything else direct-wired. Therefore your current instrument cluster is pretty much just not going to work.

The ABS, airbag and BCM modules will probably complain that they can't communicate with the ECM, because it's not on the Class 2 data bus anymore.

As for whether you still need the body computer module (I assume that's what you're referring to by BCM), the things they usually run are stuff like the audible warnings, power locks, and the interior/exterior lighting functions. It also helps with the Passlock anti-theft system.

Basically, this is going to be a lot of work, most likely, unless you're willing to gut a lot of the current wiring and body control system.

Also, I don't know if it matters much where you live, but this sort of conversion (installing an older engine) is NOT emissions-legal.