Running Dual PCMs with a FiTech Ultimate LS (and Keeping HVAC + Gauges Alive)

I just wrapped up a setup using the FiTech Ultimate LS kit (the higher HP version, I can't remember the advertised output) and ran into a problem: once the factory PCM was sidelined, the A/C doors, A/C compressor clutch, and even the gauge cluster stopped working. There as no communication with (or through) the old PCM despite having left it nearly completely connected and powered.

Turns out the stock PCM won’t talk on the network at all if it can't see certain sensor inputs. If it thinks those inputs are missing, it basically goes dead to the rest of the truck, and takes out everything that needs to communicate with the PCM to function, including climate control and gauges.

Here’s what I learned:

• ECT sensor can’t be shared. It’s a thermistor, and splitting it between two ECUs gives garbage readings. The trick is simple: the LS blocks/heads have a second M12×1.5 port. I dropped another stock GM coolant sensor (ACDelco 213-928 / GM 12551708) into the other head. Now both PCMs get their own ECT.

• CKP & CMP: Only one ECU (the new one, obviously) should supply the 5V reference. The signal and low wires can be spliced to both PCMs. The stock PCM just "listens.” I built all new metri-pack 150 series waterproof pig tail connectors to do this because I hate soldering.

• MAP & TPS: Same as above but be advised that the MAP uses a reversed-keyed metri-pack 150 series connector and these are a bit harder to find than the traditional ones used by the CKP and CMP. It might be easier to just find an open port on your manifold (there are several unused ones on the FiTech) and run a second MAP, skipping the splice entirely. The TPS uses a special type of triangular three-way connector. Mot quite as hard to find as the MAP.

• Find the wiring products here: http://eficonnection.com/

Once the factory PCM saw ECT, MAP, and timing signals, it woke up and started talking on the network bus. Suddenly the A/C doors worked again, the A/C clutch began cycling, and the cluster was alive.

At this point, I could technically let the stock PCM/TCM handle transmission shifting, but I prefer the level of control for shift points and lock-up that FiTech gives. Having full adjustment ability over the trans is worth keeping that side of the system on the new PCM.

So if you’re doing a similar swap and wondering why your climate control and cluster is dead: it’s not wiring or a bad PCM. The legacy PCM just needs enough sensors to stay happy on the network. Feed it those critical few, and the rest of the truck comes back to life.