I'm looking at replacing the ECM computer in my 1994 F150, doing the ever popular SD to Mass Air convsion. After crawling around under the dash and looking at the engine bay/fire wall I have a question or two regarding the wire harnesses, or perhaps some confirmation on my observations. I know there are many wire harnesses for a vehicle, but the one in question I have is what I believe is called the main engine wire harness. This is the harness that connects to the ECM computer which in my case is located behind/to the left of the emergency brake on the drivers side, which of course totally sucks to get to.

The question I have is I'm trying to see how this darn thing passes through the fire wall into the engine bay. It appears that it may break up into two separate wire bundles and pass through two holes in the fire wall going into the engine bay. At first, I thought that there were connectors at the fire wall and that the wire harness didn't really pass through it, but there were actually two separate harnesses that connected together at the fire wall. Maybe that was wishful thinking on my part. Any ways, after closer and closer inspection and I now think the wire harness is a single harness that does pass through the fire wall, perhaps through two separate holes, but it does through what appears to be a very speciallized square rubber grommet with many individual holes in it. Am I correct in this? If so, how does one go about pulling that darn thing out when replacing a wire harness, and how would one go about putting a different wire harness in it's place? Does this rubber grommet come apart so that new wires can be removed and added, or do they have to be run through the holes before they are connected to their appropriate connector and sensor?

I'm just trying to size up the task I have at hand and any insight into this would be most appreciated.

Thanks,

MrG4x4

 

Hi and welcome to the forum

The connectors you're looking at do join up at the firewall. One side, I dont remember which, clips into the firewall and the other attaches to it. There should be a screw in the middle of the connector.
To remove the PCM you can stick a pry bar in between the inner fender liner and the fender and pry the inner fender down enough to give you more room for the PCM.

 

I did some more poking around tonight and I have now come back to my original opinion which is what you just mentioned. The wire harness does connect to the outside of the firewall and apparently the ECM computer is on the other side mounted against it. Since the computer is behind the emergency brake I can't see if very well and I can't tell what position it is in, so my first thought is there was a separate wire bundle on the same side as the computer. However, looking at home much space there is back there, there is no way there would be enough room for another wire bundle, so the computer must be fastened or clipped in against the firewall.

I do see the bolt in the middle of the wire harness connector and I also see a bolt at each end, or more like at the end of the hard plastic plate that covers the hole. I'm wondering if I remove these two bolts if the computer and wire harness will pull back out into the engine bay, or is the computer also fastned to something else on the other side? Any thoughts on that? Sure would be nice not to have to take that darn emergency brake out just to get at the computer. Thanks for the help.


MrG4x4

 

The connector on the PCM has the one 10mm head bolt holding it into the PCM. Loosen the bolt and the connector will pull out. Remove the nut on each end of the plate that holds the PCM into the firewall and then the PCM will pull out into the engine compartment.