Firewall penetration point for ECU wiring

[Steve McMahan]

I was wondering if anyone had an illustration, or something visual or dimensional, of where on the firewall you penetrated to run the ECU inside? I was hoping it’s on the passenger side, as that’s where my 87 TBird wiring made it’s home (kick panel). I’ve examined that area, and I cannot determine what point avoids the HVAC unit under the dash. Thank you in advance.

 

[Steve McMahan]

Is this a really hard question or am I asking the wrong forum?

 

[Bentworker]

Might be more the wording of the question.  A quick read of it by someone may lead to the impression you are not working on a 71-73.

The way the heater box is on an AC car you may be able to sneak the harness over it between the box and the lower cowl itself.  How far apart do you want to take things?  How big is the harness and plug?

 

[Bill73Ragtop]

Since 71-73 cars did not come equipped with ECU's its pretty much a custom job.  You will need to determine where the ECU will fit under the dash.  The size of the wire loom will determine the size of the firewall grommet and where that will best fit.  Holley EFI wiring looms are huge as are loom sizes when retro fitting something from the 90's and up.  Is the ECU (and the loom) shielded from heat / electronic sources?  Are you going to use the ECU wiring looms as they are made by the manufacturer or are you going to custom terminate each wire in the loom?  Does the ECU need easy access to plug in diagnostic leads? 

These are all basic questions that need to be considered when determining where to locate the ECU and where to route the looms.

 

[Don C]

A little more information would help. Like how big of a hole will it take. Does it have a bulkhead connector, and if so, does the ECU connect directly to it? Or, is it just a cable and grommet, same question, how big is it? Are you retrofitting a fuel injected engine into your car?

If anyone else had done what you're wanting to do, they probably would have chimed in by now. I haven't retrofitted a fuel injected engine into an early Mustang, but have to other vehicles. Finding locations for the ECU and cabling is very timing consuming, requires a lot of measurements. I would remove the dash and heater/AC after outlining the box on the firewall. That will give you a better view of what you're trying to accomplish and better access to measuring points.

Depending on what you're installing, you may be better off using a retrofit harness, designed for more flexibility than the OEM harness.

 

[EdM]

In the cab, Pro-Flow 4, behind the glove box.

 

[Steve McMahan]

Thanks for the questions. I am retrofitting a stock 1987 TBird 5.0l complete drivetrain, including the existing wiring harness into my 73 Grande. The TBird’s ECU is located in the passenger kickpanel (the ECU was attached in that “well” established by the design of that panel).



The HVAC unit on that car does that occupy the entire area under the dash, and it left room to its right, from the passenger seat perspective, which allows a rather large firewall penetration point to be at the passenger side, corner of the engine bay (see the pic). The OEM harness has that very large plug for the ECU, and the hole is also large to allow it to fit through the firewall. The grommet is also very large. It doesn’t look like that kickpanel location would work, mostly because the harness is too short to reach, assuming I could come at the point Bentworker illustrated. It might work if I could completely redo the harness and move the firewall grommet to allow a much longer lead from that point (of those three holes, that large hole on the far left of the photo is the ECU wiring penetration)


If I could come through that location, I wonder if I could make a mount that would attach to the metal above that penetration location? I was hoping I would be following in the footsteps of an already successful retrofit and benefit from the experience. Well, I wanted a project I could really learn from.

I was hoping to make as few as possible removals to the interior, mostly because of limited garage space, but it seems I need to at least remove the dash and the from seats so I can see what I’ve got to work with.

I’ll try and take pics and note the mistake for the next time. Thanks again for the questions.

 

[tony-muscle]

Depending on if you have A/C/heater or not you will have more or less space there to make an opening and locate your ECU. I suggest you mock up your parts and figure out where the opening for the wiring has to go. You have a lot of space below the A/C box or between the box and the outward panels. It also depends on having a charcoal canister or not which is in that general area. If you go below the A/C box you can't go too low since you need to leave space for the feet of whoever seats in the passenger side. However, you should have some space between the box and firewall. 

 

[Steve McMahan]

Do you know if there is room between the top of the AC unit and the bottom of the cowl? I’ve searched online to find pics, and the ones I’ve seen show some room, but it’s difficult to know how much. Since it’s not really a part you service, I’ve thought about mounting the ECU to the bottom of the cowl section. Here is a still from a remove video that shows the space:



That space above the air box looks like the ECU would fit there, but I don’t know if that cardboard or metal above it. Also, how close to the glove box could I get it, in case I would want and try to have access to it?

 

[tony-muscle]

Do you know if there is room between the top of the AC unit and the bottom of the cowl? I’ve searched online to find pics, and the ones I’ve seen show some room, but it’s difficult to know how much. Since it’s not really a part you service, I’ve thought about mounting the ECU to the bottom of the cowl section. Here is a still from a remove video that shows the space:

That space above the air box looks like the ECU would fit there, but I don’t know if that cardboard or metal above it. Also, how close to the glove box could I get it, in case I would want and try to have access to it?

You do have some space there. I know because I used it in the past before I replaced the OEM A/C with the CAA. I will guess between 3/4" and 1". You can even move the box a little lower after you remove the bolts that attach it to the cowl intake - at least that was the case with mine. You would need a thicker gasket between the box and cowl intake to seal any gaps if you move it lower. How thick is the ECU?

Edit: I found this picture. You can see the gap with some foam wedged in there left of the cowl's intake. 

 

[tony-muscle]

You also have some space between the box and the exterior side where the ECU may fit vertically. There is also room behind the kick panel where it also could fit vertically. I have a subwoofer there. Here is a picture of the subwoofer. It only fits at an angle to clear the recirculation flap actuator of the A/C box. As an idea, the subwoofer box is (W)11 x (H)2-3/4 x (D)7-1/2. The kick panel then fits over it so it is hidden.

 

[Steve McMahan]

I really like the woofer location. It’s easier to access. The ECU is about 8x6x2”. Thanks for the pics and info.