Hello all.
I recently completed the Holley Terminator X Max integration in my 2003 Silverado. Before I began I tried doing some research and found that there is very limited information out there on the integration of Holley EFI on 2003-2007 Silverado trucks.

This thread is a compilation of everything I have learned integrating Holley on my project and its for people that want to keep the factory ECM and will be doing the wiring themselves instead of buying a pre made engine harness.
I have a couple YouTube videos that I made on this subject. Its mainly me rambling on but there is some useful information and the visuals may help some of you. Jacob Seaton YouTube Channel
You can also follow my Instagram to see the whole build. Fast_sss

Let me start by saying I am new to Holley EFI and I am not a professional so just take this information and apply what you can to your builds!

Background on this truck
I started getting a little more serious with my build and competing here lately. The combo has been 9.41 @ 142 and has seen 30+lbs of boost on the street. I was at the top of my VE scaling with the stock ecu and the need for a boost controller was also arising. I was also getting to the point where I wanted more data. So I thought it would be a good time to look into swapping to Holley.

My requirements were I had to keep the fully functioning stock gauge cluster including the PRNDL switch, Heat and A/C and the Stock key and ignition.
I was able to keep everything listed.


I started with ordering a Terminator X Max Part #: 550-926. 24x/1x EV1 with DBW throttle control and transmission control. I swapped to an 87mm 4 bolt throttle body and I have a Jakes level 5 4L80e that it will control.
I also bought a CAN Input / Output Module Part #: 554-165. This gave me 8 more inputs and 8 more outputs.




I wanted to mount the Holley ECU somewhere easily accessible and where I wouldn't have to cut the dash up or drill holes in the firewall. So I decided to mount it behind the knee panel below the steering wheel.
I built some brackets to hold a sheet of 4mm thick carbon fiber as a backer for the computer. It was a tight fit but turned out really nice. And yes, the panel still fits and did not require any trimming.
I was also able to fit the Holley harness through the firewall without cutting any holes. Since i am no longer using the TAC module, the harness from the pedal can be removed leaving a 2" hole for the new engine harness.





If you like the carbon fiber idea you can get sheets on Amazon for about 100 bucks. Its messy but super easy to work with. A regular cut off wheel on a grinder will make smooth cuts and regular drill bits do a good job of drilling it.

The next step for me was to remove the factory engine harness and start de looming and prepping to de-pin the wiring that i did not need. Worst part of the whole job!
I would recommend leaving the connectors to the sensors you are splicing until you are ready to cut them off and tie them into the Holley harness that way you can easily tell where it goes later.
You will need to remove all of the unused wiring all the way back to the ECM and fuse panel connectors.
I also de-pinned the signal wire that tells the gauge cluster when to turn on the Check Engine Light. Now i dont have to worry about an annoying engine light since the stock ECU will be in a constant state of error.

LT1swap.com is a huge recource for truck wiring schematics and pinouts. Here is the link to the 03+ ECM pinout so you can figure out what is what when you go cutting into your harness. lt1swap.com/silverado




Now lets talk about what sensors you need to keep and what you can get rid of. The chart below lays out everything you need in order to keep your gauge cluster and HVAC.

I have heard of some people letting the stock computer control the throttle body so they can keep cruise control. I have mixed feelings about that. Any little upset in the TPS in the pedal or throttle body will throw the truck into limp mode and it will slam the throttle body shut. Which is something that could be really bad for someone on 30lbs of boost in the middle of a race. I would be open to going back and giving this a try once I see a few people having success. So for now, I removed the TAC module and stock TB wiring and I am letting the Terminator control the throttle body.
Everything else is pretty basic. Remove the stuff that the Holley will control and keep or split the sensors that you still need to stock computer to see for normal operation.

If you plan on using the stock key and ignition you will need to keep the PRNDL switch mounted and connected. The ecu will use this information to shuffle through the gears on your gauges and its also used for a neutral safety switch for starting the engine.
Also, you need to keep the large purple wire that goes to the starter. The factory fuse panel and relay will control the starter as usual.

Now lets talk about how to split the sensors. I did not purchase any splitters or adapters. I spliced into my Holley harness and added a single Delphi weather pack connector for easy removal of either harness in the future. If you do not feel comfortable opening up the Holley harness then there are plenty of companies out there that made pre terminated splitters that you just click together and tuck the slack away.
If you do not wish to purchase any splitter harnesses then do the following. You will use the Holley connectors to connect into your sensors. You will only split the return signal wire on each sensor.

The crank position sensor uses 3 wires. 5v, ground and return signal. You can remove the 5v and ground from your stock harness. The Terminator will supply the sensor with 5v and ground. All you need to do is splice into the return signal wire on the Holley harness so the stock computer can see that signal.
Same principle for the oil pressure. It has a 5v, ground and return signal. Remove the 5v and ground from your stock harness and splice into the return signal wire in the Holley harness. Some people add a second oil pressure sensor near the oil filter but I kept it simple and had success splitting the oil pressure signal wire.
VSS is simple, just splice into both wires on the Holley harness.

I purchased this and used the single connectors to split the sensors. This kit makes it super easy. You will also need these special pin crimpers
Now lets talk about fuel pump control. Once you do the swap the stock ecu wont turn on the fuel pump any more so your truck probably wont start. Most people at this point already have some type of aftermarket fuel pump relay or controller so this is no big deal to them. The Terminator harness has a fuel pump output wire built in and all you need to do is use it as a signal for your relay. If you happen to still use the stock relay and harness then all you need to do is tap into the factory relay trigger wire under the fuse panel so the Holley will turn the relay on instead of the stock computer.

If your new to wiring and want to learn before you tackle this project go check out Devin Vanderhoof channel. He is a Holley wiring guru and he gives loads of information on how to properly wire this stuff and what tools will make your life way easier.






Once you're done splitting and de pinning your ready to start routing the harness how you see fit.


This is a suction mount from Carchains3d. I got rid of my 3 pod gauge pillar and went with this.


Holley also recommends that you verify your timing and make any corrections in the ECU before you start tuning. This is a timing pointer from Motion Raceworks for the truck style accessories. It was cheap and super easy to install.

 

Definitely saving this for future reference, thank you so much for taking the time to document everything and post it for us!

 

Back to the subject of Check Engine Lights, on the 2003 and up trucks you will get a Reduced Engine Power message on your message screen on the gauge cluster if you disable the stock throttle body and remove the TAC module. This is an easy fix in your stock ECM HP Tuners tune file.

Open your last tune file, go to the systems tab, then select system options, then find the ETC column and follow that down to the GMT 800 row. Your stock file should have a (1) in the cell. Select that cell and change it to a (0). Then save the file, flash it back in and that should be the last flash you ever have to do on your stock computer.
Take a look at the screen shot below for directions.



Other things i ran into that dont nessicarily relate to trucks but might be useful to some of you was the Terminator Power Tap Plug. In my case i have 6 inputs and 3 outputs so far and the Terminator harness only has one pin for you to pull yout 12v, 5v, sensor ground and chasis ground from. So i made a splitter harness that gave me 3 extra pins to pull sensor power from. I dont have room to mount 3 terminal blocks and run the wiring all fancy like you see in race cars so you may not like the way i did it but sensors draw very little amperage so this worked out great for me.





Another thing that wasnt well documented is Flex Fuel wiring for Holley. You cant just run 12v and ground to the sensor and run a signal back to the terminator. The holley requires an extra 5v voltage boost with a 10,000ohm resistor tee'd into it. Its kind of mind boggling but it works. I fought it for a good 5 hours and finally found a diagram of the correct way. Added the 5v and resistor and it instantly started working.
Take a look at this flex fuel wiring diagram for Holley below and it will save you a headache later.

 

I vote sticky

 

I vote sticky also. Thanks for the write up!

 

This is great info, sticky material for sure.

 

Just to add, Current Performance has a crank splitter adapter as well as a plug n play A/C integration harness (haven't hooked mine up yet). Sloppy mechanics has a FF harness for the Term X too (uses one output IIRC).

To retain the OEM oil pressure sensor for cluster and no SES lights I'd suggest something like an ICT billet oil adapter (ex. P/N 551541) to allow you to connect another sensor for the Holley to see pressure. If you have the room at the P/S rear cylinder head there should be a provision for another CTS for the Holley to run off of.

Other bonus is that if you did not have E-fans controlled via OEM ecm you can now set that up through the Holley with an output and not have to worry about it in factory tune.

Depending on which generation motor you have, if running 1x cam sensor (rear of engine) I believe you should be able to run a Gen 4 cover (get an 06 Trailblazer SS as they are 24x/1x on front cover) to allow the Holley to read cam position while OEM still uses rear sensor location.

Good info here Blue03sss, glad to see some more modern day write-up.

 

This is great info

 

I see a sticky... good job man!

 

Thank you for the write up on this