Why We Created This Write-Up

Our 1999 GMT400 Suburban was purchased with a Whipple supercharger mounted to the 454ci BBC engine. This Whipple kit is from the 90s and used a very rudimentary 18V "Boost-a-Pump" piggyback system on the OEM pump to force more fuel into the engine under boost. This system was sketchy when it was new 30 years ago, and now more than ever it is due to be replaced with a more efficient and safe system. We decided to go with Holley EFI for the build due to our familiarity with the system and their gracious sponsorship of the ECU for this build.

Core Components Used in the Swap

These are most of the new parts selected to upgrade the truck.

Desired Features and System Goals

Before cutting a single wire, we defined what we wanted the finished system to do. That kept the project focused and helped us decide which factory systems still mattered.

Uncommon Parts and Tools We Needed

Fuel System Notes for a Boosted GMT400 Holley EFI Swap

The fuel system was one of the most important parts of the entire project. The Terminator X documentation recommends a 0–100 PSI pressure gauge or transducer. 554-102 is a 0-100 PSI pressure sensor that can be purchased that will plug into the TERMINATOR X™ harness to check and monitor fuel pressure. We ended up adapting the intake rail Schrader test port to support a pressure sensor via an adapter fitting.

You need to determine how much injector you need for your application and select a fuel pump that is appropriate. For us, the selected 340 LPH QFS pump had enough flow for the application, and was drop-in. The pump is rated at up to 40 amps peak draw, which is well beyond what the OEM wiring or the ECU’s direct trigger output can handle alone. The solution was to run a relay triggered by the Terminator X fuel pump output in conjunction with a dedicated pump harness with appropriate wire gauge capable of the required current.

The factory fuel hard lines were left unchanged. An inline 4-micron WIX 33481 filter in the OEM location was already present, well below the Holley recommended 10 micron minimum. OEM service manual notes indicate the factory fuel regulator will hold 56-62 PSI - perfectly within the range needed for the Holley 60 PSI target. We replaced the regulator with a new OEM one to ensure consistent regulation.

The Holley kit can be used to set prime for the fuel pump with IGN ON. The OEM was set to 5 seconds prime. Use the terminator fuel pump output (J1A-pin 2) to connect to the fuel relay trigger wire DK GN/WHT (465). If we keep the GRY (120) wired as it is from the factory, there will be a backup 12V to the fuel pump as long as there is oil pressure.

Exhaust and Wideband O2 Placement

The included oxygen sensor should be placed where it can read a reliable average from one bank, ideally just after the cylinders merge and before the catalytic converter. If the vehicle uses an AIR pump, the O2 sensor must be placed before AIR injection or the AIR system must be disconnected. We had a port in a header collector we used since our AIR pump system was deleted.

Cruise Control Strategy: Keeping a Commonly Deleted System

Cruise control is one of the first systems many people give up on during an EFI conversion. Here are some technical notes from service manual literature:

The Cruise Control Module uses a Stepper Motor to adjust the throttle position via a cable, similar to the Accelerator Cable. When engaged, the motor controls the speed by turning a reel, and when the system is inactive, the Stepper Motor disengages, allowing the cable to move with the Accelerator Cable. When engaged, the motor maintains a set speed, but releasing the Accelerator will not decrease speed below the set point. The cruise control module (CCM) is powered by the CRUISE Fuse 6 (with IGN on) through the BRN (41) wire. This fuse also provides operating voltage for the Cruise Control Switch located on the Steering Column. CCM grounding is via the BLK/WHT (451) wires at Grounds G103 and G104. The Cruise Control Switch, located on the Steering Column, has two switches: a Set/Cruise (S/C) button and a three-position Off-On-R/A slider. The GRY (387) wire sends an "ON" input to activate the system, while the S/C button, connected via the DKB (84) wire, sets the speed. Pressing the S/C button briefly decreases the speed by 1.6 kph (1 mph), while holding it disengages the throttle and allows coasting.

The R/A (Resume/Accelerate) input, via the GRY/BLK (87) wire, controls the speed. If the brake pedal is pressed, the system disengages, but the previously set speed can be resumed once the brake pedal is released. Pressing the R/A button briefly increases the speed by 1.6 kph (1 mph); holding it accelerates the vehicle until released, with the new speed becoming the set point. Prolonged use under heavy load may trigger a transmission downshift or engine damage due to excessive speed.

The Cruise Control Module also monitors two Stoplamp Switch inputs. The PPL (420) wire sends an enable signal from the Stoplamp Switch, disengaging when the brake is applied. The WHT (17) wire checks Stoplamp operation before activation. The vehicle speed signal from the VCM is received via the DK GRN/WHT (817) wire, providing data to set and control the speed. Power to Stoplamp Switch Pin B is available when the Ignition Switch is in RUN through the ORN (140) wire. After the Ignition Switch is placed in RUN, the Brake Pedal must be actuated to provide cruise input through the W HT (17) wire. After Stoplamp Switch operations are confirmed, the Cruise Control Module will respond to a set input.The module signals the VCM on the LT BLU/BLK (396) wire, which may influence transmission shifting. The Cruise Control Module monitors a vehicle speed signal from the VCM on the DK GRN/WHT (817) wire. This 4000 pulses per mile input is used for setting and for controlling vehicle speed. This input will generate an R M S voltage of between 0-5 volts depending upon speed when measured with a high input impedance voltmeter. The Cruise Control Module signals the V C M that Cruise Control is engaged on the LT BLU /BLK (396) wire. The V C M may use this information in control of transmission shifting. If a new VCM is installed, it must recognize the Cruise Control Module by engaging and disengaging the system.

The Cruise Control Module monitors a 0-5V 4000 pulses-per-mile vehicle speed signal and uses it to set and maintain speed. Our working theory was that a proper 4000 PPM output from the Terminator X could satisfy the CCM input requirement. A summary of pins and the FSM diagrams are below.

To get cruise control to work, it appears that all the CCM needs is to have a VSS output of 4000PPM (pin in cavity K) & should be over a 0-5v spread. We should be able to output this from the terminator box. The unknowns are:

0) if the VCM can be disconnected from the CCM (outputs J and K)

1) if disconnecting outputs J and K out of the VCM will break the VCM (the speed output is also used by the radio, security system, and transfer case if used)

2) if the (J) Cruise control signal engaged output is required

3) what this output of (J) is (5v, 12v, PWM, switched ground, etc)

Vehicle Control Module (VCM) Considerations

The VCM on these trucks handles more than most swaps account for. In this build, the VCM mattered because it tied into ABS, EVAP behavior, and cruise control support. The core challenge was deciding which inputs had to remain connected, which could be discarded, and which would be replaced by Holley.

Below is a compiled diagram of the basics of the VCM and nearby, supported components. This figure is not holistic of all the adjacent systems, however it should give you a good idea of the inputs and outputs of the VCM.

ABS System Notes

The ABS strategy in the notes was simple: retain the VSS signals the VCM needs and let the factory ABS continue doing its job. Several EBCM outputs could potentially be repurposed to drive dash warnings through the Holley display if desired - this may be explored in furher tuning of the Holley system.

Instrument Panel Connector Pinout and Signal Strategy

We’ll need to depin the connector and take some of the signals for the Holley 12.3. We can set up the 12.3 for custom inputs and have nice displays for almost everything that the OEM dash would have to include warning lights. Below is the FSM cluster pinout and a following table describing which of th epins we plan to use.

EVO, Passlock, EGR, and EVAP System Decisions

EVO Delete Considerations

Electronic Variable Orifice steering was introduced to vary steering assist based on vehicle speed. In practice, it became a common source of complaints when it failed. It was common enough that GM eventually issued a TSB on the issue and also created a delete kit. The delete kit runs the system on constant ‘max’ pressure, so it may put more wear and tear on the pump in the long term. However, lots of users have no complaints. If you want to to 100% OEM correct, take a 1996 pump, lines, etc and it will not have an EVO port and will be at an OEM designed pressure. Regardless, for our build, we installed the EVO delete kit, Dorman 926-049.

Passlock Notes

Passlock verifies the key and communicates to the VCM whether the vehicle should start. In this build, the Holley standalone strategy was expected to make disconnecting and bypassing the EVO/Passlock side of the original arrangement a non-issue.

EGR Delete

The EGR system was deleted using standard -14 JIC pipe plugs and block-off plates, available on our website.

EVAP Notes

The evap system has a few complex modes where the VCM senses coolant temperature, tank pressure, tank level, or fuel trims to vent the gas tank via the purge valve. With many of these signals removed, we are hoping the EVAP will still work with limited data input from VCM once the Holley is integrated. If it doesn’t work, we should still be OK. We’re just leaving the evap system untouched for now. More info below.

Holley 12.3 Pro Dash Wiring Strategy

The Pro Dash uses a 34-pin CAN/power harness for power, communication, and local I/O. The connector pinout and our adaptation to the OEM IP harness is shared below.

This link will show the full pinout diagram: full pinout diagram

Terminator X Wiring Overview

The Terminator X Max strategy in this build centered around understanding the role of each major connector and then mapping OEM functions around them.

REVurban-Specific Installation Phases

Breaking the swap into phases made the project easier to manage and made the notes more useful later. The process below reflects the documented structure from the internal reference.

Phase 0: Remove the Whipple Kit and Piggyback System

1. Disconnect battery cables.
2. Remove the vacuum line from intake manifold to the FMC.
3. Access the VCM, remove the red plug, locate the relevant modified wiring, and reverse the Whipple-related splices.
4. Remove the auxiliary fuel pump relay harness connector.
5. Remove the Whipple red power cable from the underhood fuse box stud as documented.
6. Remove the FMC, its plugs, grounds, and relay below the VCM area.
7. Trace and remove the Whipple Boost-A-Pump harness along the frame.
8. Disconnect the fuel solenoid wiring and leave the solenoid in place as a union if appropriate.
9. Find the BAP, remove its frame and electrical connections, and identify the OEM grey wires that will need to be repaired later.
10. Identify any additional Whipple-related gauges or sensors, such as the EGT setup, before final exhaust work begins.

Installation Phases

PHASE 1 - Fuel Pump

These tank instructions are for a Subruban. If you have a pickup your tank will not match the diagrams and info below.

Try and get that 42 gallon tank near Empty.

When I dropped my tank, I had a half tank. It was not too bad to remove. I had the truck on the ground, and used a pump jack below the tank to ease it down. The empty tank probably weighs around 50 lbs.

The tank is held on with 2 straps. It may have a skid shield over it. The fuel pump wiring and lots of rubber hoses and clamps sits above the tank. The connections look like this:

The bad news is the hoses and connectors on my truck weren’t long enough to fully let the tank drop down, so I kinda had to balance it on the jack and disconnect it all. You’ll be removing the fuel filler neck hose (big 2.5” hose with clamp on LHS). There are I believe 2 electrical connectors, maybe a ground, and also feed, return, and evap hoses.

Once the tank is free, clean the top around the pump before you open it up as to not drop in a bunch of junk into the tank

Use a hammer and punch to drive off the locking ring and pull the pump from the tank

Inspect tank and pump condition.

Important: compare the new pump to the old pump. Take pictures, note connections. We will need an ohm-out of the fuel level sender to verify the resistance sweep. The MFG should be able to provide this information, but at least try and get an empty, full, and mid range reading.

Install the new pump with the fresh seal. Use specialty tank sealer / grease for the big O ring, to mimic the factory installation. This isn’t required, just insurance.

I also had a little rust around the top of the tank, from condensation. I bought spray fluid film and doused the metal on top after it was all re-assembled and hooked up to slow it down.

Swing the tank all the way back in and fill er up.

PHASE 2 - Engine Bay

Time for the fun stuff. Pulling the blower off and fan out should make wiring easier.

• Pull the upper rad shroud, and mechanical fan out to make a lot more space
• Remove coolant hose(s), and any other components that will make a big mess, before you pop open the engine
• Pull air intake hose, blower throttle body, blower / intake, upper and lower intake manifold
• Remove the ignition control module, keep the coil itself. Remove injectors, etc etc. Make sure to label your parts and take your time

This might be the best time to replace the fuel pressure regulator.

If you decide to delete the EVO, this might be a good time to do that too.

PHASE 3 - Wire Removal

Reference the VCM diagram earlier. Start to depin the unneeded components and strip them from the factory harness.

You can either leave the unused wires in the harness, or to clean it up, remove it all entirely.

• Remove the transmission sensor plugs from the 4L80e
• If you deleted EVO, remove the solenoid and wiring.

PHASE 4 - Wiring Instructions

→ Start by reading the Holley Terminator X manuals, Holley 12.3 dash documentation, and watch some videos ←

Get under the truck and install the VSS splitters on the VSS (ISS and OSS). Plug OEM VSS (OSS/ISS) into one, the other 2 will be used by the Holley kit.

The VSS may be pinned backwards - double check your adapter harness!

Find a happy place to mount the terminator, such as in the glove box as we did. We have a glovebox mount for the 88-94 trucks as well as the 95-98 trucks.

Pass the wires through the firewall using our firewall grommet.

Pull the Dash apart and remove the I/P. Start thinking about where you’re going to put the GPS sensor and where you’re going to run the OEM and Holley harnesses.

Honestly, the Terminator X and 12.3 Wiring shouldn’t be that bad - reference the pinouts above in this document.

The Holley harnesses for the term X should be pretty much plug and play. For the additional I/O we used wires pulled from the VCM since they used the same pin as the holley connectors. This saved us time from not having to re-crimp all the wires, just cut what was needed and add it to the Holley connector.

You’ll probably need to swap pigtails for sensors that you can steal from the OEM harness.

Use the right crimping tools for the Holley connectors if you are not using original wire like we did.

Install your Holley Pro Dash with our dash kit for 88-94 trucks and 95-98 trucks.

Most holley kits come with the IAC connector pinned for a Chrysler, make sure you check the pin out on your harness vs the harness and sensor you have. This may have been because we used a Universal Terminator X Max kit for the big block, it may be different for the LS kits.

Be considerate of wire strain relief and vibration, as well as noise in the circuits - try not to run coil wires and shit alongside a CAN cable.

To connect the Holley 3.5, we used their Holley EFI CAN Splitter (558-465) so we can also use their CAN to USB-A cable for tuning without disassembly.

Install your Holley 3.5 with our vent gauge pod and 3.5 adapter.

Who This GMT400 Holley EFI Resource Is For

This write-up is for builders who are serious about planning a GMT400 Holley EFI swap correctly, especially on 1995–1999 trucks and Suburban-style configurations where factory systems, accessory logic, and cluster behavior all matter. It is especially useful if you want to keep more than just engine function and care about cruise, ABS, dash behavior, transmission control, and clean system integration.