Wiktor Wise
Wiring an electric speedometer can be a tricky task due to the variety of engine control modules (ECMs) available. The two most common ECMs are the GM controller and the Holley Terminator & Dominator systems, both of which can be easily connected to the transmission speed sensor. However, other ECMs may require additional steps, such as connecting the speed signal to the engine controller or using an intermediate signal converter. It's important to understand the basics of the relevant wiring and connectors to ensure the speedometer functions correctly and displays the right output speed.
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What You'll Learn
Understanding speedometer signal transmission to the ECM
Understanding how the speedometer transmits signals to the ECM is essential when hooking up an electric speedometer. The ECM (Engine Control Module) is a crucial component in modern vehicles, often referred to as "the brain" of the vehicle. It receives input from various sensors, including the speedometer, processes this information, and sends commands to regulate the vehicle's performance.
The speedometer signal transmission to the ECM can be achieved through different methods, depending on the vehicle's make and model. For example, the GM controller and the Holley Terminator & Dominator systems are two commonly used ECM types. In these systems, there is no need for complex setup procedures; simply plug the ECM directly into the transmission speed sensor and configure the transmission programming.
Another example is the Compushift system, which allows the speed signal to be fed into the GM ECM via the "speedo out" wire included in the harness. The connectors labelled TISS (Transmission Input Speed Sensor) and TOSS (Transmission Output Speed Sensor) are connected directly to the transmission speed sensors. It's important to note that early electronic transmissions may only use the TOSS, leaving the TISS connector unplugged.
In some cases, a signal converter may be required if the speedometer needs a specific signal pulse. For instance, the Tremec Magnums for GM engines use a Ford-style speedometer plug with a 17-pulse VSS output, which is incompatible with the GM ECM. To address this, an All-in-One harness can be used to convert the 17-pulse output to the required 40-pulse signal.
Understanding the speedometer signal transmission to the ECM is crucial for proper speedometer installation and ensuring the ECM receives accurate data for optimal vehicle performance and diagnostics.
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Using a GM controller or Holley Terminator system
Using a GM Controller
The GM controller is used to drive a speedometer electronically. The Quick 2 controller, for example, has a speedo output wire that connects to the speedo. However, it is important to correctly identify which wire on the PCM harness to connect it to. The original output speed sensor wires are light green with a black stripe and purple with a white stripe. These wires can be spliced in to run in parallel with the new controller's output speed sensor wires.
The DRAC, which is located behind the glovebox, converts the signal for the speedometer (2000ppm), cruise control (4000ppm), and to the PCM. To run the DRAC, you need to hook up the factory wires to the output speed sensor.
Using a Holley Terminator System
The Holley Terminator X Max outputs the speedometer as a PWM- signal, which means the expected +5V or +12V that most aftermarket speedometers require is not there. To resolve this, you can use a relay to convert PWM- to PWM+.
To enable the speedometer output, you must first pin map the output and then connect it to your speedometer. In the Pin Map, assign the speedo to Output #4, pin B3. Then, take the wiring harness that came with the kit and hook the designated wire to the speedometer input wire in your dash.
If you are using a VSS in the TH350 transmission, the signal wire must be hooked up to one of the available inputs and configured according to the instructions. You should then use the Dakota Digital Holley BIM Module to transmit the speed information.
The Holley BIM module comes with Terminator X-specific instructions and will transmit the speed information and other relevant sensor data over CAN Bus.
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Connecting Compushift harness to transmission speed sensors
To connect a Compushift harness to transmission speed sensors, there are a few steps to follow. Firstly, identify the connectors in the Compushift harness labelled TISS (input speed from the engine RPM) and TOSS (output speed of the transmission). These connectors will be directly connected to the transmission speed sensors. It's worth noting that some early electronic transmissions may only utilise the TOSS, leaving the TISS connector unused.
When connecting the Compushift harness, ensure you do not mix up the TISS and TOSS wires. If you do, you may encounter issues during the first test drive, such as the transmission refusing to shift or displaying an erratic speed signal. The Compushift system can read both input and output speeds, helping to identify any potential issues within the transmission.
For certain transmissions, like the Ford 4R70W or 4R100, the Compushift II transmission harness may require an additional connector. In this case, you'll need to connect the Digital Range Sensor connector, which is a square 4-pin plug, directly to your transmission. Similarly, for Ford AODE or E4OD transmissions, you'll need to connect the Manual Lever Position Sensor connector, a male 2-pin plug, separately to your transmission.
When connecting a Compushift to a Holley Sniper EFI or Holley Terminator X, the two control systems are linked via a CAN bus. Signals like throttle position and engine speed are automatically shared between them. Connect the HGM CAN-18 cable between the Holley EFI and its display, then join the grey Deutsch connector on the CAN-18 cable to the CAN bus connector on the Compushift controller harness. Ensure both the Compushift and Sniper are powered by the same switched ignition power.
Additionally, the Compushift transmission pressure sensor kit can be used to display line pressure. This kit includes a sensor harness and a pressure sensor. Locate the pressure test port on your transmission and thread the sensor into it, being careful not to over-tighten it. After installing the sensor, add the sensor harness to the main Compushift connector and route the wiring to the pressure sensor.
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Setting up a different speed signal for dash speedometer
Modern cars with electronic dashboards have a speedometer that uses a sensor in the output shaft of the transmission to calculate the speed of the car. This sensor could be a simple reluctor wheel and Hall effect sensor, or a gear-driven motor. This produces a signal for the PCM to interpret and display on the dashboard.
There are several types of engine control modules (ECM) available, but the two most common are the GM controller and the Holley Terminator & Dominator systems. Both can be purchased with an integrated transmission control module for an automatic transmission. In this case, you simply need to plug it directly into the transmission speed sensor and set up your transmission programming.
The Compushift harness has connectors labelled TISS and TOSS, which are connected directly to the transmission speed sensors. The TOSS is the output speed of the transmission that is displayed on the dash speedometer. The All-in-One harness can also feed speed signals directly to any electronic speedometer.
If you need to send a different pulse count to your dash speedometer, you can set up a secondary output with a different speed signal. The purple/white output wire on the All-in-One is connected to the purple/white (VSS TOSS Hi) in the GM harness, and then the light green/black wire is grounded.
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Using an electronic speedometer with a carbureted engine
The car industry has been increasingly adopting electronic controls, including fuel injection and digital/electronic dashboards. This has created some confusion about the best way to get the speedometer signal from the transmission to the ECM (Engine Control Module) and then to the actual speedometer.
There are several different types and styles of ECMs available, but the two most common are the GM controller and the Holley Terminator & Dominator systems. Both of these systems can be purchased with an integrated transmission control module for an automatic transmission. In these cases, you simply need to plug the module directly into the transmission speed sensor and set up your transmission programming.
For those using an electronic speedometer with a carbureted engine, the setup will not need to receive a speed signal into an engine controller, but it will need an electronic speed signal for the speedometer. Both the Compushift and the All-in-One harness can feed a speed signal directly to any electronic speedometer without the need for any other intermediate signal converters.
The connectors in the Compushift harness labelled TISS (input speed from the engine RPM) and TOSS (output speed of the transmission) will be connected directly to the transmission speed sensors. Not every transmission will utilize the TISS, and early electronic transmissions may only use the TOSS. If you mix these up, you will know right away during your first test drive when the transmission refuses to shift or shows an erratic speed signal.
The All-in-One harness can also be connected to the GM ECM VSS input in the same way as the Compushift. The purple/white output wire on the All-in-One is connected to the purple/white (VSS TOSS Hi) in the GM harness, and then the light green/black wire is grounded.
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Frequently asked questions
The best way to hook up an electric speedometer depends on the type of engine control module (ECM) you have. The two most common ECMs are the GM controller and the Holley Terminator & Dominator systems. If you have one of these systems, you can simply plug it directly into the transmission speed sensor.
The connectors in the Compushift harness are labeled TISS and TOSS. TISS stands for transmission input speed sensor, and TOSS stands for transmission output speed sensor. The TOSS is the speed that you want to see on the dash speedometer.
Yes, you can use an electronic speedometer with a carbureted engine or a throttle body injection kit. The Compushift and the All-in-One harness can feed a speed signal directly to any electronic speedometer.
To connect the All-in-One harness to the GM ECM VSS input, you need to connect the purple/white output wire on the All-in-One to the purple/white (VSS TOSS Hi) wire in the GM harness. Then, you need to ground the light green/black wire.
TISS stands for Transmission Input Speed Sensor and it measures the input speed coming from the engine RPM. TOSS stands for Transmission Output Speed Sensor and it measures the actual output speed of the transmission, which is the speed that is displayed on the dash speedometer.
