Universal Pump Harness Diagnostics"Universal" Powerstroke Fuel Pump Wiring Harness Diagnostics

From time to time we have customers that either need some help understanding how this harness works, or need some help figuring out why it isn't working correctly in their truck. This article will cover the complete operation of our Universal Fuel Pump Harness so that even those with limited automotive electrical understanding can locate and fix a problem if needed.

NOTE: If diagnosing electrical issues, we always recommend having a good DVOM (digital volt ohm meter) handy. A simple "test light" can help with some problems, but leaves a lot to be desired when trying to figure out some types of problems. Also, all descriptions of operation as well as diagnostic tests assume that the truck has good batteries, good battery cables, good chassis grounds, tight battery terminals, etc. Before attempting to diagnose ANY electrical issue, it's a good idea to make sure that you are not dealing with a simple low battery, loose cable or bad ground first.



BATTERY PIGTAIL: This is shown at the top left in the above diagram. This pigtail consists of a 2-Wire Weatherpack connector with about 12" of wire that is terminated with large ring terminals. It is important to note that the Positive wire in this pigtail contains a 30A Fusible Link (not shown in diagram). PURPOSE: The battery pigtail makes the connection to the battery and provides fused protection for the harness.

PUMP RELAY: Shown as "Relay" on the diagram, this component is the box with the metal tab and 4 wires coming from the bottom. PURPOSE: The relay is the switch that turns the fuel pump on and off based on input from the Blue Wire.


RED: In the Battery Pigtail and the Main Harness leading to the Relay, the Red wire is connected to the (+) terminal of the battery and is always hot (unless the fusible link blows). After the Relay and all the way to the fuel pump, the Red wire is Switched 12v+ to run the fuel pump only when the relay is activated.

BLACK: Once the Battery Pigtail is connected to the (-) terminal of the battery, every black wire in the harness is connected to full time ground.

BLUE: The blue wire is a key switched 12v+ trigger for the relay. Anytime you want the pump running, the blue wire should have 12v+. In 99-03 7.3L and 03-07 6.0L Powerstroke applications, we recommend connecting the blue wire to the 12v+ wire from the OEM fuel pump. This ensures that the PCM and Inertia Cutoff Switch still maintain control of the fuel pump(s) in the vehicle. If this harness is being installed in a vehicle that doesn't have a factory fuel pump control wire, the blue wire needs to be connected to a KEY-ON or switched 12v+ source, and the pump will run whenever the key/switch is on.

HARNESS OPERATION (Once Fully/Properly Installed):

At Key-On, the Blue Wire will get 12v+. Since the black wire at the relay is already connected to ground, this will cause the Relay to activate and the pump will start running. In a 99-07 7.3L/6.0L Powerstroke, if you do not start the truck within about 20 seconds, the PCM will time out and kill power to the blue wire, which will cause the pump to stop running. This is correct/normal operation.

When you turn off the key to shut off the truck, the Blue Wire loses 12v+ from the ignition switch being turned off and the pump will stop running.

If you are using this harness in an application other than a 99-07 7.3L/6.0L Powerstroke, whatever method you choose (ignition source, toggle switch, etc) to control 12v+ on the blue wire will determine when the pump runs and stops.


Step 1: Check all harness connections (at the battery, pump and blue wire) and make sure they are secure, connected and that no connector pins have pushed out. Repair any issues found and re-test.

Step 2: Using a DVOM set for DC Volts, check the voltage at the pump (+) and (-) terminals WITH THE PUMP CONNECTED and the key on. If you have battery voltage (12v approx) at the pump, but the pump is not running, you more than likely have a fuel pump problem, not a harness problem. If you have significantly less than battery voltage at the pump, you will need to look at the harness more. The next steps assume little to no voltage at the pump.

Step 3: Check for Relay Operation. Hold on to the Relay while another person turns the key on and off. You should be able to feel the relay "click" on and off as they cycle the key. Keep in mind that this is NOT a guarantee of relay functionality, just a guide for where to move next.

RELAY NOT CLICKING: either the blue wire does not have battery voltage at key-on, the black wire is not connected to ground or the relay has failed.

RELAY IS CLICKING: either the red wire is not connected to 12v+, fusible link is blown, the relay pins are not making good contact in the socket or the relay has failed.

From here we get down to the basics. Do we have connectivity and voltage where expected. A Digital Volt Ohm Meter (DVOM) will be needed, and this article assumes you have a basic idea of how to use one.

Step 4: Disconnect the Battery Pigtail from the Main Harness at the 2-Wire weatherpack connector. Using a DVOM (set for DC Volts), check the voltage on the PIGTAIL pin for the Red wire. If you do not have the same voltage in the connector on the Red wire that you do right at the battery (12" of wire away), the Fusible link needs to be replaced. We have not had to replace these often, but it is possible. Contact Strictly Diesel at 623-582-4404 for replacement information. The next steps assume that you have the same voltage at the 2-wire connector as you do at the battery. RECONNECT THE PIGTAIL TO THE HARNESS! Make sure none of the pins push out!

Step 5: Push the release tab on the relay and separate the Relay (top) from the Socket (bottom). Visually inspect the relay socket and relay pins for signs of arcing (burn spots). If the relay and socket are clean and appear normal, proceed to the next step. If they do not appear normal, take some pictures of what you found and send them in to us at sales@strictlydiesel.com. Follow up by contacting Strictly Diesel at 623-582-4404 to discuss repair options.

Step 6: The tests in the next 2 steps will use the exposed terminals where the relay plugs in. Using the DVOM (set for DC Volts), check the voltage on the red wire that goes to the battery, it should be the same as battery voltage. Note: there are 2 red wires, one will have battery voltage and the other will show 0v, this is normal. Next, put the test lead on the exposed terminal for the blue wire. It should read 0v with the key off, and battery voltage with the key on. If you do not have battery voltage on the red wire to the battery (full time) and the blue wire (key on), you need to investigate these wires and their connections.

Note: in 99-07 Powerstroke applications, don't forget that the blue wire will go dead after about 20 seconds of key-on. You may need to turn the key off and back on to make sure the circuit is live for the above test. For any other applications, you need to make sure that whatever causes the blue wire to receive 12v+ (key on, toggle switch, etc) is enabled for the above test.

Step 7: Now, switch your DVOM to continuity (tone) mode and check the connection between the battery (-) terminal and the black wire lead at the socket. You should have tone and/or a very low ohm reading. If the black wire doesn't have continuity to the battery, it needs to be checked at the battery or inspected for breaks because the harness has no ground.

Step 8: With the relay still removed from the socket, use (2) short pieces of wire to connect relay pin 85 to the (+) battery terminal and relay pin 86 to the (-) battery terminal. This will cause the relay to activate, and each time you make or break the connection, the relay should "click". If you want to take this test a bit further, you can put the test leads of your DVOM (set for continuity/tone) on relay terminals 87 and 30. Any time you make contact to the battery and the relay clicks closed, you should have tone. The tone should stop when you break the connection with the battery. If all of the checks in this step are good, plug the relay back into the socket and make sure it snaps fully into place.

By now, you should have identified the problem in your harness.

Author: Dennis Schroeder - Co-Owner of Strictly Diesel

Dennis has been Designing, Building and Supporting Aftermarket Fuel Systems for 7.3L and 6.0L Powerstrokes since 2001.