In modern vehicle design, 5-volt reference signals/circuits are a key aspect of vehicle command and control. As summarized by Gary Goms, the 5-volt reference signal typically

“flows through a sensor containing a resistance that varies according to changes in temperature, pressure or position. Due to this variable resistance, the signal return voltage to the ECM is always less than the reference voltage… [many] OBD II-related engine sensors are used to control an engine’s spark, fuel and throttle maps.”

But, since these sensors are part of larger subsystems, involving components, wiring harnesses and wiring, not to mention the modules they feed data back to, how does a technician or ambitious vehicle owner pinpoint a problem in the 5-volt reference circuit? This month, we’ve gathered several resources related to this topic with a goal of helping you craft (or hone) your diagnostic routine when it comes to problems related to the 5-volt reference circuit.


A 5-Volt Reference Primer


We start with an article by Gary Goms from UnderhoodService.com. In his comprehensive look at these circuits, Gary outlines their function, the ways in which they interact with the rest of the vehicle and identifies how a problem circuit can impact vehicle behavior. As he notes,

“When the sensor system is monitoring data correctly, the ECM automatically adjusts the spark and fuel maps and throttle openings to their desired settings. When one or more sensors fail, we begin to see various intermittent and continuous drivability complaints occur… if a single sensor shorts the 5-volt reference to ground, the ECM might resort to using an engine ‘limp’ mode that drastically reduces power.”

He then uses two case study examples to show how he resolved two customer complaints that ultimately involved the 5-volt reference circuit, one on a two-wire configured circuit and one on a three-wire configured circuit. It makes for a great introduction on the topic.


Developing a Diagnostic Strategy


Now that we have an understanding of how these circuits are configured, what they do and how they fit into the overall vehicle control system, we turn our attention to diagnosing and overcoming problems in these circuits. In researching this topic, the most common problem we found in these systems (in articles, forum posts and how-to videos) was a short to ground. This great article by Tom Tillman in Motor magazine outlines a diagnostic strategy for spotting 5-volt circuit shorts to ground, tracing them back to their cause and subsequently repairing them to restore the vehicle to proper function.

Of course, at the outset, he stresses the importance of starting with a complete, accurate wiring diagram (a common point of focus for all resources referenced in this article). He reviews the difficulty in sorting out whether the problem stems from the sensor, the wiring or the module that is communicating with the sensor and then outlines his strategy of using amperage draw to identify which wire/sensor path is the likely source of the problem. He points out that, although the absolute amp draw on the wire involved in the shorted portion of the circuit is still extremely low, requiring a milliamp meter to properly see its difference from the rest, it will be significantly higher than the amp draw on the wires on the properly operating portion of the circuit. He also stresses that the amperage reading should be taken on the wire(s) at the computer, not elsewhere along its length.

As he points out, once you have properly identified which wire is the one with excess current draw, you are only getting started. Now, you have to find the actual source of the problem. He provides a checklist of steps to take to try to isolate the source of the issue, with things to remember for successful completion of each step. This is a great article to help to develop or further refine your diagnostic strategy when you suspect you have a vehicle problem stemming from an issue with the 5-volt reference circuit.


Understanding Sensor Operation and Diagnostics

As we noted above, it can be difficult to pinpoint the root cause of a problem in the 5-volt reference circuit: is it a sensor, the wiring or a problem with the module itself? Understanding sensor operation and diagnostic strategies can be very helpful. In this quick article by David Bry on alldata.com, he sheds light on this diagnostic area. As he states:

“In many cases, by looking at the wiring diagram, developing an understanding of how the circuit works, and doing a few simple tests, you will determine the source of the fault. Ultimately, your task is to determine if the problem is related to a primary ECU, the wiring, or the sensor itself.”

He takes a dive into sensor operation, using the Engine Coolant Temperature (ECT) Circuit as a representative temperature sensing circuit and the Throttle Position Sensor (TPS) Circuit as a representative position sensing circuit. For each, he outlines a diagnostic strategy to confirm proper operation as well as identifies common error patterns. As he states, “Armed with these hints you’ll save time and increase your speed and accuracy while diagnosing temperature and position sensing circuits.”


Video Case Study

If you’ve stuck with us this long, you’ve done a fair bit of reading. So, we’ll wrap up with a case study video by Scanner Danner, addressing a problem in the 5-volt reference circuit on a Chevy Silverado. As he shows, the vehicle has clearly identified a problem with the 5V REF2 circuit (P0651). He then walks through his diagnostic process for finding the source of the problem. While it is a long video, it is very instructive, taking a step by step approach and showing how he uses his scan tool and other tools throughout the diagnostic process. If you have a little time, it is a good investment toward refining your diagnostic approach.


We hope this look at the 5-volt reference signals/circuits was helpful for you. The common denominators here are to start with your vehicle data, codes and a complete wiring diagram for the vehicle. Then, each of these resources provides a perspective on working back to the original source of the problem, culminating in the video case study above. Have you run into this as a common source of customer complaints? If so, we’d love to hear about it in the comments.