A tale of two circuits

Jan. 1, 2020
The current required to turn a relay on or off usually is no more than a few tenths of an amp, a level the controller can handle.

The automotive A/C system has several high current components that the electronic control units (ECU) manages, including the A/C compressor coil, blower motor and fan motors. Current flow through these components can run from three amps or so, to better than 20 amps. Current flow of that kind would spell almost certain death if it passed directly through the driver of a typical ECU, so a neat little electrical device known as the relay acts as an intermediary.

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The current required to turn a relay on or off usually is no more than a few tenths of an amp, a level the controller can handle. While the use of relays to control high current devices is certainly not limited to the A/C system (fuel pumps and window motors jump to mind), because this is our A/C issue, we’ll focus on those applications.

One Fault, Two Circuits?
If you remember your electrical fundamentals, you know that every circuit has five basic elements: a source (typically the battery), a load (the component that the circuit is designed to operate), a circuit protection device (a fuse, circuit breaker or fusible link), a control (a means to open or close the electrical pathway and turn the load on or off) and a complete path that connects all of them together. When a load doesn’t work the way it’s supposed to, the fault has to lie in one of these five areas. Easy, right?

How does all this apply to a relay-controlled circuit? The relay is an electrically operated switch that closes and opens the current path to the primary component (the compressor coil, blower or fan motor) we’re trying to operate, doesn’t it? That makes it a control in the primary component’s circuit.

But it is electrically controlled. Something else is turning the relay on (the electronic control unit that is managing the primary component). Doesn’t that make the relay a load, too? That’s another circuit entirely.

The first step in diagnosing a problem in a relay-controlled circuit is to figure out whether the problem is on the primary component side of the relay or the load side of the relay. Most of us understand this instinctively when we glance over the schematic. The relay marks an electrical crossroads of sorts. I know I’m not the only one who has stolen a relay from another section of the fuse box and swapped it with the suspect relay. If the primary component now works, though, does that mean it was the relay’s fault? And what did we learn if the component the relay is supposed to control still doesn’t work?

What’s Working?
If an electrical circuit is complete and operating, current will be flowing through it. Measuring the amount of current can provide a quick answer to what is working and what’s not.

To use current to isolate which of the two circuits to focus on, go back to the schematic for the component you want to troubleshoot and identify its relay control. Trace the two power feeds at the relay (one for the relay, one for the component) back to the battery. Along the way, you’ll pass through at least one fuse, and that makes a great test point for your current measurement. Often, the same fuse will be protecting both power feeds to the relay, allowing you to identify the problem side with one measurement. (If not, you’ll need to take a measurement at each fuse. For the purposes of this discussion, we’ll assume that one fuse feeds both.)

Remove the fuse and replace with a fused jumper wire. Place your low amp current probe around it, turn the primary component “on” and read the current measurement. A reading of 0.0 amp tells you how the circuit where the relay is the load is working. No current is flowing, so the relay is not on when it is supposed to be, and you need to diagnose why the circuit is open. There is either a physical “open” in the circuit or the control module in charge is not “closing” the circuit path on purpose.

To remove the relay itself as a possibility, swap it out with an identical relay and repeat the current test. If the current reading remains 0.0 amp, verify power to the relay. Once power is verified, connect your scan tool to see if any circuit-related codes are stored. Are all the input parameters that the control module uses to decide when to issue the on command intact? Can you command the relay on with the scan tool? Is the circuit path to the control module intact and free of excessive voltage drop?

A reading of less than 0.5 amp or so tells you that the relay is on and that whatever parameters the control module needs to see to turn it on are all present and accounted for. The primary component, however, is not on, so focus instead on the circuit that uses the relay as a control. Again, you’re looking for an open circuit condition, just this time it’s focusing on the path where the primary component is the load. To isolate the relay as a cause, simply jumper the relay panel’s terminals with a fused jumper wire to see if the primary component starts working. If not, verify the integrity of the circuit paths on both the power and ground side of the primary component using the voltage drop testing method. If power and ground voltage drops are in specification, the fault has to lie in the component itself.

The last possible measurement will be in the whole amp range (1.0 to 20.0-plus, depending on the primary component type). That tells you that, electrically, all is well. If the primary component isn’t working, it is likely a mechanical fault with the component and not an electrical one. Some examples are A/C compressor clutches that have an excessive air gap, or blower motors that are mechanically seized.

Voltage What?
Using current is an easy, convenient way to isolate which circuit you need to focus on, but many of us don’t own a low-amp current probe. And even if you do, you still will need to perform some traditional tests to pinpoint the exact problem. Let’s start with the circuit where the primary component is the load, because that is the simpler of the two to troubleshoot.

With just three measurements, two taken as close to the load as you can get, you should be able to narrow the problem down very quickly. The main issue will be accessing the component. Some compressors are buried in the engine compartment, making access to the connector a little difficult. Ideally, you want to backprobe the wiring directly at the component’s connector. If you must pierce the wiring, be sure to use some liquid electrical tape to seal any holes you made, or you will have set the stage for a future failure. One alternative I also have used successfully is to disconnect the harness from the primary component and reposition it for easier access and then substitute a different load (like a sealed beam headlight). This still allows you to test the circuit path for voltage drop issues. Just be sure to make up a jumper harness that uses the correct size terminals so you don’t create any new problems while trying to solve the original one.

With the circuit on, measure the voltage directly at the battery and record your reading. Now measure the voltage available on the power side of the primary component, as close as you can get to the actual component itself. Keep the negative meter lead at the battery’s negative terminal to insure you are testing as much of the circuit path as possible. Record your second reading. Last, measure the voltage available at the ground side of the component and record that reading.

Voltage available at the power side should be nearly equal to what you measured at the battery, while voltage measured on the ground side should be nearly non-existent. That is basic electrical theory. Voltage is used to overcome resistance and allow current to flow. Whichever side of the load isn’t reading as it should is the side with the problem. Start backtracking along the circuit path towards the battery until your reading does return to normal. Your problem lies between those last two measurement points.

If the power side and ground side readings are OK, the problem lies between your first two test points or in the component itself.

No power to the component? Bypass the relay to eliminate it as the problem, being sure to use one identical to the one you’re testing. If the relay is OK, you’ll need to perform the same type of voltage drop tests on the relay control circuit. Make up a load substitute using an old turn signal bulb. Just be sure to use terminal ends that match up with the female relay connector sockets to avoid damaging them. You should see the bulb light when the control module turns the relay on. If not, command the relay on using your scan tool and perform the same two load-side voltage measurements you performed on the primary component earlier to identify which side of the circuit path has an issue.

Testing Electronic Controlled Variable Compressors
Ever-increasing fuel economy requirements as well as stricter emissions standards impact nearly every system on new cars. The automotive air conditioning system is certainly no exception. Today’s mobile A/C systems provide better cabin cooling using smaller amounts of refrigerant in more tightly sealed systems, run by more efficient compressors; all leading to reduced emissions and better gas mileage.

Electronic Controlled variable displacement compressors (ECVs) are becoming more and more the norm for this very reason. Controlled by the Powertrain Control Module (PCM), compressor displacement (and pumping ability) can be precisely controlled to deliver the most efficient operation possible under any given combination of conditions.  Unlike variable displacement units that are controlled as a direct result of the heat load on the evaporator (GM’s V5 or V7, for 

example), ECVs are controlled through an electronic solenoid that receives a pulse width modulated command from the computer. Displacement on an ECV can be quickly and seamlessly controlled in a matter of milliseconds. And since most ECVs have no clutch, traditional methods of diagnostics may lead to unnecessary compressor replacement.

That’s where the CLT1 ECV compressor driver tool (from Four Seasons, a Standard Motor Products company) comes in. This tool allows the user to command the compressor displacement up or down while monitoring system pressure and temperatures. The kit also includes a compressor solenoid simulator that plugs into the main harness to prevent the accidental setting of HVAC-related trouble codes.

A Breakout Box For Relays
That is only one advantage to the AES uActivate. This tool provides a few advantages when diagnosing any relay-controlled circuit.

First, it provides manual control of the circuit’s primary load using a toggle-style switch. Second, it has four test points corresponding to the four pins on most relays; power in to the relay’s control circuit, ground out from the relay’s control circuit, power in to the relay’s switch circuit and power out to the primary component. Third, there is a built-in current loop that provides a convenient access for your low amp current probe making it easy to get a current reading or scope waveform of any component that relies on a relay to turn on.

One distinct advantage with the uActivate is the ability to quickly and easily perform a ground or power side voltage drop test on the relay control circuit. An LED on the front of the tool, between the two test points, illuminates when the ECU has commanded the relay circuit on and provides instant verification that the relay control circuit is working. A green light means that the green test port is the power side of the control circuit while a red light means that the red test port is the hot side. The box itself is the substitute load, allowing you to test the entire circuit path in the palm of your hand.

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