Stop the Clicking: How to Diagnose and Fix Mustang Relay Problems

How to Fix Relay Issues

Electrical relay problems in your 2020 Ford Mustang can manifest in frustrating ways, from a no-start condition to accessory failures. These small, switch-like components are critical for managing high-current circuits for items like the fuel pump, starter, and convertible top. When they fail, the symptoms can be intermittent and confusing. The good news is that diagnosis and repair are often straightforward. As one owner, searlesscott, shared after a fix: "Took the car to a shop that specializes in convertible tops...since I had the seat removed and had already done a lot of trouble-shooting...it was a quick solve...a bad relay in the connection next to the motor...$75, good as new!"

Symptoms

The most common symptom reported by owners is a rapid, repetitive clicking sound when attempting to start the engine. This is the classic sign of a starter relay or solenoid engaging but not sending sufficient power to crank the motor. As GeneralGumdrop described, "But now I’m getting a rapid clicking when I try to start. what I’ve ruled out I tested the battery it has a little over 12 volts." This indicates that even with a battery showing adequate voltage, a faulty relay can prevent the engine from turning over.

Intermittent operation of major electrical accessories is another hallmark of relay failure. This is especially prevalent with power features like the convertible top motor or power seats. The system may work one day and be completely dead the next, or it may struggle and act erratically. Owners often describe this as a "pain in the butt" to diagnose because the problem comes and goes, pointing away from a simple blown fuse and towards a component like a relay that is failing under load or when hot.

You may also experience a complete loss of power to a specific system with no warning. For instance, a relay controlling the fuel pump or fuel pressure regulator can fail, cutting power entirely. This leads to a no-start condition or severe drivability issues like engine stalling or "choking out." Hammer6950 identified this exact scenario: "Hey guys, I have likely finally found the issue behind the excessive fuel dumping into my cylinders. I found that I'm getting no power to the fuel pressure regulator connector." A failed relay is a prime suspect for such a total loss of power to a critical component.

Finally, relay issues can contribute to parasitic battery drain. A relay that is stuck closed or malfunctioning can keep a circuit active even when the vehicle is off, slowly draining the battery. While not the most common symptom from the data, it is a known failure mode. As SolarJoe noted while diagnosing a drain, "With EVERYTHING, including the fuse block disconnected in the car, the alternator still draws 4+ amps, so I will simply install a RELAY in series..." This shows how relay logic is integral to managing electrical loads when the vehicle is parked.

Most Likely Cause

The primary cause of relay failure in these vehicles is electrical degradation within the relay itself. A relay is an electromechanical switch. Inside, a small electromagnet pulls a set of contacts together to complete a high-amperage circuit. Over time and with repeated use, these contacts can become pitted, corroded, or carbon-fouled. This increases electrical resistance at the connection point. When you try to send high current (like for the starter motor) through a high-resistance contact, it can’t flow properly, resulting in the rapid clicking sound as the electromagnet engages but the power can't cross the gap.

This degradation is often heat-related. The engine bay fuse box, where many critical relays live, is a high-temperature environment. Repeated thermal cycling weakens internal components and accelerates contact wear. A relay for a high-draw item like the fuel pump or cooling fan is under particular stress. The failure can be intermittent because sometimes the contacts might still make a partial connection, especially when cold. As they heat up from electrical resistance during operation, they may expand and lose connection entirely, causing the system to cut out—a symptom owners describe as happening when things get "hot."

While the relay itself is the component that fails, the root trigger can often be traced back to issues within the specific system it controls, particularly the fuel system. As indicated by owner reports identifying "fuel system" as a cause, a failing fuel pump drawing excessive amperage can overload and destroy its control relay. Similarly, a short to ground or high resistance in the wiring harness for an accessory like a power seat motor can place abnormal stress on its corresponding relay, leading to premature failure. The relay is the failure point, but it’s frequently a symptom of a deeper electrical fault.

How to Diagnose

Diagnosis starts with identifying which circuit is malfunctioning. Is it a no-start with clicking? Focus on the starter relay. Is the convertible top dead? Find its relay. Consult your owner's manual to locate the fuse and relay boxes (typically one under the hood in the Power Distribution Box and one inside the cabin) and identify the specific relay for your problem circuit. Having a wiring diagram for your 2020 Mustang is invaluable for this step, as it will show you the exact relay location and the circuits it controls.

The most effective diagnostic tool is the simple swap test. Most relays in the fuse boxes are identical. Find a relay that controls a non-essential but similar amperage system (like the horn or rear defroster) that you know works. Swap it with the suspect relay. If the problem moves to the other system (e.g., the horn now doesn't work), you’ve found the bad relay. If the original problem persists, the relay is likely good, and you need to look elsewhere. As optionizerSS described, "I tested all circuits detailed in that thread and tested the relays based on youtube videos. Even swapped a couple of relays back and forth and no change." This process of elimination is key.

If swapping doesn’t yield answers, you’ll need a digital multimeter (DMM) to test further. With the relay removed, you can test its coil resistance across the two smaller control terminals (usually pins 85 and 86). A typical 12V automotive relay coil should show between 50 and 120 ohms. An open circuit (infinite resistance) means the coil is burned out. Next, with the relay energized (you may need a 9V battery to power the coil), test for continuity between the two large switched terminals (pins 30 and 87). There should be continuity (near 0 ohms) when powered and open when not.

Finally, if the relay tests good, the problem is in the wiring or the component itself. You need to check for power and ground at the relay socket. With the ignition in the appropriate position (e.g., "Run" for fuel pump), use your DMM to check for 12V at the power pin in the socket (often pin 30 or 86, refer to a diagram). Check for a good ground at the ground pin (often pin 85). As LukeG detailed in his diagnostic process, "I swapped relays and checked all fuses. I checked for continuity of the harness at pin #87 to power at the connector, and chassis ground to ground at the connector." This methodical approach isolates the fault to either the relay, the power supply, the ground, or the component downstream.

Step-by-Step Fix

1. Safety First: Park on a level surface, set the parking brake, and disconnect the negative terminal of the battery. This prevents accidental shorts or electrical shocks while working on the fuse boxes.

2. Locate the Faulty Relay: Identify which circuit is failing. Using your owner’s manual or a fuse box diagram (often on the underside of the fuse box lid), find the exact location of the relay. For example, the starter relay is in the under-hood Power Distribution Box. The diagram will label it clearly (e.g., "Starter Relay," "Fuel Pump Relay," "PWR TOP").

3. Perform the Swap Test: Before removing the suspect relay, identify a compatible donor relay from a non-critical system. Grip the suspect relay firmly and pull it straight up and out of its socket. Install the known-good donor relay in its place. Reconnect the battery and test the previously faulty system. If it now works, you’ve confirmed the relay is bad. If not, reinstall the original relay and move to electrical testing.

4. Remove and Inspect the Relay: Once identified, remove the bad relay. Visually inspect the terminals on the relay and in the socket for any signs of melting, corrosion, or green oxidation. Clean the socket terminals with electrical contact cleaner and a small brush if needed. As one owner shared: "I tested all circuits detailed in that thread and tested the relays based on youtube videos. Even swapped a couple of relays back and forth and no change." This highlights the importance of moving beyond the simple swap if the problem is more complex.

5. Install the New Relay: Purchase a new relay with the correct part number. It is crucial to match the amperage rating and terminal pattern. Orient the new relay correctly by matching its terminal layout with the socket (most have a keyed or notched corner). Press it down firmly until it seats fully. You should feel a solid click.

6. Test the Repair: Reconnect the battery terminal. Operate the system to verify the fix. For a starter relay, turn the key to "Start." The engine should crank normally without rapid clicking. For an accessory like the power top, run it through a full cycle. Listen for smooth operation.

7. Address Underlying Causes (If Necessary): If the relay failed due to an overload, the new one may fail again. If you suspect an issue—like a fuel pump drawing too much current or a motor binding—it’s wise to investigate further. Monitor the system after repair. For parasitic drain concerns, as SolarJoe addressed by planning to install a relay, consider if the circuit logic needs modification, though this is an advanced step.

Parts and Tools Needed

Parts:

• Replacement Relay: Must be the correct specification for your Mustang. Common OEM part numbers are often something like BR3Z-14N089-A or DR3Z-14N089-A, but you must verify using your VIN or the number on the old relay. A standard 20/30A ISO 280 mini-relay is common for many circuits. A pack of assorted relays from a reputable brand (like Bosch) is a good investment for diagnostics and future repairs.
• Electrical Contact Cleaner: Such as CRC QD Electronic Cleaner, to clean the relay socket terminals if corroded.

Tools:

• Digital Multimeter (DMM): Essential for testing voltage, continuity, and resistance.
• Fuse/Relay Puller: Often included in the fuse box lid or a basic automotive tool kit. Needle-nose pliers can work in a pinch but be gentle.
• Basic Socket Set & Wrenches: For disconnecting the battery terminal.
• Shop Manual or Wiring Diagrams: Access to factory service information or a high-quality aftermarket manual for your 2020 Mustang is extremely helpful for identifying circuits and relay locations.
• Flashlight: For seeing into deep fuse boxes.

Real Owner Costs

The cost to fix a relay issue varies dramatically between DIY and professional repair, largely due to diagnostic labor.

DIY Repair: This is very inexpensive if you can diagnose it yourself. A single replacement relay typically costs between $10 and $25 from an auto parts store or dealership. If you purchase a multi-pack for diagnostics, you might spend $20-$40. The only other cost is your time, which could range from 15 minutes for a simple swap to a couple of hours for more involved electrical testing. The financial outlay is minimal.

Professional Repair: Shop costs are driven by diagnostic time. A straightforward case where the problem is obvious (like rapid clicking) might take a technician 0.5 hours to diagnose and replace the relay. At a shop rate of $150/hour, that's $75 in labor plus the $20 part, totaling around $95. This aligns perfectly with searlesscott's experience: "a bad relay in the connection next to the motor...$75, good as new!" This $75 fee likely represented one hour of diagnostic and repair labor at a specialty shop.

However, for intermittent issues, costs can climb. If the symptom is an intermittent no-start or accessory failure, a shop may need to book more diagnostic time. If the relay tests good and the fault is in the wiring, diagnosis becomes more complex. You could easily be billed for 2-3 hours of labor ($300-$450) plus parts. This is why learning basic relay diagnosis can save you hundreds of dollars. The repair itself is cheap; finding the problem is where the cost lies.

Prevention

While relays are wear items, you can extend their life. Minimize unnecessary cycling of high-amperage accessories. For example, avoid repeatedly testing a malfunctioning power window or seat motor before diagnosing it, as a stalled motor draws excessive current that can weld relay contacts. When installing aftermarket electrical components (lights, audio amplifiers), always use a properly rated relay wired with an inline fuse—never tap directly into a factory circuit not designed for the load.

Keep the fuse box areas clean and dry. Ensure the seals on the under-hood Power Distribution Box are intact to prevent water intrusion, which leads to terminal corrosion. During routine maintenance, such as when checking fuses, it doesn’t hurt to firmly press down on all relays to ensure they are fully seated. A loose connection can cause arcing and heat damage over time. Addressing electrical gremlins promptly is key; a short circuit or a component drawing too much current will kill a relay quickly.

What Owners Say

Real experiences from FORD owners:

Success Stories

"So update I fixed the shifting issue all I had to do was tighten some of the bolts on the arm connected to the neutral safety switch they were lose. But now I’m getting a rapid clicking when I try to start. what I’ve ruled out I tested the battery it has a little over 12 volts." — GeneralGumdrop (source)

Owner Experiences

"It reads accurately down to .1A. With EVERYTHING, including the fuse block disconnected in the car, the alternator still draws 4+ amps, so I will simply install a RELAY in series with the middle pin on the alternator connector and this will solve the problem." — SolarJoe (source)

"Hey guys, I have likely finally found the issue behind the excessive fuel dumping into my cylinders. I found that I'm getting no power to the fuel pressure regulator connector." — Hammer6950 (source)

"Connector to Fuel Pressure Regulator Has No Power Hey guys, I have likely finally found the issue behind the excessive fuel dumping into my cylinders." — Hammer6950 (source)

Real Repair Costs

"Posting an update here for the benefit of others. Took the car to a shop that specializes in convertible tops...since I had the seat removed and had already done a lot of trouble-shooting (thanks for the help here!), it was a quick solve...a bad relay in the connection next to the motor...$75, good as new!" — searlesscott (source)

"Took the car to a shop that specializes in convertible tops...since I had the seat removed and had already done a lot of trouble-shooting (thanks for the help here!), it was a quick solve...a bad relay in the connection next to the motor...$75, good as new!" — searlesscott (source)

FAQ

Q: How long does it take to fix a relay problem? A: The physical act of swapping a relay takes less than five minutes. The vast majority of your time will be spent on diagnosis. If you get lucky with a simple swap test, the entire job can be done in 15-30 minutes. If you need to break out a multimeter and follow wiring diagrams to track down a power or ground fault, it could take 1-2 hours for a diligent DIYer.

Q: Can I drive my Mustang with a faulty relay? A: It completely depends on which relay has failed. A bad starter relay means you cannot start the car—it’s not drivable. A failed fuel pump relay will cause the engine to stall and not restart. A faulty relay for a non-critical accessory like the horn or power seats may not prevent driving, but it’s not advisable to ignore it, as it could be a symptom of a larger electrical fault that could strand you.

Q: Is this a common issue on the 2020 Mustang? A: Based on owner discussion data, electrical issues involving relays are a frequent topic. Relays are common failure points across all modern vehicles due to their electromechanical nature. The 2020 Mustang is not uniquely prone to it, but with complex features like power convertible tops and seats, there are simply more relays that can potentially fail as the vehicle ages.

Q: DIY vs mechanic - what’s recommended? A: Relay diagnosis and replacement are one of the most DIY-friendly electrical repairs. The parts are cheap, the tools required are basic (a multimeter is the most advanced), and the process is largely non-destructive. If you are comfortable locating your fuse boxes and using a simple puller, you should attempt the swap test yourself. However, if the problem is intermittent and you’ve swapped relays with no change, or you are uncomfortable with a multimeter, taking it to a professional with proper diagnostic tools may be the faster, albeit more expensive, solution.

Q: What if swapping the relay doesn’t fix my problem? A: If a known-good relay doesn’t solve the issue, the problem lies elsewhere in the circuit. The next steps are to use a multimeter to check for constant power (usually pin 30) and switched power/ground (pins 85/86) at the relay socket with the ignition on. If power or ground is missing, you have a wiring, fuse, or control module issue upstream. If power and ground are present at the socket, the fault is in the wiring between the relay and the component or in the component itself (e.g., a seized motor or a failed fuel pump).

Q: Are all the relays under the hood the same? A: No, but many are identical. Your owner's manual will have a diagram showing the position and function of each relay. Circuits with similar amperage draws (like the horn and rear defogger) often use the same relay type. High-amperage circuits for the starter or cooling fan may use a physically larger relay. Always check the diagram before swapping, and never swap a relay from a safety-critical system (like fuel pump) into a non-critical one during testing unless you are sure they are identical.