Acura MDX (2008-2016) Relay Guide: Diagnosing A/C and Fuel Pump Issues
Before you suspect a major failure in your A/C or fuel system, check these common, easy-to-replace relays.
- Two relays commonly fail on the 2008-2016 MDX: the A/C relay (causing no cold air) and the fuel pump relay (causing no-start or stalling).
- Before replacing an expensive A/C compressor or fuel pump, always check the corresponding relay first.
- Relay replacement is a simple, plug-and-play DIY job that takes less than 15 minutes and requires no special tools.
- A quality aftermarket relay is a cost-effective and reliable alternative to the more expensive OEM part for this component.
Is a Bad Relay Causing Your MDX Problems?
A faulty relay can mimic more serious and expensive component failures. For the 2008-2016 Acura MDX, two relays are common culprits for specific issues: the A/C compressor clutch relay and the PGM-FI (fuel pump) main relay. Understanding their failure symptoms can save you time and money on diagnostics.
Symptoms of a Failing A/C Relay
If your MDX's air conditioning suddenly stops blowing cold air, a bad A/C relay is a likely suspect. You might have even paid for an A/C system recharge only for the problem to persist. The key symptom is that the A/C compressor clutch does not engage when you turn the A/C on. The relay, which is responsible for sending power to the clutch, can wear out from constant use, especially during hot summers.
Symptoms of a Failing Fuel Pump (PGM-FI) Relay
A failing fuel pump relay is more serious and can leave you stranded. The most common sign is an engine that cranks but refuses to start. Other symptoms include:
- Intermittent Starting: The car may start fine sometimes and fail to start at other times, especially after it has been turned off for a few minutes.
- Engine Stalling: The engine may suddenly stall while driving because the relay cuts power to the fuel pump.
- No Fuel Pump Noise: When you turn the key to the "On" position, you should hear a faint hum from the rear of the vehicle as the fuel pump primes. If you hear nothing, the relay may not be sending power.
On many Hondas and Acuras, the PGM-FI main relay can fail due to cracked solder joints on its internal circuit board 🎬 Learn how to test and replace a faulty main relay. from years of heat cycles.
Known Issues and Investigations
While no direct recalls exist for these specific relays on all 2008-2016 models, a Technical Service Bulletin (TSB #SB-11-031) was issued for some 2011 MDX models where the A/C blows warm air or the battery dies, which can be related to a faulty relay. 🎬 Watch: Fix A/C issues and battery drain with this relay swap. Additionally, 2016 MDX models are part of an NHTSA investigation into engine start-stop failures, where a potential fix involves replacing starter relays.
Choosing the Right Aftermarket Relay
When shopping for a new relay, you'll find both OEM (Original Equipment Manufacturer) and aftermarket options. For a simple electronic switch like a relay, a quality aftermarket part is often a smart, budget-friendly choice. Brands like Standard Motor Products are common aftermarket suppliers for the MDX. Since the function is straightforward, an aftermarket relay from a reputable source should provide reliable performance without the higher cost of an OEM part from the dealer.
Cost of Replacing an Acura MDX Relay (2008-2016)
| Part Type | Price Range |
|---|---|
| New OEM Relay | $44 - $74 |
| New Aftermarket Relay | $18.08 |
| Shop Labor | $37 - $54 |
Prices are estimates and may vary based on the specific relay type, supplier, and your location. The aftermarket price reflects our current inventory.
Installation: An Easy DIY Fix
Replacing a relay is one of the easiest repairs you can do. Most are plug-and-play and located in easily accessible fuse boxes.
- Locate the Fuse Box: The A/C compressor relay is in the under-hood fuse box, typically marked with a snowflake icon on the diagram inside the lid. The fuel pump relay for the second-generation (2007-2013) MDX is a blue relay located in the driver's side interior kick panel area. 🎬 See this walkthrough for locating and replacing the fuel pump relay.
- Identify the Relay: Use the diagram on the fuse box cover to find the correct relay.
- Swap the Relay: Pull the old relay straight out. You may need a pair of needle-nose pliers to get a good grip. Push the new relay firmly into the same spot. It will only fit one way.
As a quick test, you can swap the suspect relay with an identical one from a non-critical system (like the horn). If the problem moves to the other system (e.g., the horn stops working but the A/C now works), you've confirmed the relay is bad. Just be sure to put the good relay back in its original spot and replace the faulty one.
Frequently Asked Questions
My MDX cranks but won't start. Is it the fuel pump relay?
It's a very strong possibility. A failed PGM-FI (fuel pump) relay is a classic cause for a crank-no-start condition in Acuras. Before replacing the fuel pump, which is much more expensive and difficult, checking this relay is the first step.
Where is the A/C relay on my 2012 Acura MDX?
The A/C compressor clutch relay is located in the main fuse box in the engine compartment. The diagram on the underside of the fuse box lid will show a snowflake symbol indicating the A/C relay's position.
Can a bad relay drain my MDX's battery?
Yes. A relay can fail by sticking in the "closed" or "on" position, causing the component it controls (like the fuel pump or another module) to run continuously even when the car is off. This creates a parasitic draw that will drain the battery.
Is an aftermarket relay good enough for my Acura?
For a simple component like a relay, a quality aftermarket part is generally a reliable and cost-effective alternative to OEM. The technology is not complex, and as long as the part meets the correct specifications, it should perform just as well as the original.
Helpful Videos
Technical Specifications
OEM Part Numbers: 39794-SDA-A03 39794-SDA-A05 39794-SDA-004 39794-S5A-003
Multi-purpose 4 or 5-pin relay. The most common relays for the A/C and fuel pump systems are micro ISO relays.
The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- Acura MDX: