That relay really stinks!

By Steve Schaeber, MACS Technical Editor

Here’s a situation where your nose can be the best tool in your box. A fellow technician was diagnosing a “high hi-side” issue on a Pontiac Sunfire, when he asked me for some advice. I explained that one of the most common causes of “high hi-side” pressures is due to condenser air flow issues. That being said, it’s not like there’s only one thing that can affect how air flows across a condenser. There are actually quite a few reasons why air flow may be impeded, causing elevated high side pressure. For example, there can be dirt or debris actually clogging the passageways, or sometimes even a plastic shopping bag could get sucked up onto the condenser inlet. That would really block the flow of air, likely causing the engine coolant temperature to increase as well. Perhaps those rubber or plastic air shrouds are not in the correct position to properly direct air towards the heat exchangers. Many times they are even missing, perhaps damaged or thought unnecessary by a previous technician. But as is usually the case, suspect something with the operation of the fan itself.

Figure 1 - vlcsnap-2014-10-08-10h23m49s85 High Hi-Side due to Blocked Condenser

Figure 1: A tell tale sign of condenser air flow problems, low side pressure will look normal while high side pressure will be much higher than usual.

In this particular case, it was mostly luck that helped me find the problem. It appeared that the cooling fan was not spinning fast enough, but it was hard to tell for sure. I happened to be standing out along the driver’s side front fender, so I crouched down to gaze at the fan. The cover was off of the under hood fuse / relay center at the time, when my nose got a bit close to the bank of relays. When breathing in, I noticed a “burnt electric” smell, which immediately reminded me of all those relays I had replaced in the past. I went over to the work bench and grabbed an old relay of the same type (they’re always handy to keep around). I swapped it out with the fan relay, and voilà! It began spinning noticeably faster than before. Upon further inspection of the disassembled relay, I found the corroded contacts, likely offering resistance enough to slow down the fan and reduce the volume of air passing over the condenser. With a new relay, that fan was blowing strong, and the high side pressure came back down to about 160psi.

Figure 2 - S1600036

Figure 2: Burned relay contacts can provide enough resistance to slow down a fan motor.

Ever have a “dumb luck” moment that saved the day in your shop? Send an e-mail to and let me know!

If you’re a service professional and not a MACS member yet, you should be! Become a member and receive a technical newsletter with information like what you’ve just read in this blog post visit for more information.

You can E-mail us at . To locate a Mobile Air Conditioning Society member repair shop in your area. Click here to find out more about your car’s mobile A/C and engine cooling system.

Mobile A/C professionals should plan to attend 35th annual Mobile Air Conditioning Society (MACS) Worldwide Training Conference and Trade Show, Meet me at MACS Make Connections that Matter, February 5-7, 2015 at the Caribe Royale, Orlando, FL.


About macsworldwide

Mobile Air Conditioning Society (MACS) Worldwide Founded in 1981, MACS is the leading non-profit trade association for the mobile air conditioning, heating and engine cooling system segment of the automotive aftermarket. Since 1991, MACS has assisted more than 600,000 technicians to comply with the 1990 U.S. EPA Clean Air Act requirements for certification in refrigerant recovery and recycling to protect the environment. The Mobile Air Conditioning Society (MACS) Worldwide’s mission is clear and focused--as the recognized global authority on mobile air conditioning and heat transfer industry issues.
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