DIY: Fan Control Unit Repair

My 91 NSX started having issues with the AC, where sometimes the compressor wouldn't come on when commanded and would stay on when I turned it off (the compressor itself, not just the fan). Also, the condenser fans would come on sometimes when the AC wasn't on. The consensus seems to be the fan control unit failing, so I took it apart to see what was wrong. From a first glance, the electrolytic cap seems to be bulging:



Upon further inspection with a microscope, it also seemed that some of the solder joints were cracking:



I replaced the capacitor with a 5000-hour rated part (Digikey P14373-ND, $0.21) and reflowed the joints with a soldering iron, adding a little flux-core solder in the process to break the surface tension. Note that half the board is conformal coated, but I was still able to solder through it. Initial tests today seem to be promising, in that all the fans were off initially and turned on when they were supposed to. I need to do some more testing, but I'm fairly confident this will solve the issue. Took maybe an hour or two and $0.21. See below for re-flowed joints, hope this is helpful to someone else in the same boat.

This is a super common problem and you should replace all of the electrolytic caps on the board. Otherwise they are prone to leak and destroy the PCB traces, making repair a lot harder. Here is a list of all of them. I did all of mine recently before there was a problem.

Edit: maybe you are talking about a board different from the Climate Control Unit (CCU)?

jwmelvin said:

This is a super common problem and you should replace all of the electrolytic caps on the board. Otherwise they are prone to leak and destroy the PCB traces, making repair a lot harder. Here is a list of all of them. I did all of mine recently before there was a problem.

Edit: maybe you are talking about a board different from the Climate Control Unit (CCU)?

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My mistake, I should have specified that this wasn't the CCU. This is the box behind the passengers seat, next to the main relay. I replaced the only electrolytic on the board. The CCU in my car has already been repaired.

crxguy52 said:

This is the box behind the passengers seat, next to the main relay. I replaced the only electrolytic on the board.

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On reflection, this was reasonably inferred from your OP. Thanks for the update. Seems like a constant campaign of capacitor replacement would be wise. Kind of a pain but I suppose there are worse things.

Conformal coatings make repair a pain! If you try to de solder using an iron as opposed to a hot air rework tool I have found that it tends to foul the tip of the iron making it really hard to maintain a tinned tip. I have found the best solution is to try and remove the coating at the repair site. Acetone using a cotton swab to clean off the repair site has worked for me.

North Carolina is kind of steamy (at least compared to Saskatchewan). Warm moist air entering a cold interior when you exit the car could eventually result in condensation and corrosion. Best practise would be to touch up the conformal coating in the repair areas. MG chemicals and others sell the coatings in spray cans.

Old Guy said:

Conformal coatings make repair a pain! If you try to de solder using an iron as opposed to a hot air rework tool I have found that it tends to foul the tip of the iron making it really hard to maintain a tinned tip. I have found the best solution is to try and remove the coating at the repair site. Acetone using a cotton swab to clean off the repair site has worked for me.

North Carolina is kind of steamy (at least compared to Saskatchewan). Warm moist air entering a cold interior when you exit the car could eventually result in condensation and corrosion. Best practise would be to touch up the conformal coating in the repair areas. MG chemicals and others sell the coatings in spray cans.

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It wasn't too bad to reflow everything - the tip kept fowling, so I had to clean it every 3rd or 4th joint. I thought about removing all the coating, but I didn't have anything to replace it with and it would have been a mess (not to mention would have looked bad).

I thought about replacing the coating, but it seems like they wanted to protect the ICs (as opposed to the solder joints) since only half of the board was coated. The coating on the ICs was intact, so I left it how it was.

I Love nice cheap DIY fixes like this! Thanks!

fyi your pictures aren't loading. best to upload them directly to NSXPrime for posterity

vivanshah said:

I Love nice cheap DIY fixes like this! Thanks!

fyi your pictures aren't loading. best to upload them directly to NSXPrime for posterity

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Photobucket blocking everyone's pictures bothered me enough to go ahead and upload them to Prime. Hopefully this will help others in the future.

Also a quick update, the AC compressor has come on (and stayed on) whenever I turned the AC on - I'm going to call this one a success.

Mine started doing this about 6 months ago and i think it finally blew. This is a great writeup

Just started getting this intermittently, was driving me crazy. Would just not blow cold on startup without a pattern. I could leave the house with it blowing cold, park at the store, come out and it not be cold anymore. Then sometimes it would suddenly get cold after 15+ minutes of driving, or not all. Next morning, cold from the start and no issues all day throughout trips.

Strange part is that it would never stop being cold, or at least I never felt that. Looked through the service manual under the Climate Control section and couldn't find the fan control module listed as a section. Turns out it's on page 22-60 under "AC Compressor (cont'd)", middle small box next to the ECU behind the passenger seat.

There's also an interesting AC troubleshooting guide and self-diagnostics on 22-12.