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I have a 2008 Fit Sport with a manual transmission. My AC died last summer. After going through two independent mechanics both leaned towards a bad ECM but mostly gave up. I finally took it to the dealer, and after them badly troubleshooting it to the AC switch on the dash and charging me $280, they then said it was the computer (much longer story). Their quotes far exceeded the value of the car to replace the ECM and I wasn't very happy with their troubleshooting, so I was thinking of eBaying a computer, but wanted to find out what is needed to program it to work with the immobilizer. The computer is part 37820-RME-A17, and I can get one around $100 off eBay. How involved is flashing the VIN & programming the keys? Is it something a locksmith who does keys can do? Would most shops be able to do it? Or am I stuck with the dealer. Is there any software I can buy for a reasonable price to do it myself? Any help would be appreciated...AC season is rapidly coming on the mid Atlantic!
I'm surprised a PCM could go bad on the A/C circuitry,
The computer basically activates the A/C compressor clutch and radiator fans according to the A/C switch status on the dash (and the A/C pressure switch)
You could hard wire a switch (or even the actual dash switch) to do exactly the same.
The A/C switch is plugged in series with the pressure switch, telling the PCM you want to activate the A/C system.
Then, the PCM activates the compressor clutch relay (Red wire) and the radiator/condensor fan relays (Green wire). It will deactivate them under certain circumstances, like when you floor the engine so you have more power.
When the pressure switch reaches a certain value, it's the same thing as if you just pressed out the A/C switch on the dash. The PCM cuts the compressor.
Make sure the switch is good and the relays work. If the A/C fans come on when you press the switch, the PCM "should" be ok.
If you want to bypass the PCM, you can do this:
You can splice the blue/white wire at the PCM and connect it with the red and the green wire. Of course, you'll need diodes on each wire so current from the green wire cannot go into the red wire of blue/white wire if the PCM activates the radiator fan to cool the engine.
I would leave the blue/white wire connected at the PCM so it know when the A/C is actually engaged and will compensate for idle speed.
First of all, does OP have automatic or manual a/c??
If manual refer to schematic below:
With ignition on, a/c OFF, engine running or stopped check for 4.4 Volts on the E28 (red box) of PCM
If voltage is NOT present check the evaporator sensor and radiator fan sensor lines for continuity (should be less than infinity)
(blue box)
If voltage is present, disconnect the battery, wait 30 minutes and unplug E connector from PCM
Push a/c switch in, move blower to 2nd and check for continuity between E28 Harness side and a ground
If no continuity check the switches as indicated:
Check and reply!
[QUOTE=carotman;1426806]I'm surprised a PCM could go bad on the A/C circuitry,
It can if someone messes it up during testing
The computer basically activates the A/C compressor clutch and radiator fans according to the A/C switch status on the dash (and the A/C pressure switch)
Not just that, it needs analog data (variable resistance) input from 2 thermistors (ECT2 -in radiator) and Evaporator thermistor (so later will not not freeze)
You could hard wire a switch (or even the actual dash switch) to do exactly the same
The A/C switch is plugged in series with the pressure switch, telling the PCM you want to activate the A/C system.
Then, the PCM activates the compressor clutch relay (Red wire) and the radiator/condensor fan relays (Green wire). It will deactivate them under certain circumstances, like when you floor the engine so you have more power.
About right
When the pressure switch reaches a certain value, it's the same thing as if you just pressed out the A/C switch on the dash. The PCM cuts the compressor
(this happens if one of the fans are not working and condenser cannot cool the gas)
When thermistor's (at evaporator) resistance increases in response to the temperature drop and goes past threshold value, the PCM will remove + power from terminal E28, switching the a/c off (cycling) to prevent ice building up on evaporator fins
First off, thanks for all the replies! It is a 2008 US Fit Sport with around 115k miles. From my limited poking around I believe the schematics match the two that Carotman posted. I'll give the backstory...
Around the end of 2015 the AC cut out on the drive home from work. While still driving I waited about 20 minutes then tried again and it ran for a bit then died again. I dropped it off at the mechanic and he said the compressor was bad, $700 later and a new compressor, and the AC was back in business.
Fast forward to about summer 2017 and in similar fashion my AC died after driving for about 20 minutes. While still driving I left the AC off for around 20 minutes then tried again and it worked for a little while before dying again. I initially thought the AC clutch relay was bad, so after seeing the fan and clutch relays were the same I swapped them, but the clutch still failed to pull in. The condenser fans also don't come on.
Thinking there may be a small leak from when the compressor was changed a few years back I bought one of the recharge kits. The gauge on the recharge kit showed it low, so I added about 3/4 of a can worth of refrigerant. The AC ran great for about 3 weeks. After about 3 weeks I was having the same issues, where it would run when first started, then stop running after the car had been running for a few minutes. Thinking I had a leak, I dropped it off with my mechanic. He put a real gauge set on it and said that the pressures looked good, if anything a bit overcharged. I explained what I had done to that point, and he still suspected the relays, so he ordered a set. After swapping them with no luck he next thought it was the pressure sensor. After waiting a few weeks for the part to come in, he changed it, but the AC still won't run. He was next thinking it could be the compressor again, but not wanting to spend $700 again I declined to go further. I tried another independent mechanic, who I believe tried the same things but in the end gave up. After I got it back I checked that jumpering the compressor relay output pulls the clutch in and the system cools. My girlfriend's mom had a CRV that also had a hard to troubleshoot AC problem. It ended up being the evaporator thermocouple. I tried checking mine, and both got a reasonable Ohm value for it based on the ambient temperature, and also jumpered it with a 10k Ohm resistor back to the ECM and still it didn't work.
Over the winter I took it to a Honda dealer. Their initial notes are "AC issue duplicated. Connect HDS to vehicle found every time vehicle AC switch on dash is push on under fuse no. 22 keeps blowing rec replacing switch to start with and retest vehicle for any other related issue." The dealer called and I carefully questioned that if they jumpered the switch everything ran. They swore up and down that it did. Once the switch came in I got a call that it was now the computer along with a $1700 quote to replace it. Not thrilled that they didn't diagnose past the bad switch properly I opted to get the car back. Their notes after changing the switch state "Replaced AC switch fuse stopped blowing still no command for AC compressor clutch on at switch. Under fuse box has 12 volts at no. 2 terminal clutch clicks when jump at PCM 31P connector and at relay PCM not sending signal to command compressor rec R/R."
The car isn't worth a ton, so it isn't worth spending over $1700 the dealer wanted to replace the ECM. I bought one from eBay last night for $50, and just need to get it programmed. I know everyone thinks the ECM is a long shot, but it is worth trying another few bucks to see if it can get it going, as it doesn't appear to be anything else, but if anyone has any other suggestions, I'm happy to try them! My other weakness is I'm not great on automotive electronics... I'm great with 125VDC contorl systems, but this frame ground thing has me scratching my head! Thanks for all the input thus far!
04-08-2019, 05:05 PM
