Hi all. I've been relying on this forum to help me with past issue but this one has me stumped. My 96 LX a/c quit working last month and an indy suspected a bad ecu after several hours of diagnostics. I installed a used ecu (numbers matched) from ebay but now the clutch is always engaged, even with the switch turned off. Could this be a bad ecu or perhaps an ecu for a non a/c car?

 

Ok guys, DOES the ecu have any thing to do with the ac circuit on this model? My limited troubleshooting skills would suggest either the on/off ac switch is stuck on or the ac relay is stuck in the on position. Time for me to learn also...opinions please.....

 

Thanks for your input Bandit. Prior to changing the ecu, the only way I could get the clutch to engage was by jumping the relay. Additionally, the low pressure switch had juice with the key on but didn't when the car was running.

I will swap relays tonight to see if that helps.

 

Yes. When the A/C button is pushed, the ECU grounds the compressor clutch relay so that the compressor clutch can engage. The purpose of including the ECU in the A/C circuit is to allow the ECU to compensate for the increased engine load from the A/C by ramping up the idle speed up.

 

A stuck closed relay would have caused the clutch to engage regardless of what ECU were installed or even if the ECU were unplugged.

Did you just forget to unjump the relay socket?

 

No. The jumper was removed and relay inserted. Is it possible to be the incorrect ECU? I just noticed non a/c written in yellow paint on the side but the numbers match.

Here is the ebay description: Up for bid is an ECU *Engine Control Unit* out of a 1996 Honda civic LX model with the D 16 non-vtec motor this ECU will work with all non vtec OBD2 engines code on the ecu is 37820-P2E-A71

 

If you remove 7.5A dash fuse 17, does the compressor clutch now disengage?

Does the A/C button light turn on and off when you press it? Or does it remain on?

 

Clutch disengages with fuse out. A/C button light does turn on and off.

 

This rules out a stuck open relay. Check for a short the Blk/Red wire that runs between the compressor clutch relay and ECU terminal A17.

 

Ron, thanks for your input ! I will look at it tomorrow evening and report back.

 

Here's what I've found. Obviously when I unplug the low pressure switch, the compressor stops. According to Haynes, the blue/white wire should have 5 volts but I get around 2. On the other hand, the blue/red has about 4 volts.

I grounded my test light and probed the blue/red wire, the tester lights up and the compressor comes back on as long as the probe is touching. If I probe the blue/white wire the test light comes on but no compressor.

Somethings off but where?

 

Does the condenser fan also run continuously with the pressure switch plugged in? Or does the compressor clutch only engage and the fan does not run?

Grounding the Blu/Wht wire with the pressure switch unplugged should NOT cause the compressor clutch to engage. So that's expected.

 

Wow, condenser fan runs all the time period. Even if unplug the compressor relay, low pres switch and switch the fan motor off. So basically the compressor and fan run all the time.

Grounding the blu/wht wire doesn't engage the compressor, it's the blu/red wire which can't be right.

 

Relevant circuit:

Pressure switch & fan relay ---> Blu/Wht wire ---> thermostat ---> Blu/Red wire ---> A/C switch

The problem is that there is a short in the Blu/Wht wire, thermostat, Blu/Red wire, or A/C switch. Use a multimeter to pinpoint the location of the short.

Incorrect. Grounding the Blu/Red wire SHOULD cause the compressor clutch to engage.

 

Thanks for your help. Electrical issues aren't my strength but sounds like an easy fix one I find the short.

 

Start by unplugging the thermostat and then the heater control panel to see whether the clutch and fan turn off. The results will help to pinpoint the short.

 

2 steps up 1 step back. Unplugging the thermostat stopped the comp and fan. Just to further diagnose, from the plug I jumped the blu/wht and blu/red wires and everything came back on. For some reason now the fan doesn't work at all and the compressor only comes on when the fan relay is removed. Checked all fuses under the dash and in the relay box, nothing blown.

 

This test^ was done with A/C button off, right? This tells you that the short is in the Blu/Red wire or the heater control panel.

Tests^ done with A/C button on or off?

If you remove the fan relay and jump terminal 4 to 1 in the socket with the key in ON(II), does the fan run?

 

Went back out and comp and fan won't come on at all...even if I jump the terminals in the relay box (terminals do have juice). I've double checked all of the plugs and everything is correct. No more juice to the low pres switch with key on or running.

 

Use a test light or multimeter to test for voltage to body ground at installed 20A hood fuse 56 and installed 7.5A dash fuse 17. Click the Test Fuses link in my signature.

 

Fuses good per test light. Other than fuse box or hood, are there any other fuses?

 

What exactly did you find when testing the fuses with a test light? It's important to KNOW the fuses are good before considering anything else.

 

Test light came on when touching the service terminals on each side of the fuses.

 

Unplug ECU connector A and then test for battery voltage to body ground at ECU terminal A17 with the key in ON(II). Do the same voltage test at the Blu/Wht wire terminal of the unplugged A/C thermostat connector.

 

I don't get any power from ECU A17 providing it is the widest plug then count pins starting from top left. This was also confirmed by a diagram I found on the net.

blu/wht a/c therm has power.