ECM Schematic/repair

[tk]

On my 1996-T the A/C clutch output (connector 1 pin 8) is no longer working. My A/C compressor blew up and after replacing it, the output from the ECM no longer goes low. I can see the input from the ECU change and if I ground the wire to the clutch the A/C engages.
Does anyone have a schematic for the ECM for a 1996-T or know what part needs to be replaced? I have not pulled the computer out yet to see where the circuit trace goes.

 

[Old Guy]

On my 1996-T the A/C clutch output (connector 1 pin 8) is no longer working. My A/C compressor blew up and after replacing it, the output from the ECM no longer goes low. I can see the input from the ECU change and if I ground the wire to the clutch the A/C engages.
Does anyone have a schematic for the ECM for a 1996-T or know what part needs to be replaced? I have not pulled the computer out yet to see where the circuit trace goes.

and if I ground the wire to the clutch the A/C engages

There is an interposing relay between the compressor clutch and the ECU. There is no 'wire to the clutch'. Do you mean that you grounded the wire at A8 on the ECU which is the wire from the ECU to the clutch relay. If that is what you did and the clutch engages, it confirms that the clutch and relay are operational. If you did something different you need to elaborate on what you did.

The ECU output on terminal A8 is a simple open collector transistor which applies a ground to the clutch relay to engage the clutch. Because of the interposing clutch relay it would be just about impossible for a clutch problem to cause a transistor failure. If the clutch relay coil shorted out and applied 12v directly (no relay coil resistance) to A8 that would overload the transistor and cause failure. However, if your relay is working that does not appear to be the case.

The signal from the ACC is not the only signal to the ECU. There is also an input from the fan control unit / triple pressure switch. I don't know what "A/C compressor blew up" entails; but, I think it is much more likely that there is a problem with your triple pressure switch or a problem with your charge pressure. Get a copy of the factory service manual (free .pdf copies are available) and go through the trouble shooting procedure set out in the service manual for testing clutch operation.

The chances of finding a schematic for the ECU are pretty much zero. However, if it turns out that the ECU A8 output has failed it should be very easy to find the output transistor by tracing from the A8 pin connection on the circuit board. That trace will go directly to the collector connection on the transistor. However, I would look everywhere else for the problem first because the probability of transistor failure is exceedingly low. If the transistor is a through hole mount, careful work with a soldering iron will do the job. If it is an SMD you will need a hot air rework station to do the repair and you run the risk of messing up lots of other things while attempting the repair.

 

[tk]

By compressor “blowing up” it was the bearings giving out. And you are right, I also can not figure out how the bearings going out would blow a transistor in the ECM.
I did follow the trouble shooting guide in the Service manual (1995) and stopped on page 22-50 after doing the ground A8 and C2 steps and it said substitute “know good ECM.” I should have continued on to the next page. That will be tomorrow’s exercise.

 

[Kaz-kzukNA1]

This is how the A/C compressor CL is being controlled.

The Fan Control Unit (FCU) decides whether to engage the A/C compressor CL (clutch, ACCL) or not depending on the multiple inputs.

As you stated, the ECU (Engine) is the one controlling the ACCL relay.


FCU receives the following inputs;

A/C ON request from the CCU (Climate Control Unit).
Rad Fan control temperature sensor at the thermostat cover (coolant temperature).
Triple Pressure SW - A: Hi/Lo pressure protection


Based on these, FCU sends the command to the ECU (engine) to switch On or Off the ACCL.

And the ECU triggers the ACCL relay On or Off accordingly.

Question:
While the compressor didn't engage, how about the R & L condenser fans?

Since you replaced the compressor, how much (gram or oz) refrigerant did you charge?
Unless the triple pressure sw sees about 225kPa (about 33psi, 2.3kgf/cm2), FCU won't allow the ACCL to engage.



I believe you haven't touched the evaporator or the thermostat cover so followings are just side notes;

CCU prevents the evaporator from freezing by switching Off the ACCL if the evapo sensor detects below 37degF (3degC).

FCU will switch off the ACCL AND the R&L condenser fans if the thermo sensor exceeds 266degF (130degC) but unless it's sensor/wiring issue...... Time to forget about the A/C issue......


Kaz