1999 Z3 Roadster A/C problems

[senikc]

I have a 1999 Z3 A/C, took it to shop, they report the compressor is controlled by some sort of computer which is tied into the ECU, which is also connected to the outside air temp sensor, and instrument cluster, when jumping out one of the wires from the snow flake switch to the ac head end, the compressor did run, but pressures were not right, and air temp from vents was only 70 degrees, switch did control a/c compressor. Upon picking car up at shop, the a/c was putting out 46 degree air, cooled great, however the compressor would not shut off and the light on the snowflake switch would not go out, took car back to shop, they removed the jumper wire, found the compressor clutch had too much air gap, so they took out a shim, they then found a/c to work correctly with exception when blower is on high, the compressor will drop out and not run until you go back to a lower speed, this is with the recirulate button on or off.
Refirgerant circuit is charged correctly.

Does anyone have any input or ideas on this one?

Thanks in advance for any suggestions.

[saneesh8]

As far as i can remember, the AC is controlled directly by the DME (ECU) on MS42 DME. When you press the snowflake button, it actually turns it on via the DME.

From MS42 - X6000 connector
16 E Activate A/C Pressure switch
19 E A/C-ON-signal Air conditioning operating unit
74 A Activate A/C compressor Compressor control relay

Where E = Input, A = Output, M = Ground

I think it can turn on the compressor from switch and can turn it off from one of the low/high pressure switches and from the evaporator sensor. All these inputs might be taken as input for DME and to control it via some other circuits. It would be nice if you can monitor different sensor values and see which one is turning off the AC. It might be the high / low pressure switch or the sensor in the evaporator.

Buy putting jumper wire, it was bypassing the DME?

[328 Power 04]

Sounds like maybe the pressure switch is the problem?

Not a good idea to jump it and bypass the ECU control.
The ECU turns off the compressor at high RPM and/or hard accelerations to prevent damage

[senikc]

Thanks for the tips, does anyone know the location of the A/C control unit (p/n 64 11 8 367 931)

[saneesh8]

Some where close to the evaporator unit? Just my guess. There are lots of things which looks like this over there. If you remove the glove box you can see it.

[TommyMaddox]

Sounds like maybe the pressure switch is the problem?

Not a good idea to jump it and bypass the ECU control.
The ECU turns off the compressor at high RPM and/or hard accelerations to prevent damage

Hey can you think of any effective way to separate AC control from the ECU for engine swap purposes? I initially was thinking of moving the activation signal to our AC switch/snowflake button, but I was wondering if you had any other ideas?

[mpire]

Check your pressure switch, if that shows no pressure or is bad, then the compressor doesn't respond to controls.

[rf900rkw]

Hey can you think of any effective way to separate AC control from the ECU for engine swap purposes? I initially was thinking of moving the activation signal to our AC switch/snowflake button, but I was wondering if you had any other ideas?

What's the victim? There are both E36 and E46 versions of compressor control in the Z3 series. Does the swap use compressor control?

[TommyMaddox]

What's the victim? There are both E36 and E46 versions of compressor control in the Z3 series. Does the swap use compressor control?

Negative, this is for the E36 N54 swaps that a buddy of mine has built. N54/N55/F30 N55 have pretty complex AC compressor control systems and if you don't want to swap in the whole dash and climate control box and entire wiring harness and sensor packages it gets a bit tricky. Unlike the MS41-MS45 the ac on requests have been shifted from the DME to JBE(A glorified CAN gateway) that evaluates all the bus traffic data coming from the HVAC (IHKA/R/etc...) And from what I found, no external compressor control relay is present, instead the JBE has a PWM generator that controls the variable displacement AC compressor solenoid directly based on demand, and sends the load information to the DME, which adjusts engine performance as necessary.

From reading up on the controls, it would appear that the evaporator control and pressure sensors decide when to turn the compressor off, but basically the AC on request gets sent to the DME, which decides if it will allow this based on engine operation conditions, and will add more fuel and air, increase the idle, and delay the ac turning on if there is WOT to allow better acceleration. Makes a bit of a difference at 170whp, not sure if it makes much of one at 400whp and peak torque at 1200-1500rpm.

N54 compressors are PWM solenoid variable type, so you could probably work up an arduino controller to slowly ramp the duty cycle up over a few seconds when you hit the snowflake button and turn the compressor on manually, and could probably do the same in reverse, slowly ramping it down when turning it off. Depending on the actual performance, you could potentially just treat the solenoid as the clutch, 100% duty cycle is DC anyways.

Just crunching on some ideas here.

[BimmerBreaker]

Can you bolt the E36 A/C compressor bracket and A/C compressor to the N54 to just sidestep those issues?

E36 A/C stuff bolts onto S54s and M54s. N54 is a newer generation, but I would try that first

[rf900rkw]

Okay. The two are totally incompatible, as you said. Modern A/C compressors do not have a clutch. They have a PWM control to vary the output. Some work by a variable bypass valve, some by variable displacement. They are not on/off. They are never, or rarely at max. The output is varied to meet the system demand, no more, no less, no matter the engine speed or cooling load.

So, you would need to adapt the E36 style compressor to the N54. Then plan out a power supply that keeps the pressure switches in circuit, and add a pressure switch to shut the compressor down with any signs of boost. Is this an ODB2 era E36?

[rf900rkw]

Using a 97 328 for the wiring diagram, I would connect the afore mentioned boost/vacuum switch to the old DME compress-request and compress-enable wires, and leave the rest of the climate control alone.. Though there may be issues with the IHKA not seeing engine RPM/Temp/moonphase on the k-line. I didn't trace the wires that far.

All this assumes you can find a clutch styyle compressor that will bolt to the N54, of course.