Aircon ECU control for 1992 Soarer 1UZ 40-40-40 pin ECU

[Mat Grant]

Hi all,

I've got a 1UZ fitted into a 1970s Lotus, the 1UZ is from a 1992 Soarer with the three pin ECU (40-40-40 plugs).

I have air conditioning and it works well. However when I did the wiring on the car I didn't have any solid information on the aircon wiring for the ECU, so I just made the aircon control manually switched by the aircon button on the heater panel, and a temperature sensor in the cabin to turn it off when it reaches the target temperature. So the ECU does not know when the AC is on.

There are two pins on the ECU that seem to be concerned with the aircon. I wired these from the from the ECU pins into the connectors behind the dash.
- E9 connector pin 23 - white wire - called "AC MAG" on the Toyota wiring diagram
- E9 connector pin 34 - blue/red wire - called "AC" on the Toyota wiring diagram

I have two things I'd like to fix/change:

1. When I engage the aircon the RPM drops at idle by about 200 rpm. to compensate I have raised the idle speed a small amount, but it would be better if the ECU maintained the idle speed
2. When going to full power I can manually turn the aircon off, but it would be good if the ECU could decide to turn the aircon off during periods off full acceleration (lots of ECUs do this)

But I don't know how this works on this ECU - does anyone know how this is implemented in the 1UZ Soarer ECU?

For example if the "AC" is an input that I should put 12v on when the aircon panel control is switched on, I can test this by putting 12v on the blue/red wire and seeing if the idle speeds up (assuming that's what it does)

And if the "AC MAG" is the output to the aircon relay that drives the mag clutch, I can test this by putting a bulb to ground and seeing it comes on and off while doing some test driving, then i can wire this output to the aircon mag relay in place of the direct wire from the aircon switch on the heater panel.

But I don't want to muck about with it, not knowing how it works - the car runs well with no error codes, and I don't want to damage the ECU by guessing.

Any suggestions or help appreciated

Thanks, Mat.

 

[P Bateman]

AC Mag provides output ground to the ac clutch relay

AC recieves a 5v input from the ac ecu when the ac is off, that pulls to ground when the ac switches on.

 

[Mat Grant]

Hi, thanks for the reply, that's really helpful

So at the moment I'm not putting 5v on that wire when the AC is off - what does the 5v achieve? Is it just there to tell the ECU that the AC ECU is there?

If I just put a ground on that pin when the AC is on - will this work to make the ECU think the AC is on and increase the idle speed? I could try this if there is no risk of it damaging anything(?)

And I could take the ground connection from the AC mag relay through the AC mag relay to make the ECU control the mag clutch.

Is there any way I can work out what the behaviour of this will be? Other than maybe wiring 12v to an LED and resistor, to the AC mag pin and seeing if and when adds the ground(?)

Thanks, Mat.

 

[P Bateman]

Looking at the circuit, the ECU outputs 5V through a pull-up resistor, through a diode, then out to the AC ECU via wiring harness
The AC ECU applies another diode to it, then connects to ground via a transistor.
The pull-up resistor is there so that the ECU can measure a logical value.
By doing this, the ECU can detect if the circuit is working normally... or has an open or short circuit.
If it does, the system will throw Code 51 (Switch Signal Error)



When the ECU senses the AC ECU switch the transistor on, yes it bumps the idle up, but it also engages the AC MAG.


From my understanding of Toyotas (I've seen this on other models) if you have the throttle pedal more than XX% or the engine speed goes above XXX RPM, the ECU will switch off the AC MAG to prioritise engine power to the wheels and prevent over-speeding the AC pump.

If the ECU had logged the Code 51, it should inhibit operation of the AC MAG and protect the compressor

 

[Mat Grant]

Hi, thanks for sending this

Btw where did you get these diagrams? All I have is the (Lextreme?) black and white wiring diagram. The sections you have pasted there are a lot more informative. Are these available anywhere?

Looking at the top diagram, to "put a ground" on the AC pin of the ECU, I need to have a resistor in line before ground to limit current flow? As there is one in the AC ECU before the ground via transistor. How do I work out what this needs to be?

And on the bottom pic, presumably is that a 12v feed going into the relay coil of the mag clutch relay(current is limited by the resistance of the relay coil in this instance?)

I'm starting to get a picture of what I have to do to implement this on my car without frying anything

Thanks, Mat

 

[Mat Grant]

Btw this is the Lotus my 1UZ is fitted to

 

[Lextreme]

Thanks for sharing. Is that a Éclat (Type 76 and Type 84)?