A/C Support

[antonch]

I am looking at the DME pinout for my stock ECU that is going into the trashcan soon. It has:

• Input-A/C Pressure Switch
• Input-A/C compressor on
• Output-A/C compressor control

It seems that we can use MiscOut for A/C control. Most other standalones such as Autronic/AEM support this feature. Does VEMS?

[rob@vems.co.uk]

Supporting A/C control is possible, and as it differs slightly across various cars we dont claim to support it out of the box, but its been possible on a few cars.

We can control the compressor on using a Misc Out channel, we just need to work out the function of the inputs.

With the Nissan I have two inputs - one is the Throttle pedal (which cuts the compressor on WOT) and the other is the A/C on button.

So we need to make a truth table up like this:

Pressure Switch	Compressor On	AC control
Off	Off	Off
Off	On	Off
On	Off	Off
On	On	On

And with a table like this all we'd need is an extra relay

[antonch]

Supporting A/C control is possible, and as it differs slightly across various cars we dont claim to support it out of the box, but its been possible on a few cars.

We can control the compressor on using a Misc Out channel, we just need to work out the function of the inputs.

With the Nissan I have two inputs - one is the Throttle pedal (which cuts the compressor on WOT) and the other is the A/C on button.

So we need to make a truth table up like this:

Pressure Switch	Compressor On	AC control
Off	Off	Off
Off	On	Off
On	Off	Off
On	On	On

And with a table like this all we'd need is an extra relay

That table looks right to me. I am not sure if you need Throttle pedal input. Using MiscOut page we can just disable A/C at pressure > 100kpa and rpm > 3500 or so. I am not sure why you want to use relays. Can the ecu handle inputs?

[rob@vems.co.uk]

That's right, the pedal input is going to be controlled by one of the MiscOut triggers.  And while the VEMS hardware can handle 3 inputs the software to control the A/C is not present, which is not really a problem as the solution looks a little like this:

Driver presses the A/C switch on the dash, the coil opens the circuit for the AC control which the VEMS handles based on your MiscOut settings.  The exact wiring depends on the dashboard switch - if it switches to ground when on then the relay coil will need a 12v supply not the ground connection.

The choice of which EC36 pin to use will be made based on the current drawn by the AC control circuit.

[Bat]

Hi,
On the standard system I would expect the pressure switch to activate the cooling fans, is this not the case?
Cheers,
Gavin

[rob@vems.co.uk]

Activation of a fan could be added with a twin pole relay.

I guess it depends on what the car does as standard - if we can define it we can design something to suit.

Fan control will be provided by VEMS based on coolant temperature, it would be possible to control a second fan using the water pump control

[Sprocket]

If the Misc outputs had temperature and input selectable fields, then anything is possible, and truely would be misc outputs.

I assume this is a firmware subject

??

[rob@vems.co.uk]

If we want an aircon control feature we need to define its exact operation.

The thing is that Aircon is a road car feature and that not what VEMS was ever designed for.

[Bat]

Hi,
I know a little about AC 
The pressure switch is normally a multi function switch. HP and LP contacts kick the clutch out if too high or low pressure is in the system. These basically stop it exploding or killing the compressor.
The other function is a mid pressure contact which activates the cooling fans, usually engine fan but can be independent....
Another feature I've seen used is AC disable if engine temp gets too high. This kicks the clutch out to allow the engine cooling to take priority, when the condenser is mounted in front of the radiator.
Cheers,
Gavin

[Sprocket]

If the Misc outputs had temperature and input selectable fields, then anything is possible, and truely would be misc outputs.

I assume this is a firmware subject

??

This feature could be used in Many Many ways not just for AC.

If you have the functionality of the inputs and outputs you can do anything, not just control AC

[antonch]

That's right, the pedal input is going to be controlled by one of the MiscOut triggers.  And while the VEMS hardware can handle 3 inputs the software to control the A/C is not present, which is not really a problem as the solution looks a little like this:

Driver presses the A/C switch on the dash, the coil opens the circuit for the AC control which the VEMS handles based on your MiscOut settings.  The exact wiring depends on the dashboard switch - if it switches to ground when on then the relay coil will need a 12v supply not the ground connection.

The choice of which EC36 pin to use will be made based on the current drawn by the AC control circuit.

Adding inputs to MiscOut would open up other possibilities to users who do not need AC. And using ECU for logical control is easier than relays. I am not too familiar with various AC configurations.
But here are some things that some ecus do:

• Raise idle
• Turn on electric fan

Now these things are optional, just getting AC to work [disable/endable based on load and rpm] would be a good start. ICV algorithm should be able to cope with the changed idle speed.