OK no more quibbling, is everybody happy now.
View attachment 259909

You've made my day.

 

OK, you have an aftermarket ECU. First, is this a spark controller, or a fuel controller or just what sort of Electronic Control Unit is it? Or is it an Engine Control Unit???
I am guessing that it has a 12 volt supply connection as well as a system "Common,battery negative" connection, and so this line is not switching main power. Negative power switching is not used much in most negative ground vehicle power systems. So what is the control function of the line that you want to switch? Without more information so far there are only guesses.
MAX asked in post#2 "what actuates the switch", and now I ask "what is the purpose of the switch??" For an ECU of unknown function the switch could be for anything, not main power switching.
So then this reply: "@MaxHeadRoom there is no switch at the ecu, the 12v signal is controlled by a manual dial that turns on/off air conditioning for the car. " I did not see that previously. So it is a control signal of some sort, and not power. If it is supposed to be "low" when the A/C is off and High when the A/C is on then all it needs is to connect to the switched side of the AC power feed.
And we still do not have an explanation as to just what the function is, and what the logic sense is supposed to be. But it might not need any extra circuit at all.
Thus we do not really have enough information to allow good advice to be given.

 

Hi everyone,

I'm working on installing an aftermarket ECU in my car and need to connect a 12v input signal to it. However, the ECU can only recognize a grounded switch.

Here's a really poor diagram of how it needs to be wired:

ECU--(wire)--switch--(wire)--ground

The switched 12v is not PWM, and does not carry high amperage. I thought of using a relay, using the 12v signal to close a grounded switch but room is limited and I want to keep things as simple as possible.

Does anyone have an idea of how this could be easily done without the use of a relay? Thank you!!

How much room do you have?

You might be able to use a ready made automotive Solid State Relay (SSR). They're about $10 USD though, but they are basically a relay that uses a Mosfet inside instead of a mechanical relay.

 

If the purpose id to sense if the A/C system is on or off, consider that when the A/C is off the terminal of that switch is already pulled towards common by the devices in the A/C system. So if that wire that needs to be grounded when the A/C is off connects to the terminal of the A/C control switch, it will be pulled down to zero, or close to it with the system off, and it will be pulled up to 1 volts with the A/C on. To verify this, first check the voltage on that wire with the A/C off, then check the resistance to common, (ground) with the A/C system off. That will be just a few ohms, consisting of the blower motor resistance and probably, the compressor clutch, in parallel.
The result being that the wire that needs to be grounded with the A/C off already is, and it is pulled up to +12 with it on.
So it seems that you will not need any external circuit, except possibly a one amp fuse, in line.
Let us know if the requirement is actually for that input to be grounded with the A/C off, which sounds very reasonable.

 

I appreciate all the help everyone, thank you!

I dug some more into the wiring diagram. @MisterBill2 you were right, it just senses 12v to tell if the ECU that the A/C is on

The ECU triggers another relay which then supplies power to the magnet clutch inside the AC compressor, engaging it.

The only reason the ECU needs to know whether the AC is on or off is to slightly bump the RPM up by 200 to ensure the engine has enough power to drive the compressor.

I changed the engine so the power output even at idle is significantly greater than stock, so in the end I don't even think it's necessary for the ECU to trigger the relay at all! I'll just ground the A/C relay (air conditioning cut relay in below diagram) to ground with a resistor, so it turns on every time the dash switch is closed.

Do you gents think this will work? The low pressure switch and thermal protector is still in line and protecting the AC from low gas pressure, or some other failure.

 

How much room do you have?

You might be able to use a ready made automotive Solid State Relay (SSR). They're about $10 USD though, but they are basically a relay that uses a Mosfet inside instead of a mechanical relay.

From my cursory reading I read that most SSR's work off of AC current, not DC. I do have room for the relay, but I'm trying to keep things as simple as possible to prevent problems with my soldering down the line.

 

From my cursory reading I read that most SSR's work off of AC current, not DC. I do have room for the relay, but I'm trying to keep things as simple as possible to prevent problems with my soldering down the line.

Not true. There are SSR's for either AC or DC. Most automotive SSR's are used for headlights and are DC.

 

Not true. There are SSR's for either AC or DC. Most automotive SSR's are used for headlights and are DC.

Great to know, thanks!

 

From my cursory reading I read that most SSR's work off of AC current, not DC.

For many decades Opto22 has made industrial SSR's, including system I/O versions for DC & AC, that include low current versions for such switching as this.

 

I see that there is an A/C "cut" relay. I was aware that they used them in chase vehicles to avoid damaging the compressor at really high speeds, I am not aware of other applications using them. Other than that, you could connect that line across the clutch coil.
Or just ground the line, unless the compressor loading the engine starts to bother you.

 

I see that there is an A/C "cut" relay. I was aware that they used them in chase vehicles to avoid damaging the compressor at really high speeds, I am not aware of other applications using them. Other than that, you could connect that line across the clutch coil.
Or just ground the line, unless the compressor loading the engine starts to bother you.

I had to replace a headlight relay in a vehicle once. I took the old one apart out of curiosity. It turned out to be mosfet inside.

 

I see that there is an A/C "cut" relay. I was aware that they used them in chase vehicles to avoid damaging the compressor at really high speeds, I am not aware of other applications using them. Other than that, you could connect that line across the clutch coil.
Or just ground the line, unless the compressor loading the engine starts to bother you.

Thanks!

I could always just switch it off with the actual dashboard toggle instead of relying on the ECU to cut it, I don't mind. I'd def need a resistor to drop it to ground right? And how would I know which size resistor to pick? Sorry for all these questions, electronics/circuits are definitely not my strong point!

 

I could always just switch it off with the actual dashboard toggle instead of relying on the ECU to cut it, I don't mind. I'd def need a resistor to drop it to ground right? And how would I know which size resistor to pick

As far as the ECU is controlled why can't you just put a switch to ground in stead of inverting the 12 volts?
For the A/C cut relay just connect directly to ground if it's a 12 volt relay.

 

As far as the ECU is controlled why can't you just put a switch to ground in stead of inverting the 12 volts?
For the A/C cut relay just connect directly to ground if it's a 12 volt relay.
View attachment 260010

I am not sure if that item "3" between E27 and B73 is a resistor or not. But given tghat you told us it needs to be switched to ground, tghat is what aI would do, except just ground it. Then the engine will not speed up at idle because the AC is on.

 

I am not sure if that item "3" between E27 and B73 is a resistor or not. But given tghat you told us it needs to be switched to ground, tghat is what aI would do, except just ground it. Then the engine will not speed up at idle because the AC is on.

That 3 designation is the connector number, There are no resistors in the harness from what I know.

Is it safe just to ground it or is an inline resistor needed? It seems like it would pull a lot of current if it were hooked up straight to ground.

The ECU does ground the signal to activate the cut relay, but I would assume there is some internal resistance?

 

As far as the ECU is controlled why can't you just put a switch to ground in stead of inverting the 12 volts?
For the A/C cut relay just connect directly to ground if it's a 12 volt relay.
View attachment 260010

This is because the ECU controller needs a low side switch setup, it can't accept a 12v input signal.

as for triggering the cut relay, wouldn't a direct connection pull a lot of current? not sure what sort of resistance the relay offers. the ECU completes the ground circuit when on, but I think there is a built-in resistor.

 

the ECU completes the ground circuit when on, but I think there is a built-in resistor.

Possible. If you want to be sure measure the voltage from the ECU to ground when the relay is on.

 

Possible. If you want to be sure measure the voltage from the ECU to ground when the relay is on.
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I will definitely do this once the weather warms up, thanks!

 

When the AC compressor clutch is not powered it has a rather small resistance to ground. We know this because the clutch only draws an amp or so. Thus the resistance is probably ten to 15 ohms,which will not do a lot of current limiting. Besides that, the input does use a pull-up resistor, and also, it is a fairly high resistance input. So no, you really do not need an external resistor, although a ten ohm hhalf watt resistor will not cause problems.

 

When the AC compressor clutch is not powered it has a rather small resistance to ground. We know this because the clutch only draws an amp or so. Thus the resistance is probably ten to 15 ohms,which will not do a lot of current limiting. Besides that, the input does use a pull-up resistor, and also, it is a fairly high resistance input. So no, you really do not need an external resistor, although a ten ohm hhalf watt resistor will not cause problems.

Got it, thanks for all your help!