LS Computer outputs - tied high, floating, what?

[davepl]

I'm a software guy, not a hardware guy, so... on LS engine controllers (like the E67 for the LS3) does anyone know if an output, like the fan relay controls, are left to float or are they tied high when not active?

I want to add some LEDs to show what the computer is asking for, but don't know if the outputs are +12V or floating or what they do when they're not "ON" (tied to ground).

If I had one handy, I'd go measure it, but nothing actively running right now.

[Bagd72Chevy]

It depends on the output. You'd have to measure the ones you are looking at using.

[Livrat]

Dave, I do believe the LS ECM's use a "ground trigger" to energize the fan relays, I see two ways to put in an LED on any given circuit.

One way would be to put the LED's in parallel with each fan motor.

The second would be to put the LED's directly onto the ECM output using its "ground trigger" for the ground side of each LED and using a separate 12V power source.

Although since I do not know a complete "theory of operation" of the ECM, I do not know if it senses a voltage to ground drop or resistance changes in the fan relay circuit
(I do know it does so for the knock sensors). If so the second method may interfere with the ECM's correct operation of the relays.

[ga-stepside]

This is from the LS1Tech board.

[davepl]

Sadly if you look in the bottom left corner (and again, warning: software guy) it shows the relays as connected to dots when not triggered, but it doesn't say what, if anything, those dots are connected to. I wish it did, that's the question!

I think I'll tie the LEDs to 12V and then use the relay trigger wires as the ground. That's what an EE told me to do this morning based on the circuit, so I'll blame him if it goes bad :-)

[ga-stepside]

Yep that will work. That is not a relay, it say "relay control" its a computer circuit. My LT1 ECM in my Z is pretty much the same. All the trigger is doing is providing ground to the relay, allowing it to float is fine. There are a lot on circuits in the ECM that look exactly like that.

[dayj1]

Originally Posted by davepl

I'm a software guy, not a hardware guy, so... on LS engine controllers (like the E67 for the LS3) does anyone know if an output, like the fan relay controls, are left to float or are they tied high when not active?

I want to add some LEDs to show what the computer is asking for, but don't know if the outputs are +12V or floating or what they do when they're not "ON" (tied to ground).

If I had one handy, I'd go measure it, but nothing actively running right now.

Any outputs that I've ever checked on a GM ECM/PCM were never left floating. There has always been an internal pull-up (or pull-down) to set the state high or low when it's disconnected.

Something else that you may find useful is that the fan relay control is not only an output, but it is also used as an input. The ECM monitors the state of the relay control line which would normally be tied to 12V through a relay coil. If the ECM fails to see the voltage, it assumes that the relay coil is open and sets DTC P0480 or P0481 depending on whether we're talking about relay 1 control or relay 2 and 3 control.

[davepl]

Originally Posted by dayj1

The ECM monitors the state of the relay control line which would normally be tied to 12V through a relay coil. If the ECM fails to see the voltage, it assumes that the relay coil is open and sets DTC P0480 or P0481 depending on whether we're talking about relay 1 control or relay 2 and 3 control.

I would guess it can only monitor it when its commanding the trigger ON (grounded output, connected to the relay coil), no? When it's off, it's (arguably) left floating and certainly not connected at the relay end.

[dayj1]

Originally Posted by davepl

I would guess it can only monitor it when its commanding the trigger ON (grounded output, connected to the relay coil), no? When it's off, it's (arguably) left floating and certainly not connected at the relay end.

In its factory configuration, the fan relay control line is always tied to 12V through the relay coil. It is never floating or disconnected unless there is a fault in the system. It's a logic "high" and if the ECM doesn't see that, you get a DTC.

The same pin is a logic "low" when the ECM attempts to activate the fans. There isn't a time in a normally operating system when the fan control line is undefined (or "floating").

[davepl]

Originally Posted by dayj1

In its factory configuration, the fan relay control line is always tied to 12V through the relay coil. It is never floating or disconnected unless there is a fault in the system.

OK, I think I know what you mean: since it's connected to the magnetic coil, which is fed 12V, the computer can sense the 12V on the fan control line even when it's not pulled low to command the fan on. That's reasonably clever.

[dayj1]

Originally Posted by davepl

OK, I think I know what you mean: since it's connected to the magnetic coil, which is fed 12V, the computer can sense the 12V on the fan control line even when it's not pulled low to command the fan on. That's reasonably clever.

That's it exactly