Aux Out (Ign) voltage

[MGV101]

Was struggling to get the Tacho working on a MR2 Link V2 Pnp even after modifying the resistor on the dash.

After some diagnosis, I realized that the Ign 5 output I was using on the expansion connector was only putting out around +5V (both under the Tacho function and Test(On)/High side). After noticing the problem, I tried reassigning the Tacho output to Aux 3 which does measure +12V when set to high side and the tacho worked straight away. So it seems like there really is a voltage problem in Ign 5.

Are the Ign outputs supposed to output 5V only? I still need to look for an alternative output for the Tacho as Aux3 is currently used for the boost solenoid and I just borrowed it for test purpose. Would using an even lower resistance resistor (1.8K currently used) allow me to use the Ign output for tacho? Thank you!

 

 

[Vaughan]

14 hours ago, MGV101 said:

Are the Ign outputs supposed to output 5V only?

Ignition outputs have different circuitry on them to make them suitable for driving ignition coils and so their pullup voltage is different to Aux Outputs.

14 hours ago, MGV101 said:

I still need to look for an alternative output for the Tacho as Aux3 is currently used for the boost solenoid and I just borrowed it for test purpose

Use Ign 5 for the boost solenoid.

[MGV101]

Won't the lower voltage on Ign 5 effect the operation of the solenoid and require a higher duty cycle as a result? Tuning on the car had already been done, won't this require a retune of the boost control?

[Brad Burnett]

23 minutes ago, MGV101 said:

Won't the lower voltage on Ign 5 effect the operation of the solenoid and require a higher duty cycle as a result? Tuning on the car had already been done, won't this require a retune of the boost control?

boost solenoid aux operation drives to ground, so positive voltage level will have no effects.

[Adamw]

The other option is just add a 12V pull-up to the ign output for the tacho.  You can do this at the ecu plug or tacho end.

 

[MGV101]

7 hours ago, Brad Burnett said:

boost solenoid aux operation drives to ground, so positive voltage level will have no effects.

You are right, I completely overlooked that lol.

7 hours ago, Adamw said:

The other option is just add a 12V pull-up to the ign output for the tacho.  You can do this at the ecu plug or tacho end.

 

I will give that a try, thank you Adam.

[essb00]

If you are you use the Ign 5 for ISC, be reminded though that Ign outputs do not have flywheel diode. You need to install a diode as per below diagram.

 

[MGV101]

 

On 8/2/2022 at 6:22 AM, Adamw said:

The other option is just add a 12V pull-up to the ign output for the tacho.  You can do this at the ecu plug or tacho end.

 

Hello Adam.

Did the 12V pull up and it seemed to have worked initially. But after a few drives it started acting up and would work only intermittently with the needle bouncing up and down as the signal constantly gets picked up and dropped or simply not work at all.

Would changing the ohm value of the resistors help solve this problem? I am currently using a 1K for pullup and 1.8K on the tacho.

I am still struggling understand the principle of how the 12V pull up is working in this system. My guess is when the ECU signal is pulsed off, the connection is floating at the ECU end so the full 12V current would flow throw the pull up resistor to the tacho (or the 1.8K resistor in front of the tacho in this case). With the ECU signal pulsed on, it is connected to 5V so there is a 7V difference between the 12V pull up and the ECU so some of the voltage is diverted to the ECU and the tacho will receive a lower voltage signal. However, this voltage will not be zero. Is it possible that this voltage is still too high for the tacho to distinguish it from the 12V signal?

In this case, I will need a higher ohm resistor at the tacho I guess? That that would also lower the 12V peak going into the tacho.

 

Thank you!

[Adamw]

No, changing the pull-up resistance is not not going to help you.  Most ECU's only have a 1.5-4.7K aux pull-up so you are already stronger than the typical ecu.The problem must be at the tacho end.  With the set up I gave above you will have a 0-12V square wave pulse going out to the tacho.  

Effectively yo can think of the ignition output as a switch that is connected to ground on one side.  When the switch is "on" the ignition output is connected to ground with very low resistance (effectively a dead short), so you have 0V on the tacho output wire.  When the switch is "open" you have the pull-up resistor pulling the tacho wire up to 12V.

 

[MGV101]

Thank you Adam. So with the Tacho function selected, the Ign output actually functions as a low side output?

Sorry I thought the Tacho function was giving out a high side signal between +12V and floating for a normal Aux output instead of ground switched (as described in the help file: "This function provides a 0-12V pulsed output to drive a low-level tachometer from an auxiliary output pin.") So in the case of Ign out, I thought it was just giving out +5V instead of +12.

But from what you are describing, the Ign out actually functions as a low side output switching to ground and does not put out a +5V pulse (which I though it would, in place of the +12V) when assigned as a tacho.

Anyway, while waiting for your reply, I have tried a 1K and a 3.XK resistor instead of the 1.8K on the tacho input, none of which worked so yes, changing the resistor does not solve it.