potentiometer to min/max relay circuit help

Thread Starter

Ryan Schuermann 1

Joined Nov 18, 2015
11
@Ryan Schuermann 1 :
I think you are misunderstanding the way the ECU reads a ground...
Ok, I will admit I may be misunderstanding... but I also feel you all might be misunderstanding? What I have is a 1990 ECU that does not by oem standards use a potentiometer, and I am trying to trick it into using one. So, the ECU that I am using does NOT send any type of voltage to the 1990 throttle body that it was designed for. The t-body it was designed for has 2 microswitches that simply act as a continuity switch for a common ground. The ecu sends out a ground to pin 2 of the 1990 t-body. In fact, that ground from the ecu is irrelevant as I can supply the 1990 t-body with a ground from the engine bay and the 1990 ecu works just the same. That 1990 t-body has a microswitch that is triggered when the butterfly is closed and therefore sends ground to an ECU pin regardless if it comes from the ECU or the common car ground. The switch is released when I press on the gas pedal and now none of the 2 ECU pins in question get a ground. When the t-body is at wide open throttle, another microswitch is triggered, and a ground is sent back to the ecu to a different pin. So, in both cases the ecu is expecting a ground, not a complete circuit. If I was 'completing a circuit' by connecting either of the ecu pins to ground, I should see some mAmps as my DMM was installed inline to a potential circuit. I stick one pin of the DMM into the supplied ground and the other in one of the two pins, then the other, simulating triggered microswitchs. Since I saw no mA in both scenarios, I assume there is no circuit and the ECU just wants ground in order to trigger a logic in a chip somewhere that causes the ecu to switch between 3 fuel maps.

am I still not understanding? I profusely apologize if I do. I can take pictures of me using the DMM so that I can be advised, but I feel I should ask around here locally and keep this forum to circuits, not "DMM usage for Dummies" :)

Now, If I need to take some measurements on my 1994 t-body with the 5V TPS, let me know and I can do that once all those parts get in and I construct part of the circuit, else I have no way of feeding the TPS 5V at the moment since my auto's system is completely 12V and the 1990 ECU has no 5V out.
 
Last edited:

crutschow

Joined Mar 14, 2008
23,532
No, we didn't misunderstand.
If you don't fully understand the relationship between current and voltage then perhaps you should read a tutorial on that.
When you ground the ecu pin you are completing a circuit between whatever voltage is at the ecu pin and the battery ground (car frame). To complete that circuit there has to be a current, no matter how small. Otherwise how would the ecu detect that the switch has closed?
That's why I asked you to do the current measurement on a more sensitive range. It may be much less than a mA.
Also measure the voltage from the ecu pin to ground.

My circuit can be powered from the 12V vehicle battery since the comparator output is a open-collector which can switch the 5V from the ecu pin.
 

strantor

Joined Oct 3, 2010
5,160
No, we didn't misunderstand.
Who is we? [you and Ryan] or [you and me]?

What I have is a 1990 ECU that does not by oem standards use a potentiometer, and I am trying to trick it into using one.
got it.
So, the ECU that I am using does NOT send any type of voltage to the 1990 throttle body that it was designed for.
I disagree.
The t-body it was designed for has 2 microswitches that simply act as a continuity switch for a common ground.
got it.
The ecu sends out a ground to pin 2 of the 1990 t-body.
How does anything "send out a ground"? that does not make sense.

What's happening (as you rightly said) is that it is checking for continuity to ground. It does that the only way possible, by sending out a voltage and waiting for that voltage to be pulled low (to ground, by a switch or a transistor).
 
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