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Pull Up resistors

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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Just figured I'd post this article on " floating state " and undefined voltage for anyone who wasn't exactly sure what it meant.

Admins,if this is not aloud please delete .


Nah that's great Joe, more info is always better. I struggle sometimes finding a good balance between getting my point across and not delving into too much electronic background knowledge that isn't really relevant when wiring up a car. But tech info on the forums is always appreciated! :-).

Oh by all means this course was awesome and very informative.def worth it for the knowledge I obtained .you definitely explain things thoroughly.

Will the worked examples be released anytime soon along with advanced motorsoprt wiring ?

I will also add a comment to this subject...

The term "pull up resistor" in automotive ECU language can be used to describe a couple of quite different scenarios. One case would be something like an ECU trigger input or digital input where it is used to prevent a floating input as per the article Joe linked.

The other scenario where the term pull-up resistor is used is for analog temperature inputs for resistance type temp sensors. In this case its purpose is not to prevent floating but it is actually one side of a voltage divider circuit used to convert the variable sensor resistance into a voltage. I guess a more correct term for this kind of case maybe something like "bias resistor" but it still seems common in the automotive world to call it a pull-up resistor.

G'day Adam.

Yeah, I left voltage dividers out of the fundamentals course on purpose, as creating one manually in the harness usually leads to having to generate a custom calibration for the sensor in question, and although not incredibly complex, is a more advanced topic than I wanted to go in to in the fundamentals course.

Sorry, I havent watched the video and didnt even notice this topic was related to that originally. I was more just trying to clarify to readers of Joe's post that "pull-up resistor" may have more than one meaning in automotive ECU talk. For instance the temp channel's bias resistor inside many ECU's configuration software is often labelled "pull-up".

I have the problem where I should add the resistor and how?

If I place it inside a sealed loom it is hard to replace if faulty and also very hard to find.

If I place it close to the ECU the input channel can never be used for something else if required.

or place it close to the sensor in a wire extension or inside the plug it has to be soldered in.

are there any „gadgets“ like for inline fuses where such a resistor can be placed inside?

Thanks Bernhard


I've installed resistors in harnesses in a variety of ways. Most commonly, I will solder wires to the legs of the resistor so that the wires pass the body of the resistor, so that when the heat shrink is added the resistor and the wire the from the other leg all support one another and reduce any chance of failure, since the most common failure with a resistor in a harness is the leg braking away at the body of the resistor. Depending on what this resistor's purpose is, sometimes it will be near the component, sometimes near the device that is reading or providing the signal that needs the resistor.

Another way I do it, especially with pullups specifically, is to split the signal wire off along with a wire split from the positive voltage source, and run these to a two pin connector, often Deutsch DTM, and install a resistor in the matching connector. The DTM connector provides a good way to support the resistor as well, since the seal can be removed and the resistor crimped directly to the pins, and then potted. The advantage of the DTM route is that the resistance can be changed at any time, if there is ever a need found later, such as switching to a different model or brand crank sensor that might specify a different resistor value, this also provides a test point. I place these near the ECU or an area in the harness that is convenient to access.

Another place to add the resistor is in the short section of harness between the sensor and the main harness connection, if the sensor has a hardwired harness to it, I.E. The ZF, formally the "Cherry" Hall sensors that are popular. The disadvantage here is that the resistor will be more prone to vibration and heat, and has a higher potential to fail than if the resistor was installed nearer the ECU. The advantage is, that the resistor installed with that sensor will be the specified resistor for that sensor, and if swapping to a different sensor, then the new resistor value can be swapped in in one "motion" so to speak, or if the new sensor doesn't require an external pull up resistor then again, it's one less step overall.

Some people open up the ECU and install the pull up resistors right inside the ECU soldered between the output pin pads. I'm not really a fan of this, just because incase of the off chance that an ECU has to be swapped in, even just for quick testing then the new ECU would also require this internal modification, not to mention this will void the warranty for most ECU manufacturers.

And on that last note, some ECUs have software enabled pull up resistors, I don't know what ECU you're working with, but I would verify whether this is an option for you or not.

I'll back basically everything Chris said. I opted to crimp vs solder even at the ECU end but if done properly and sealed in heat shrink I'm doubtful it'll be a problem.

Typically I put all the external pull ups resistors at the ECU, there's more room there normally. Gets sealed under a boot and no room to vibrate apart.

DTM/ATM connector also a great idea, if I have a spare input that a use hasn't been determined for I'll provide 5V there as well so a pull up could be added in the boot of the new connector. Most of my expansion connectors are 4 pin (5V, sensor ground and 2 inputs) and have plenty of room to do this.

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