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Changing resistance of fuel level sender

Changing resistance of fuel level sender

(OP)
Hi,

I need to increase the resistance of the signal coming from the fuel level sender in a Miata going into the ECU of an LS2 engine. The Miata ranges from ~5 ohms (full) to ~115 ohms (empty). The LS ECU expects a range from 40 ohms (full) to 250 ohms (empty). Reading the full point is most critical because the fuel level sender is used by the ECU to trigger the evap solenoid. When the tank is full, it absolutely should not run. So if I have to pick one accurate point, it would need to be full. Of course it would be nice if both full and empty points are accurate. So, can a simple mix of resistors accomplish this change in range? If not, perhaps just target the full point; go from 5 to 40 ohms?

Thank you in advance,
Jon

RE: Changing resistance of fuel level sender

(OP)
Thanks, Greg. I'll go that route if I can't find an inexpensive solution for the entire range, and assuming the ECU is happy. If it continues to throw a code, I may need to find a complete solution. What about an adjustable potentiometer?

Jon

RE: Changing resistance of fuel level sender

I would use a 135 ohm resistor to get the correct empty reading because we all know that's where our fuel gauge spends most of its time.

RE: Changing resistance of fuel level sender

(OP)
Thanks, TurboEng. The full mark is most critical because of the evap purge valve. If it runs with a full tank, it will suck fluid into the evap system. No bueno. But yeah, most of the time the tank is empty, especially the way I drive.

RE: Changing resistance of fuel level sender

What are your constraints? You could certainly put in a semi-programmable widget that would map the resistance of your actual fuel level sensor to the desired output for your ECU. Something like an Arduino could read the sender's resistance and drive a FET or programmable resistor

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Changing resistance of fuel level sender

(OP)
Thanks, IRStuff. So something from here: https://www.arduino.cc/en/Products/Compare
feeding into something from here: https://www.maximintegrated.com/en/pl_list.cfm/fil...
That's getting a little beyond my skillset, but I can give it a try.
Constraints: it's an auto application, so 12 volts, roughly. It's going from a sender with a 5-115 Ohm range to an ecu looking for 40-250 Ohms. I'm not sure what else to look for.

RE: Changing resistance of fuel level sender

(OP)
Cool! I now have a couple of variables to work with: offset and mx. I don't know where those are used at this moment, but I'll research the above widgets and hopefully figure it out. Thanks!
The expert video is hilarious!

RE: Changing resistance of fuel level sender

(OP)
I ended up ordering a conversion box that I found for $45. yes, the other parts could have cost less, but I would have also needed to sort the correct parts, build a waterproof/shockproof box, and figured out how to program/setup the parts. So I went the easy route. Of course I have yet to see if it works; arriving today. Here's the company: http://www.technoversions.com/MeterMatchHome.html

Thanks,
Jon

RE: Changing resistance of fuel level sender

Sounds like a plan. Very likely that the price is way less than your hours at cost, assuming you value your time at all.

The processor is possibly something like: http://ww1.microchip.com/downloads/en/DeviceDoc/30...

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Changing resistance of fuel level sender

Buying a proven off the shelf unit has another big advantage versus a roll-your-own electronic circuit.
That is available voltages. Your auto voltage may vary from less than 12 Volts, when cranking to 14 V, 15 V, or 16 Volts when charging.
Then there are load dump transients that may hit 100 Volts.
A proven commercial unit will have these issues considered and protected.
A roll-your-own unit may let the smoke out the first time you turn off the headlights with the engine running fast.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Changing resistance of fuel level sender

(OP)
We're still working on this and the vendor is helping us. It wasn't as simple as plug and play. We're going to modify the unit a little, per the vendor's instructions, and see if that works. Will update more...

Yes, time is money. Combined with the additional benefits of an OTS item as mentioned above, the cost is well worth it. plus it supports an industry expert.

RE: Changing resistance of fuel level sender

(OP)
I think I forgot to mention that in addition to converting the Miata fuel level sender to the GTO sender, the Miata cluster fuel gauge needed to continue to function. So basically we needed to splice the GTO PCM wire into the wire going from the Miata sender to Miata gauge. This presented problems due to varying signal strengths from both the Miata gauge and GTO PCM.

We ended up using a modified version of the converter unit I mentioned above called MeterMatch from TechnoVersions. Brian from TechnoVersions worked with us to remove one of its resistors. He was a big help. We then ran the engine and adjusted the output of the MM by reading the percent fuel tank level on a OBD reader with the tank full. Knowing the tank was full, we adjusted the MM until the OBD read nearly 100%. That meant that the MM was sending the correct signal strength to the PCM. When the tank is empty we will set the other extreme. Quarter and three-quarter tank levels can also be set. Uneven tank shapes can be accounted for with that. Pics to follow. Feel free to ask me if you want more details.

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