Honda Magna Low Fuel Sensor Thermistor Test

[@deankay4434]

As an ASE Master Tech since 1978, the low fuel sensor lamp was most used on imports. This is an NTC thermistor that is the variable switch to ground for the dash light. While a 194 peanut bulb works as a current limiting device, when illuminated, it consumes all the voltage but allow only 0.25 amps to pass thru. Today, LED’s are used and depending on the color require 0.24-0.30 amps to operate. So 12-14 volts applied to the proper resistor passes thru the LED to the tank. Inside the can, one end is soldered to provide ground. Fuel keeps it cool and as it sloshes away, it drains out the tiny holes. This allows the lamp to come on dim to medium bright. When uncovers and fuel is low, the resistance drops providing enough ground to fully turn on. Placed near the bottom is the installers choice if not factory. It does not get hot at all as current is limited, besides, at room temp, the one in the video is 1k (1,000 Ohms) and works as a current limiting device itself. 15 gallon tank may be positioned to come on with 3 gallons or more to come on as the fuel gauge shows almost “E”. CJ Ponies sells one for the Mustang and can be found online. That’s all. Retired Tech, Instructor.

[@Tobinindustrial]

Thank you for this. it is very informative.

[@deankay4434]

@@Tobinindustrial > As not to be for the general public consumption but I use to teach a method on how to remember an (NTC) thermistor. I told them to think of their wife or girl friend, the hotter you got them, the lower their resistance was! It stuck with all of them... PTC’s are use inside many automotive devices including power window motors for example and most seat circuit breakers. When too much current passes thru them, the carbon impregnated threads get warm and push apart from each other. When real hot, they open. The clue to figure out which type you have, is to release the switch and current stops, it cools off then goes back together and the circuit works again. It will stay open until you let off the switch. Either fully bottom or fully up, it won’t work unit amperage stops. It is easy to play with and wire up. Their are few videos on this subject, but I added one to my 1967 C10 PU and found a 4 gallon reserve spot and LED on dash next to fuel gauge. Works great and use an old metal turn flasher can, clamp it to the pickup tube with a hose clamp until Happy with position. Hope all this helps as ACDeclo had us teaching 6 electrical classes, with lab-scopes the last.

[@CB-RADIO-UK]

Nice meter Andy. Fluke have great reviews. Hope al is well buddy.

[@Tobinindustrial]

Yes Fred, I was lucky to score this Fluke meter for a real good price. I probably wouldn't own one if I had to pay full price. HEHEHE

[@richw1907]

PTC stands for "positive temperature coefficient" and NTC stands for "negative temperature coefficient", positive the resistance goes up as the temperature goes up and negative means the temp goes down at the temperature goes up

[@cjflintstone9509]

The reason your 1K reading was unsteady is because every time you touch the thermistor your body heat warms it up a bit.

[@Tobinindustrial]

Of course.... You nailed it. Thank you for that my friend. We learn something new every day.

[@frankosmith6718]

I have an old Yamaha YZF600R and have just discovered by trial and error, that when you swap the old dash bulbs for LED ones. The sensor still feeds a minute amount of current, enough to light the LED weakly, say by half power, even with fuel covering the thermistor. Long story short: I swapped the LED for the older bulb and tried it dry. I got no light until I held the sensor in my hand for 35 seconds, then it slowly began to get brighter = success = the older thermistor variances are designed to suit the higher resistance of the older bulbs. So yes, do swap for dash LED if desired, but keep your old bulb in the fuel warning circuit. Wait for 30 seconds to a minute for the warning light to appear, and again every time you turn off the ignition. As soon as you dip the sensor into cold fuel the light goes out immediately = working fine. No applause, please. No applause.

[@Tobinindustrial]

Awesome finding Franko. Very intetresting stuff. Thanks for your comment and information Sir. God bless you.

[@ShaneDobbie]

thanks for the info, i had the same issue in my car after changing to LEDs and I am stubbornly trying to make it work with the LED bulb.

[@achad256]

hi Andy , my readings are the same as yours , but the the discription on my packet is different to yours , is there any way i can email you a photo of my packet ?

[@Tobinindustrial]

Sure, can you send me a private message here in YT?

[@darioareviche5316]

Thank for the video useful for me biker brother

[@Tobinindustrial]

I am so glad the video was helpful to you brother. God bless you.

[@SpectreOZ]

Honda made a Magna as well? I only thought Mitsubishi had a model called the Magna... interesting :)

[@Tobinindustrial]

Yes they did Sir. They ran Magna's in the 1980's and again in the 1990's. Sadly, I am without a motorcycle now. Had to sell everything but one day I hope to be back in the saddle again

[@riccardogiuffrida9632]

sorry, but the problem is: when you apply 12 volt at the thermistor it's very hot (about 85 C)!!! since immersion in the fuel........ isn't dangerous ? ciao

[@Tobinindustrial]

If the thermistor is not faulty, it should be fine. The fuel keeps it cool and when it is not in fuel it will warm up and allow current to flow and then the light gets power and begins to glow. 85c may be a safe level of heat. I didn't test for heat in mine but the sensor works just fine. I am sorry that I cannot be of more help to you in that regard but I think you'd need a much higher heat level for it to become a concern.

[@Tobinindustrial]

of the specs on my thermistor which is 20F735 NTC Thermistor; Thermistor Type: NTC; Resistance :1kohm; Thermistor Tolerance: ± 2%; Beta Value (K): 3972K; Operating Temperature Range: -50°C to +150°C; Thermistor Case Style:Disc; No. of Pins:2; Beta Lower Temperature:25°C

[@deankay4434]

riccardo giuffrida > In a working circuit, it is not the voltage but the current. The 12-14 volts B+ applied goes to the light. The bulb, either a 194 peanut bulb limits current when cold. As the thermistor drops in resistance, current flows thru the bulb to the NTC thermistor to ground. As it’s resistance drops, the light starts to come on. Fully on & hot, only 0.25 amps pass thru acting as a current limiting device. The NTC thermistor never gets hot or even warm. Any vehicle with an LED has a current limiting resistor in series with it. The value depends on the color. So an LED limited to 23-30 milliamps is perfect in these circuits as the NTC thermistor is simply the variable switch that provide ground. One wire in and soldered to metal case, then grounded thru the fuel sender’s pipe. Try it! It works in water as well as fuel. I used an old turn signal case and drilled holes in the bottom and soldered one led of the NTC thermistor to it. Place a worm type hose clamp and move it up or down depending on how much reservoir fuel you want. Hope this helps!