Perplexing "FUEL" light malfunction

[Dave WM]

2 hours ago, Captain Obvious said:

Dave, that's a good question about the cold resistance. That would be good to know. I guess I could check mine too without too much trouble. It's a little bit of a pain to take the stuff out of the hatch area to get to the connector, but if I get that curious, I'll do that. I bet there's a connector up under the dash I could use too if I dug into the wiring diagram.

I'm assuming your 3K is not correct. At 13V supply, that will dissipate just over 50mW. I don't think there could possibly enough internal heating at that low power level to change it's resistance. It would have to be uber sensitive, and have an almost vertical knee in the curve. I'm with you... I think you've got connection issues elsewhere.

its is high but it does go into run away in about 5 min. I agree it does not seem like enough internal heat to boot strap it but it did work. I suppose the upside to it taking this long is you will never get a warning until the fuel level is solidly below the thermistor. My starting voltage at the lamp was about 30mV, it take a good 4-5 min to get to about 200mv with a uncooled thermistor,  from there things go pretty quick, maybe a minute to get to 2-3v  which is when the lamp becomes visible.

I am pretty sure my rear harness to lamp wiring is ok, a jumper at the harness results in full voltage at the lamp. That leaves only the thermistor itself and the connections from it to the various points on the sending unit. I should have made a ground wire to get around that clip in connection. I cant account for the connections on the hot side but they are soldered. The only other place for resistance to creep in could be the ground terminal on the sensor, not soldered but riveted to the metal shell. Perhaps there is corrosion under that rivet, creating a high resistance, It does not look bad so I assumed it was ok.

 

 

Edited July 27, 2020 by Dave WM

[ETI4K]

On 7/25/2020 at 10:39 PM, Captain Obvious said:

@ETI4K  We were having a discussion some time ago and the million dollar question you didn't ask me is "OK, smart guy... Then if the headlights aren't an inductive load than why do the switches burn up like that? Huh? Why's that?" 

Yep, had my chance and I blew it.  So, arc suppression diodes all around!  Did you see the Combination switch teardown info I posted in Knowledge Base Electrical.  Good pics of eroded contacts.

Sorry for off topic commentary ?

 

[Captain Obvious]

But that's the problem with DC non-inductive loads. I'm saying that I don't think suppression diodes are the answer. I started typing a whole bunch of stuff about inrush currents, arcs, plasma, and ionization, but worry that we're getting too far off topic and stopped. Let me know if you really want a bunch of details.

Transient suppression and mitigation is not an area that I ever specialized in (it's one of the mysterious black arts of engineering), but I just can't picture diodes being an answer for DC loads unless you are trying to deal with the flyback from an inductive load.

Sorry OP for the diversion as well. I'll try to keep my mouth shut now!  

[Captain Obvious]

2 hours ago, ETI4K said:

Did you see the Combination switch teardown info I posted in Knowledge Base Electrical.  Good pics of eroded contacts.

Oh, and about this. Great pics and write-up.

I went through all of this a bunch years ago and only took a couple pics. I replaced my loose turn signal pivot with a threaded in version I made on the lathe and I cleaned out all the switches and put in headlight relays. Hopefully both of us are done with issues on any of that stuff!!

[dmorales-bello]

I have no problem with the temporary digression from the original topic. (I do wish I had a better understanding of electronics). I consider it an "intermezzo" while I wait on my new bulbs to arrive and I can proceed with my relatively simple experiment.

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[Dave WM]

CO how about doing that back probe, I am curious what the lamp off voltage reading is on yours. Hopefully you cable connection is as easy to get at as is mine (the lamp harness to harness that goes along the back of the firewall) for mine they are just kind of stuff under the back of the center console. I do need to work on that area since a couple of the connectors have lost their locking tab (very easy for that wire to come unplugped).

[dmorales-bello]

5 minutes ago, Dave WM said:

CO how about doing that back probe, I am curious what the lamp off voltage reading is on yours. Hopefully you cable connection is as easy to get at as is mine (the lamp harness to harness that goes along the back of the firewall) for mine they are just kind of stuff under the back of the center console. I do need to work on that area since a couple of the connectors have lost their locking tab (very easy for that wire to come unplugped).

My bulb plug has a 5 inch long harness that plugs into another harness above the HVAC vent. Is that where you want me to check? I have a duplicate sender out on my workbench. Is there something I can do with that sender that will provide the info you need instead?

[Dave WM]

it can be kinda hard to hold the dvm probes on the sending unit. some jump wires (alligator clips on wires) are a handy way to grab the pins on that sending unit. Just clip onto the black wire and the yellow/blue wire, set dvm to ohms and if auto range you should get the reading. If its not auto ranging start out at the 1k setting and go up or down from there until you get a reading.

Test the setup of the meter by shorting the two leads, should be 0 ohms. always test... test equipment before relying on results.

this is to test the resistance of the sending unit on the bench. My sending unit was about 3kohms (3000 ohm).

all the above test are done with the sending unit out, no voltage required, you are just testing resistance.

 

The back probing I was talking about was aimed at capt obvious, the assumption is his setup is working so I was wondering what the key on/lamp off voltage was. Mine is about 30-50mV as long as the thermistor is in the gasoline. This test requires everything hooked up and the key in the (run) position.

(.03-.05v), starts there and takes about 5 min to get to .250v (250mV) once there it quickly goes to 6-8v  that is after its not immersed in gas (low fuel, I presume about less than 2 gallons usable fuel left in tank).

 

Edited July 27, 2020 by Dave WM
clarity

[dmorales-bello]

I bench tested the resistance of my sending unit with 2 different DVMs and got very similar results: 2.99k ohms and 2.98k ohms.

 

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[Dave WM]

hmmm well there you go, very close to what I got. for cold resistance.

So just that I am clear, right now with all hooked up and gas in tank say at least 1/2 way full, the light will come on after about 25 min and stay on?

If that is the case, seems like the next logical step will be to back probe, and check the lamp voltage from off to on over the 25 min period. will talk about that later, but its what I was doing in the video. You just have to find that plug, set the voltmeter to volts, and insert in the back so the probes contract the inside of the plug. I am assuming starting with the "2000m" to see if you get a reading if not switch down to "200m"

Edited July 27, 2020 by Dave WM

[Captain Obvious]

Well OK. I stand corrected, 50mW IS enough power to heat that thing up! 

Dave, I'll check my cold resistance too when I get a chance just for another data point.

[ETI4K]

21 hours ago, Captain Obvious said:

Let me know if you really want a bunch of details.

Absolutely!