As some of you might remember, I swapped my original fuel sending unit on my "78 280Z a few weeks ago. Although the original sender measured fuel level adequately, the "FUEL" warning light had stopped working a couple of years ago so I decided to spend some quality quarantine time swapping it for a new reproduction unit from Z Car Depot. I had already tested the yellow/blue lead and assured I had continuity from the "FUEL" light in the dash all the way back to the sender plug.

I plugged in the Z Car Depot sender to the harness before inserting it into the tank and upon turning the car key to the "ON" position the "FUEL" illuminated and I could change the position of the needle on the fuel gauge by moving the floater arm on the sender. Everything seemed to work as it should. Once I put the sender into the tank and locked it, the needle in the dash positioned itself to the right (I had an almost full tank of gas) and the "FUEL" warning light turned off (as it should). Needless to say I was very happy with the way everything went.

I drove the car for the first time since the sender swap about a week later and went on a nice 25 mile run. After driving for about 15 minutes the "FUEL" warning light came on! The tank was still pretty much full and the needle on the gauge was close to the extreme right. I drove another 15 minutes to get home and the "FUEL" light did not go off until I turned off the ignition. I turned the key back to the ON position to power the fuel gauge and the "FUEL" light did not illuminate BUT upon turning the key to START and running the engine the darn light came back on.

I performed the same routine a few days later and the malfunction remained exactly the same. "FUEL" light comes on after the car has been running for about 15 or 20 minutes. The tank being over half full. The "FUEL" light will turn off when I shut the engine it and will remain off even with the car key turned to "ON" with the fuel gauge working but will illuminate as soon as I start the car again. After the car sits overnight the light will be off until the engine runs for about 15 minutes and the whole "Groundhog Day" scenario repeats itself. In other words, the engine needs to be running for the malfunction to occur. Leaving the key in the "ON" position (which powers the fuel gauge) will not trigger the "FUEL" light. I'm totally stumped!!

I contacted Scott at Z Car Depot and he said he hadn't had that malfunction reported before and he immediately sent out another sender. Amazing customer service!!

I installed the "new" sender the day it arrived and the malfunction remained exactly the same. I made sure all the contacts on the plug were clean, applied dielectric grease, looked for anything that might look strange, successfully tested the sender by plugging it into the harness before putting it in the tank and nothing changed. I'm even more perplexed by the malfunction. As always, any help from the vast knowledge base will be greatly appreciated.

Keep safe everyone. 

very well so far I will post up a video tomorrow demo of it working. Right now I am testing by letting is set in water for an extended period of time, monitoring the current (it seems to idle at about 30ma when submerged). One thing that is different is the fact that the donor had a rather elaborate plastic container (the before mentioned 2oz plastic container) This container was mounted on a wire just like the stock Datsun ( a stiff wire to the mount flange). I can only assume the OE Datsun was designed to operate with out said container, perhaps that has something to do with the hole size etc...

I did not notice any filter media on the plastic container, but then again I did not look at it too closely. The OE Datsun unit drained VERY quickly.

Edited August 9, 20204 yr by Dave WM

Do you need two bulbs, more resistance, with that Toyota thermistor also?

Edited August 9, 20204 yr by Zed Head

I think @Dave WM was using two bulbs because they were 6 volt bulbs.

No offense to Dave, but is there more info here than for the ZCD thermistor?  I'm looking for that set of DIY parts that a person could put together on their own.

If he used just one bulb would he have the same problem that the ZCD sensor has?

I checked all the available information on each one of the 5 bulbs I'm running the tests on. The number imprinted on the base of the bulbs does not coincide (with two exceptions) with the number assigned to it on the company's web site or on their sales receipt. However I think I was able to match them by cross referencing information and images. Remember that I assigned numbers 1 through 6 to the bulbs related to their measured cold resistance in descending order. "Glow" is subjective and refers to the perceived intensity of illumination when the erroneous signal is triggered by the Zcar Depot thermistor. Here's the full info on each bulb:

Bulb #______Volts______Watts_____Amps_____Resistance_____Glow_____#on base______#online___

1                     14               1.12            0.08             19.8                      +               756                    9428049

2                     14.4            1.44            0.1               12.7                      ++             813                    813

3                     14.4            1.728          0.12             11.5                      ++            1445                  274020

4                     14.4            1.872          0.35             11.4                      +++          1892                 274004

5                     14                2.8              0.2                 6.3                      ++++         363                   363

6  (OEM)        12                3.4                ?                   3.8                     +++++       NA                    NA

 

Bulb 6 has been ruled out since it illuminates very brightly when triggered.

Bulb 5 Illuminates more softly but is still barely visible in a dark cabin. Could work as a "monitor" of the system in working order (see previous posts).

Bulb 4 illuminates softly enough when triggered that it is not visible during daylight and very hard to see in the darkened cabin. If it brightens enough once the fuel level falls beneath the thermistor, then this bulb would be a reasonable and simple solution to the problem. I'll report on that once my tank is empty.

Bulbs 3, 2 and 1 have yet to be tested but they might illuminate too dimly to be effective as a low fuel warning once triggered by the thermistor even when the tank is empty.

Edited August 16, 20204 yr by dmorales-bello

27 minutes ago, Zed Head said:

No offense to Dave, but is there more info here than for the ZCD thermistor?  I'm looking for that set of DIY parts that a person could put together on their own.

If he used just one bulb would he have the same problem that the ZCD sensor has?

That's an excellent question. It seems from all our musings and experiments that the thermistor has to be appropriately matched to a specific enough bulb for the system to work properly.

more results,

yes the bulbs are GE47 (standard 6.3 volt used in most vintage electronics, series connection results in approx. same resistance as the bulb in the fuel indicator light.

late Saturday night experiment was disappointing. While the setup worked perfectly at 1st (out of water light comes on quickly 30s or so, in water light goes out within 1 second), continued testing showed that if left in the water for an extended period (in my case 45 min) the light was on. It was somewhere between 15 min and 45min when the light came on (I was not watching constantly).

while monitoring it I noticed the mA slowly going up, I incorrectly assumed it would stop at some point as it entered a steady state of water temp. Wrong. I left it at 15 min at about 60mA up from starting at 30ma. Came back at 45min and I was up to 190ma an bulbs lit.

Oddly I could not get the lights to go off even after blasting it with contact cleaner (very cold), Thinking I had destroyed the thermistor I considered an autopsy but decided to bag it for the night just in case I was missing something obvious. oh and the resistance was fixed think about 200 ohms but cant recall.

Next day everything is back to normal, hmmm, same working comes on out of water goes out in water. Thinking maybe I should test in fuel rather than water JIC it makes a diff (surface tension, cooling effect, etc...) so this time I use some fuel, works as expected, on out of gas, off in gas. left it go for at least 45 min, still off in gas, lift it up, goes on quickly, back in gas off again.

I also had this gas rigged up with a heat source to elevate the gas temp to upwards of 130f. Thinking it would not be uncommon for the gas in the tank to be  warm. No effect works fine.

Right now I am testing again with the intent to operate for hours with warm gas and see if it stays off.

I will post up the videos I on the testing later today after the long term test is complete. I think I will also try the test with just one light (the 6.3v) just to see how sensitive the choice of the bulb is (big change from 2 bulbs in series to a single bulb), after I complete the long term test that is.

 

 

 

Edited August 10, 20204 yr by Dave WM

16 hours ago, dmorales-bello said:

That's an excellent question. It seems from all our musings and experiments that the thermistor has to be appropriately matched to a specific enough bulb for the system to work properly.

I will be addressing this specifically. IF my long term test are successful with the current testing using the 2 GE47 6.3v series bulbs, I will remove on bulb from the series (that's a drastic change) and see how the system works. I will be posting an interim video in a few mins to set the stage.

 

Edited August 10, 20204 yr by Dave WM

17 hours ago, Zed Head said:

No offense to Dave, but is there more info here than for the ZCD thermistor?  I'm looking for that set of DIY parts that a person could put together on their own.

 

not really. but I will test the system to see just how sensitive the setup is. IF it turns out that a wide range of bulbs works then perhaps we can assume that maybe its not as critical as it may seem. in which case a simple resistance/physical size may be enough. I may end up just buying the specific thermisor referenced by some of the motorcycle guys to test as well. that will give us an exact part number to use. At this point I just want to see if I can replicate the prob Dr. Dave is having.

Edited August 10, 20204 yr by Dave WM

What I am also wondering is the thermistors use in a circuit. Was the later model stuff (like the 91 Toyota) really just the same deal a simple bulb/battery/thermistor setup like the Datsun OR was it more complicated perhaps a latching relay or some transistor bias adjustment or such... anyway testing will continue.

I kinda like the idea of a latching relay, trigged on just like the bulb, but once on break the current flow to the thermistor, leave the light on until the ign key is turned off to reset the latch. This take the thermistor out of the high heat for an extended time as well after the latch is set.

for more reliable that the relay perhaps and SCR could be triggered by the thermistor. I like it since once triggered like the latching relay, it would stay on until the ign key was turned off.

Edited August 10, 20204 yr by Dave WM

after about 1.5 hours (and no lite, about 30mA) I have shorted out one of the GE47's now my current is about 40mA, fuel temp about 118f, will monitor to see if things remain stable (lite off).

Just finished testing. Results are in. works the same with one or two lights (in series). That is it stays off in gas (warmed to about 118f) but will come on within seconds of removal of the sensor from the gasoline. I had to cut the on time short for the single bulb as I did not want to overload the filament. Based on my test I can definitively say that it works with either one or two GE47 bulbs. Are they the same as the stock unit, no, but I feel the cover a broad enough range that its safe to say the circuit should work with a low amp 12v bulb. the GE47 is approx. 1 watt at 6v from what I can find.

The only other test I can think of is to try the setup in the car (I will set it up on the floor board and directly connect to the factory light). I will set that up later today and report back I have a video coming on the test I just reported on.

below is a few hours later. What I am not clear on is I tested it for well over 1 hr with 2 bulbs and then well over 1hr with a single bulb. The overall current was pretty much the same, I suppose due to the ratio of the bulb resistance to the thermistor resistance. The only diff was if I had allowed it to go on much longer on the single 6v bulb I suspect it would have burned out.

Another observation, when testing with water vs gasoline, the thermistor can dumped MUCH faster with gas vs water. I suspect a surface tension issue.

Las thing, I forgot the thermistor was in warm gas when I checked the resistance, about 1k I took it out sprayed out the warm gas cooled with the solvent it was over 2k, I suspect when it settles down to room temp (aprox 78f inside the house AC) it will return to the 1.7kish resistance.

 

Edited August 10, 20204 yr by Dave WM