Perplexing "FUEL" light malfunction

[chaseincats]

Just now, AZDatsun said:

I have the zcd sender but I have too much gas right now to test about how much before the empty line will it start to glow? If I just lift the sender up out of the tank the light should come on right?


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For me it is on regardless of fuel level

[dmorales-bello]

I have the zcd sender but I have too much gas right now to test about how much before the empty line will it start to glow? If I just lift the sender up out of the tank the light should come on right?


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Because the wiring harness isn't long enough you'll have to disconnect the sender in order to pull it out far enough for the thermistor to be above the surface of the fuel. Reconnect to the harness once the sender is out of the tank (tap out any fuel left inside the can) and turn the key to "on" and the "fuel" light should come on very soon.

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[dmorales-bello]

I have the zcd sender but I have too much gas right now to test about how much before the empty line will it start to glow? If I just lift the sender up out of the tank the light should come on right?


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When my original unit was working, the light would go on once the needle was past the E by maybe 1/8", supposedly with about 2 gallons of fuel left in the tank.

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[Dave WM]

1 hour ago, AZDatsun said:

I have the zcd sender but I have too much gas right now to test about how much before the empty line will it start to glow? If I just lift the sender up out of the tank the light should come on right?


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Just don't expect it to come on instantly, two things have to happen, fuel has to drain out, depending on the crud status this may be slower if there is a build up of crud on the drain holes. Second the thermistor has to self heat, this will take time how much is debatable, but should not be more than 30 seconds once all the fuel is gone. That was my experience when messing with them on the test stand. Actual usage will be more intermittent. Fuel sloshing around in the tank has the effect of turning the light on and off when the level at a point that its just low enough to allow time for the fuel to drain, the heat to build the light comes on then you start rolling or turn and there is a splash of gas and its off again. drive a few more miles and as soon its solidly below then the light will tend to stay on. during the in between full on an full off there is various intensity as well, again having to do with some cooling from the limited gasoline splashing around.

Edited August 16, 2020 by Dave WM

[AZDatsun]

Ok I pulled the sender and the light works I originally had an led in there that didn’t work so I popped the bulb out of my glove box. Not sure what the bulb specs are but it says 1445 and Taiwan on it.
If anyone needs more detailed pictures let me know









I






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[Zed Head]

Not sure what you're showing.  You showed a bare bulb, and the Charge warning light.

The Fuel light is down lower.  Here's 1976, they give more detail than 78 but it's in the same spot.

 

[AZDatsun]

Sorry I was just showing my gas gauge is on empty and the fuel light that I pulled from the back of the red fuel gauge in the center next to the map light actually lights up. I guess I can plug it back in and take a picture of the light shining through the red plastic if you wanted but figured this way it might help because it’s day time and you can see the glow easier

 

The bare bulb in my hand was the bulb from your diagram

[dmorales-bello]

Update on my "bulb" trials.
Bulbs 2, 3 and 4 are acceptable as workarounds for the disfunctional thermistor in the 2 ZCD fuel senders that I received.
Those bulbs have such a faint glow when the malfunction happens (after about 25 minutes of running the engine) that they are pretty much invisible in daylight conditions and very hard to detect in a darkened cabin BUT they glow brightly once the fuel level drops below the thermistor, maintaining their functionality as a "low fuel warning" light.
Interestingly they have cold resistance values that are very similar (12.7, 11.5, and 11.4 ohms respectively).
BTW, bulbs 5 and 6 were very bright at the moment of malfunction. Bulb 1 was too faint when the tank was empty. Therefore those are not acceptable.

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Edited August 18, 2020 by dmorales-bello

[SteveJ]

43 minutes ago, dmorales-bello said:

Update on my "bulb" trials.
Bulbs 2, 3 and 4 are acceptable as workarounds for the disfunctional thermistor in the 2 ZCD fuel senders that I received.
Those bulbs have such a faint glow when the malfunction happens (after about 25 minutes of running the engine) that they are pretty much invisible in daylight conditions and very hard to detect in a darkened cabin BUT they glow brightly once the fuel level drops below the thermistor, maintaining their functionality as a "low fuel warning" light.
Interestingly they have cold resistance values that are very similar (12.7, 11.5, and 11.4 ohms respectively).
BTW, bulbs 5 and 6 were very bright at the moment of malfunction. Bulb 1 was to faint when the tank was empty. Therefore those are not acceptable.

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To make it easier for people to find the bulbs that worked well:

https://www.bulbtown.com/1813_Miniature_Bulb_Ba9S_Base_p/1813.htm

https://www.bulbtown.com/274020_GM_General_Motors_Replacement_Bulb_p/274020.htm

https://www.bulbtown.com/274004_GM_General_Motors_Replacement_Bulb_p/274004.htm

[Captain Obvious]

Couple days ago I picked up a spare fuel sending unit to mess around with. First thing I did was checked the thermistor, and it's open circuit. So unfortunately no help there with analyzing a working one, but at least I get to see the thing with my own two eyes.

So since the thermistor was already dead, I figured no harm in starting the autopsy. Desoldered the end lead:

And pulled (what's left of) the thermistor out of the can. The thermistor nugget itself is completely vaporized. Just like what you guys found, all that was left was the two leads:

I still contend that there is nothing interesting inside the black plastic end plug. Here's some shots of the two ends:

A

And for confirmation, I took it apart. It's just a brass ferrule with flared over ends so it stays in place inside the black plastic cap. And then they use that brass tube as a connection point to solder the flexible wire to one of the thermistor leads. Looks like this. They just bent the ends of the two wires into "U" shapes and hooked them together and then soldered the whole thing together:

Here's the hollow brass tube after the solder has been removed:

So there's no mystery hidden inside that plastic cap. It's just transition from yellow lead wire to the thermistor lead. @Dave WM If you're going to do another thermistor swap, you don't have to splice the wire to the old stub. You can just solder it into the ferrule.

[Dave WM]

19 minutes ago, Captain Obvious said:

Couple days ago I picked up a spare fuel sending unit to mess around with. First thing I did was checked the thermistor, and it's open circuit. So unfortunately no help there with analyzing a working one, but at least I get to see the thing with my own two eyes.

So since the thermistor was already dead, I figured no harm in starting the autopsy. Desoldered the end lead:

And pulled (what's left of) the thermistor out of the can. The thermistor nugget itself is completely vaporized. Just like what you guys found, all that was left was the two leads:

I still contend that there is nothing interesting inside the black plastic end plug. Here's some shots of the two ends:

A

And for confirmation, I took it apart. It's just a brass ferrule with flared over ends so it stays in place inside the black plastic cap. And then they use that brass tube as a connection point to solder the flexible wire to one of the thermistor leads. Looks like this. They just bent the ends of the two wires into "U" shapes and hooked them together and then soldered the whole thing together:

Here's the hollow brass tube after the solder has been removed:

So there's no mystery hidden inside that plastic cap. It's just transition from yellow lead wire to the thermistor lead. @Dave WM If you're going to do another thermistor swap, you don't have to splice the wire to the old stub. You can just solder it into the ferrule.

good to know thx Capt O!

[chaseincats]

Update: I received a replacement fuel sending unit after my first one from zcardept started to engage the fuel warning light when the tank wasn't low on fuel.  After a few weeks of driving with the new sending unit, this new one started lighting the warning light even on a full tank.

I'm not sure what the issue is with the thermister on these, but it just seems to be a design flaw.