1977 280z "Fuel" warning light always on

[240zadmire]

Read in this forum that many suffer the ID10-T of the warning "FUEL" light not lid up.  My is opposite.  It always on even though the fuel gauge is half fuel.

Mind you that I haven't drove the car to gas station to get a fill up as I haven't register/insurance yet.  I made a few trip when convenience with a gas can of shy of 2 gallons.  I must have done that at least 5 times.  During my diagnose to fix other problems and driving around the neighborhood to test out brake, lights ... I put in 6 miles on the odometer.  I trust the fuel gauge  to be in working order and reports correctly.  So give or take, at least 7-10 gallons still in the tank given the tank's capacity is about 17 gallons full.  So the gauge says half full, the warning light is on.

Pull the darn thing out but didn't know what to do with it.  How do you diagnose, test to see if the "thermistor" is working/faulty?

 

regards

 

 

 

[SteveJ]

Disconnect the fuel sending unit. Check resistance on both sides of the connector from yellow/blue to black. (Note: the wire color could be different on the sending unit. Check the pin that goes to the yellow/blue wire in the body harness.)

Report your results here.

[240zadmire]

Ho @SteveJ

 

black to yellow with blue stripe got about 1K ohm

black to solid yellow got about 41 ohm

yellow to yellow with blue stripe got about 1.1k

 

see photos

[SteveJ]

So now measure resistance from the yellow/blue to the black on the wiring harness side of the connector. 

After we rule out a short In the wiring harness, you'll take some voltage readings.

[SteveJ]

Oh and you might turn the key to ON with the sending unit disconnected. If you get the low fuel light in that case, you have a cross connection. 

[240zadmire]

No light when key is ON

68 mV when connect any pin to chassis

0 when connect pin to each other

 

[SteveJ]

Well, it looks like you don't have a short to ground between the fuel light and the sending unit, otherwise you would have gotten the fuel light with the sending unit unplugged.

You seem to have good resistance at the thermistor (1kOhm between the yellow/blue and black wires).

You could pull the sending unit and see if the thermistor was covered with gas. It seems like it is not immersed in fuel with the meter readings you got.

 

[240zadmire]

Not sure if I understand you.  

4 hours ago, SteveJ said:

Well, it looks like you don't have a short to ground between the fuel light and the sending unit, otherwise you would have gotten the fuel light with the sending unit unplugged.

You mean the sending unit does not have a ground properly? Connect chassis to the black wire ?

[SteveJ]

No, that's not what I said. A short would be a bad thing. You ruled that out. That's why I gave you another test to run. Pull the sending unit and see if there is gas on the thermistor can.

[Dave WM]

there has been a lot of discussion on fuel sending lights. when everything electrically checks out I would suggest you pull the sending unit and look to see if its all crudded up with deposits.

the theory of operation is the gasoline cools the thermistor inside the thermistor housing. IF that housing is plugged the fuel will not get to it to cool off and the light goes on. A simple test would be to remove the sender, immerse the housing in a cup of gasoline (not water use real gas for this test) and look to see bubble come out, then remove it and look for a good amount of fuel to pour out of the holes in the housing (not just drip off the sides but actually dribble out of the housing holes. The housing has a filter element that is a fine mesh. this can be clogged. IF its clogged try immersing in a cup of vinegar overnight. swirl around now and then to see if you can dislodge the particles clogging, blow out the crud thru the various hole in the can. Try the gas cup test again. If its still clogged you may need to disassemble the housing. This is some delicate work that requires some soldering and is likely to destroy the existing thermistor. However if its not working you don't have a lot to lose. A possible replacement thermistor has been identified. I would suggest you rebuild the existing housing with that replacement thermistor. That housing was better designed that the replacement housings out there.

Oh and make sure the light in the indicator is NOT an LED. The car was not designed for LED lights, so things like the sending unit and the dimmer will not work with them. The instrument lights were always dim, that is what folks were used to back in the 1970's. Bright instrument lights also are not good for night vison when driving in darkness.

 

Edited August 14, 2020 by Dave WM

[240zadmire]

Hi all

 

pulled the sending unit out of the tank.

measure the length from the cap to end of the thermistor with a long stick as shown in the pictures.  Mark the stick and dip it inside the tank to make sure when the sending unit installed, it needs to submerge under the gas and it did almost twice the length of the thermistor.  That can be crossed off the todo list.

dip the thermistor into a left over plastic cup of last weekend son’s birthday. I know, glass, metal or approved plastic... all I had at my disposal at the moment.

dip it in the gas cup and see bubble rushing.  Pour gas out of the thermistor and observe gas was pouring out vs dripping.  Stir, not shake, just like Bond likes his martini, moderately not to spill gas outside of the cup. Observe stir motion had lot of bubbles and no dirt. Gas looked clean.

Measure current temperature both gas in tank and in cup.  Connect wires while the thermistor is outside.  Light on. No surprise there.  Submerge the thermistor to the gas cup. Light still on.

Been tracing wire diagrams and it lead to many places like relay under passenger seat, cigarette lighter, floor temperature, voltage regulator, condenser at voltage regulator... clean all connections.

Seem like we all agreed on the prognosis there is a problem somewhere but where exactly is the question.

Reread @Dave WM twice didn’t see anything wrong.  The third time caught my eyes

“ Oh and make sure the light in the indicator is NOT an LED. The car was not designed for LED lights, so things like“

being anal as I am that I need uniformity... I replaced everywhere possible with LED as I like it a bit brighter.  And being a hoarder like I am, keep junks until project is done many months or years before throw them away with great pain in the guts still.  Dig through the used bin and pull out a bulb.  Swap back.  Light gradually brighter while the thermistor is outside.  Submerge it to the gas cup and smoke is coming out of the thermistor.  Scary my friend.  We’re talking about highly flammable here.  Light went off. 
 

Ladies and gentlemen, I haven’t had breakfast yet and the moment I see light went off, the feeling as if I just had an all you can eat and now is sipping a piña colada somewhere in the Caribbean ?

I repeated the experience multiple times and works as expected.

boy, what a fun chasing this problem.  The old saying is truly applicable here.  If it ain’t broke, leave it alone or something along that line.

thank you everyone for the help and support.

 

regards

 

 

 

 

[SteveJ]

You aren't the first to use an LED where the incandescent bulb is needed. I doubt you'll be the last. I'm glad @Dave WM thought to mention that.

Here is another old-school application where incandescent lights are needed. In the past to synchronize an AC generator to a live system, you had two lights that are connected in series and to the same bus phase on each side of the breaker. When the sources were 180 degrees out of phase, the bulbs burned bright. The closer the generator was to being in phase with the bus, the dimmer the lights would get. When the lights were extremely dim or out, it was safe to close the breaker...if you were quick enough.

Some people still want this antiquated functionality on new equipment, but they also want LED light bulbs. I submitted a design package to a consultant complete with my parts list that had LED bulbs and the incandescent bulbs for the synchronization panel. The consultant wrote back that his specification called for LED lights. I replied that the incandescent lights were for the synchronization panel that he also specified. That was the end of that conversation.

Yes, the life of an electrical engineer can be so exciting...just like arguing on the internet.