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).

try this

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.

which is why its a good idea to always test wiper direction with the wipers not installed. as ZH pointed out, its the arm bolted to the wiper motor, its 180 degrees off.

Dr Dave, what the cold reading resistance of the thermistor in your new sending unit?

did some more test after the video, the engine on (14v vs about 12.6 engine off battery voltage) resulted in about 8v at the back probe when the same test as the video was done. I filled up the tank this morn just to make sure I did not goof up my gas gauge. The light was coming on and off while driving to the station, gas slosh I presume, this was after adding about 2 gallons to the tank. I am pretty sure I was nearly empty as it took over 14 gallons to fill (so 2 I put in at home plus 14.5, 16.5 total, I must have been on fumes). So with lets guess 2.5 gallons in the tank it would come on and off, Gauge indicated "E" even with the 2.5 est gallons. Filled up it goes right to Full. I may have cocked the sensor a bit on the install. I think it should read a bit higher on empty., I will drive it around to the 1/2 mark, to make sure my gas is below the sensor hole on the side of the tank, then release the hold fast ring, and see if I can tweek the position of the sensor in the hole. Try to get a max reading on the gauge. fyi, the cold resistance of the thermistor reading it at the plug was about 3k which seems very high to me I=E/R 12/3000 or .004 amps, seems like it should be a lower resistance to allow a faster ramp up of the heat once its clear of liquid. I think there maybe other causes for the high resistance other than the thermistor. I should have soldered on lead from the thermistor can to the mounting clip, as noted before there was a fine white residue build up that I had to sand off. So my only fix was to clean that residue and then solder the loose wire that I presume was from the thermistor to the bottom of the can (it was just bent over making a poor contact).

Turns out mine is not working, was working awhile ago. Spent the day trouble shooting. Back prob the connector at the center console. Key on fuel in tank disconnect the lamp and you should get 12v at the back prob (with out the load of the lamp you get the full battery voltage due to the high impedance of the DVM). Plug the light back in and the 12 drops down to about 20-30 millivolts (again back probing the connector from the lamp the harness that is under the dash). I next pull the sending unit. Tip remove the tank mount by the fuel pump but leave the tank in place. This will give you way more room to work around the sending unit ring (this is with the side mount sending units). Visual of the thermistor can shows a fine layer of corrosion where it snaps in. I remove the thermistor can clean both the can and the retaining clip with some fine sand paper, Try it again this time with the sending unit outside of tank. It works but not as well as I like plus I see some intermittent drops of the voltage while I monitor the back probes. down to zero... hmm must be a break in the circuit. I noticed the can has a small wire that comes out the bottom and is not firmly attached. I presume this is the electrical ground for the thermistor. I solder that to the can bottom. Much better action now. I am posting up a video will link here when done.

its not a stock thermistor? oh well that could explain things.

you would prob want to get a WW pot, those cheap carbon track pots would not like the current. I think I have some 100 or 500 ohms ones laying around if you cant get the fix with the different bulbs.

The thermistor has to have enough current flowing thru it to heat up. To low a wattage (high resistance bulb)and it will not heat up, how low wattage is to low is something the engineers can answer if they had the specs. Here is a good example, if you read the voltage at the light socket with a reg VOM or DVM you will likely see 12v, as there is no load on the circuit. Put a load on it like say a test light and the voltage will drop, not zero but some, bigger load bigger drop, as the voltage drops at the socket, more current is flowing thru the thermistor, submerged I doubt it would ever heat enough to change it resistance (lower). its a very analog setup so you may or may not see the bulb of a test light come on.

"The whole unit fits only one way and straight into the tank which was full of fuel so yes, the thermistor must be in the fuel. As a matter of fact, with the unit plugged into the harness but not installed in the tank, the thermistor triggered the "FUEL" light on. It went out as soon as I introduced the sender into the tank as the thermistor hit the fuel" Since the above was done, it pretty much means everything is working electrically. I mentioned the mount cause on mine its a side entry on the tank, and the mount wire can be bent up or down to adjust when the sensor is in the fuel. I cant see any other explanation other than an intermittent short in the harness and/or sensor wiring or a misplaced sensor. Maybe perform the test again with the engine running to confirm its not a voltage issue (higher voltage from alt running). Thing about testing is you want as few variables as possible.

One mine the onset of the light is gradual very dim and slowly brighter as the current increases. Driving the car around corners/stopping/starting, all has an effect a the gas sloshes around, so it will go out dim come back on etc.... My setup has it so it will start to light at about 14 gallons empty (that is what it will take if I fill up so I guess I have about 2 usable gallons left).

Ok I have to ask, you are sure you have the mount wire correct and the thermistor is in the tank such that its near the bottom? Next check the function of the plug by shorting out the wire harness, there is a common neg lead, connecting this to one lead will turn on the fuel light the other will peg the fuel. The last thing you can test would be to actually take out the sending unit, hook it up and dip the thermistor in a glass of water, make sure everything works as it should while you can observe the effect of the thermistor in and out of the water.

another useful purpose of these handy devices in in vintage electronics to soften the start up of power supplies. there is often a surge current when the power switch is closed, and some of those power switches are very hard to find. So to extend the life of them its common to put a thermistor like a CL-90 in series with the power line. IIRC they present with about 100 ohms at typical room temps, enough resistance to significantly reduce the surge current on power up. As current flows thru them they heat up (quite hot actually) and the resistance drops to just a few ohms. This all happens in about 10-15 seconds and is not noticeable on most tube amps. they are good for about 2 amps total which works well with most of the typical tube amps of the day.

its a resistor that changes resistance with temp. So you have a constant current flowing thru it (its in series with the bulb to the battery). Current is always flowing generating some heat in the thermistor, but since its immersed in fuel the heat it soaked away keeping the temp cool and the resistance too high to allow enough current to flow and thereby causing the light to be seen (there is always a small current flowing thru the filament, just not enough to cause it to glow). With the gas gone, the thermistor no longer has its heat sink, so it starts to heat up, resulting in more current to flow (and more heat by the way) until the is a large enough flow in the series with the lamp for it to begin to glow. You need the correct incandescent style bulb for it to work, including the correct wattage bulb. Its a VERY simply circuit as long as the correct bulb is used.

Will check it out, thanks!

Its SOO BORING driving around here, but I saw a youtube video that inspired me to use a new route when I take Dat Boi out to visit the gators in Mount Dora. Green Mountain scenic hiway, from winter garden to mount dora. I took the suburban so my gal could go (she doesn't care too much for Dat boi, prefer the rolling living room). Anyway it was a pleasant ride and has some actual hills to go up down and around (prob the hilliest spot in all of florida). So sometime next week I think I will take a long lunch and take the Z along that route. Its pretty slow (max was 55, some speed traps were seen) but that's fine, In fact I would like to make the next cross country trip on about 75% roads like this. I cam back the normal way, just have to watch my speed, Its easy to get carried away, I found myself going 85+ with the OD 5th, and while I really was not going much faster than traffic, I doubt that would be a legit excuse. With the new tires and new weather striping it creeps up on you with the low noise. My goal is to try and find more roads like this in my own backyard until the next big trip is planned. I may shoot down 27, its been a long time since I took that route south (used to take it to save a few bucks not taking the turnpike)., Use to have a bunch of little towns (Sebring being the only one notable due to the racing), but that was 40 yrs ago, so I presume its all developed now. I should prob only look north to avoid the overdevelopment that is such a problem in the southern half of the state.

For the rear end... ...is it really best practice to BURN the moustache bushings out, rather than pressing them out? Its best practice to leave them if they are ok. no more sacrifices to the PU gods... Is this a track car or are you daily driving it.

here is me messing with my spare engine, using the color tune plug to confirm proper EFI operation

the water temp sensor typically goes high (rich) but not impossible to go low (lean). As far as vacuum leaks the best practice would be just to smoke them out. So many places for problems. One major place would be the O rings on injectors, easy to screw that up when replacing. Next would be the AFM to TB hose, crack can hide in the accordion folds, then the EGR is so equipped, and any mods to the PCV system. I properly setup system will have idle issues if you remove the oil fill cap while running. even just pulling the oil dip stick should show a change in the idle if its all working correctly.

this is why is so important to use the FSM as a guide. So often it becomes a game of lets try this, or replace that, a simple check of the 35 pin connector gets you on the right track (a check of the pin 13/ground for resistance to test the water temp sensor should have shown this as an issue). Here is a tip for you, check the water pump for free play/leakage at the weep hole, Nothing ruins the day like a bad water pump.

I missed the idle screw has no effect. That is indeed a clue. Have you tried smoking it for vacuum leaks? Blocking the front of the AFM with a foam cup, then blowing smoke from a cigar into the intake manifold (at the brake booster port is a good place), look for smoke. Besides poor electrical connections to various sensors (and grounds), vacuum leaks can plague EFI, it has no O2 sensor and can not compensate for problems if there is anything that is out of spec.

the FSM is your friend. You need to start by checking resistance readings at the 35 pin connector. Don't anything more until you do that, or you could just be adding layers of errors. Main one to focus on is the temp sensor (not the gauge but the sensor that goes to the 35 pin connector and is crucial for the ECU to set the air fuel mixture). Also when ever I hear the word "deletes" it make me cringe. Deleting means the system no longer is setup to the FSM, "could" impact how the test work and the results. Reminds me of the word "upgrade" that one gets thrown around a lot, and I would say maybe "changed" would be better. sounds like it did work at one time (but for the idle, which sounds like the aux air control, covered in the FSM). So each change you make now gets you further from a once working system. Hence the need for checking before any more changes.

I used the MSA comp gasket, no sealant. Two things, don't over torque it, and if there is a leak, it maybe the side seals, they look a lot like a pan leak in the rear of the pan. If you keep having leaks, consider replacing them. You can check them easy if you are doing a clutch (rear main and side seal leaks are easy to see with the flywheel off). But if you don't want to pull the flywheel just to check you can do the side seals with just the pan off. Its a bit of a PITA but not that bad. Replacing them solved my "pan" leak.

I would have been happy to pay that for a new tank for my 75 280z. How different are they? I know in 77 they went with the space saver tire and completely changed up the tank, but this new tank looks a lot like my 75.