Fuel gauge issues - 73 240z

[Captain Obvious]

1 hour ago, JDziark said:

How about if I back feed a 12V source at the yellow wire by the Sender Unit?  That should send current through the 'Second Heating Coil' and the 'First Heating Coil' then to ground.  Shouldn't that make the needle move?  Not sure what I'd do after that even if the needle does move but if it doesn't, wouldn't that mean that that one of the coils is bad (since that would not allow current to go to ground)?

I'm not really sure how helpful this would be. Yes, it would feed current back through the system as you described, but I'm not sure how much needle movement there would be. It depends on the resistances of the heating coils and I don't remember the values offhand. I know the sender unit is about 90 Ohms, but I don't remember what the coil resistances are.

It won't hurt anything to try that, but just not sure how much value there would be.

 

I think you're going to to have to pull the gauge and clean up the regulator contacts. Start gentle... Strip of printer paper pulled through the contacts a couple times just to burnish the surfaces.

[JDziark]

Ok, thanks to both of you for the input.  Looks like I have my work cut out for me.  Not sure when I'll get to it.  I may save it for next winter's project.  I'm in upstate NY so there will be plenty of time to work on it then.

Thanks once again for all the valuable input.

John

[Captain Obvious]

G;ad to help. I wish I had some way to move forward without having to pull the gauge out, I do have one idea though...

If you can disconnect the gauge from the harness and measure the resistance between the pins on the gauge that go to 1) the power input and 2) the output to the sender unit. I believe the two colors in question are yellow/red and the yellow wires. Not on the harness side, but on the gauge side with the gauge unplugged. That resistance should be an additive total of the internal regulator switch plus the resistance of heating coil #2.

If you can tell me that number, I can measure the resistance of heating coil #2 on a different gauge here at my place. No guarantee they are identical, but they should be in the same ballpark. Then using that info, we can determine the resistance of your switch. It should be very low resistance, and my theory is that yours will not be very low.

Just one more thing to look at for now that might be easier than pulling the gauge out?

[JDziark]

Captain, a little more data.

1). When I back fed the gauge with a 12V source (momentarily since I didn't want to burn anything out), the needle behaved normally.

2). I bit the bullet and took the dash half apart to get at the back of the gauge.  The yellow wire on the back of the gauge reads < 1V, the same voltage I get all the way back to the yellow wire at the tank.

3)  The yellow/red wire appears to be the feed.  No voltage when the key is off, 12V when the key is on.

4). I got about 700 ohms between the yellow and yellow/red wires at the gauge (after pulling the harness apart).  A little hard to get a good number since the pins are set back and close together.

5). Next steps?  It looks like it isn't too much of a stretch to remove the gauge at this point.  Not sure about access to the other harnesses/lights.  It may be easier to unscrew the nuts and remove the wires from the back of the gauge.

6) lots of fun removing the heater/vent/defroster cables and crawling under the dash!

 

Thanks again for your help.

John

[Captain Obvious]

Excellent work. 

So I measured the loose fuel gauge I have here and I get about 50 Ohms. There are several possibilities to explain the difference...

1) You measured it wrong
2) I measured it wrong
3) They dramatically changed the design between your 73 and the later 280's (I think my test gauge is from 78)
4) Your voltage regulator switch is dirty and has way more resistance that it should

Of all those possibilities, I consider #4 to be the most likely. Especially in light of your result backfeeding the gauge.

So I'd try pulling a narrow strip of printer paper through the switch contacts and see what happens. This is a 280 gauge, but the switch should look something like this:

[Captain Obvious]

Oh, and I know nothing about the mechanical construction of the 240 gauges, but I can take the faceplate off my 280 gauge and get to the internals without having to take everything out of the can. Not sure if you can do that with your 240.

[JDziark]

Captain, great input!  Thanks.  I probably measured the resistance wrong, the prongs were very hard to contact with the gauge in the dash.  I wondered if the regulator switch was like a set of points.  Imagine how many times that switch turned on and off over the last almost 50 years!

I've gone this far and will continue and take out the gauge.  It will be a lot easier to work on.

Is that picture (very helpful, thanks) taken from the front of the gauge?  If so, how do you get the needles/faceplate off?

I'm the original and only owner of this car, it's been a long process to refurbish it and it hurt to take all that stuff out of the dash (again!) but part of the fun is learning a little more about the car as I go along.

Wish me luck.

John

[SteveJ]

Before you go too far, I think I have a 240 fuel gauge hiding somewhere in my parts collection. I can open it up and do a video of the measurements so you can copy the steps.

[JDziark]

Steve, thanks.  I'll wait for your video.  In the mean time, I'll work at removing the gauge.  I could just keep the tank full of gas but the thing not working is annoying.  

John

[Captain Obvious]

1 hour ago, JDziark said:

I wondered if the regulator switch was like a set of points.  Imagine how many times that switch turned on and off over the last almost 50 years!

Yes, the regulator switch is similar to a set of points. It doesn't open and close as quickly, and it doesn't handle nearly as much current, but yeah... It's opened and closed an untold number of times over the past 50 years!

So that pic I posted is from the front of a 280 gauge with the faceplate removed. On the 280, the faceplate for the fuel/volt is split down the middle and you can take either half off. Two screws and half the faceplate comes out.

Here's a pic of the gauge with the plate intact. Take out the two small screws at the bottom and the "VOLT" faceplate comes off revealing the guts insde. Be careful and gentle and make sure you don't bend the needle. Don't know if the 240 is the same, but the 280 looks like this:

If SteveJ works up a video, that would be fantastic.

[JDziark]

OK, bad contacts it appears to be.  When I apply 12V to the feed (yellow/red wire) and measure the voltage of the yellow wire, it's >1V.  If I apply a little pressure to the contact, it goes up to 12V.  Captain, as you suggested, I'll start with a strip of paper but may eventually go to a thin piece of some kind of slightly abrasive material.  I'll keep you guys posted.  This whole restoration project has been a case of 3 steps forward, 2 back.  Now I have to fix the gauge and then put everything back together again.  Do either of you have any tricks up your sleeve to reattach the cable going to the heater control valve?  Those are tough screws to get at.  When I took that cable off, I just loosened the screw holding the cable to the heater box and pulled the cable through it.  I hope I can just feed it back in and retighten the screw that's already in place.

John

[JDziark]

More info.  Played around with the gauge.  The 'stud' that the points close again is actually threaded.  I turned it in about a quarter turn to insure good contact with the other end.  Putting a current to the yellow/red wire and checking the voltage at the yellow wire gives a nice variable voltage reading.  I ordered some higher wattage bulbs for the dash and will wait for those to come in before hooking everything back up.

Great learning experience - thanks to you guys.  If anything else comes up, I'll let you know.

John