spdrcr29

No seriously! So here's where I'm at...

Fired up the engine for the first time yesterday since body-off restoration. Everything in that regard worked perfectly. Still no dash/interior in the car except for dash harness and enough to be able to use a screwdriver to engage the ignition switch. I did however temporarily hook up the center gauge cluster so I could monitor oil pressure and coolant temp. While I was at it, hooked up the amp gauge and fuel gauge too. Prior to starting the engine, I put about 4 gallons of fuel in the tank. Once I got the engine started and idling good, I was kind of just hanging out in the driver seat watching oil pressure and waiting for coolant temp to heat up when all the sudden I noticed smoke coming from the back of the gauge cluster. I pulled the gauge cluster forward and found that the smoke was coming from the back side of the fuel gauge so I quickly unplugged its connector. Smoke stopped..

So today, I have checked the fuel gauge installation and the resistance of the resistor per Willcox tech instructions. Everything looked good there and the resistor tested at 90.4 ohms like it should. Next, focused on the Signal (or Ohms wire as described by Willcox). I first pulled the signal wire off the sending unit as well as the connector from the fuel gauge and used the meter to make sure that tan wire wasn't shorted to ground. No issues. For the heck of it, also checked to make sure this wire wasn't shorted to B+ somehow. No issues. Next, reinstalled the signal wire at the sending unit and checked resistance on the tan wire at the fuel gauge with the connector unplugged. It read 5.7 ohms (again only about 4 gal of gas in tank). No issues.

Wasn't coming up with any good reason for the smoke, so I kept jacking around trying to figure out why the smoke from the fuel gauge with the ignition turned on. At one point, I had the fuel gauge reconnected with the tan wire on the sending unit unplugged. With the ignition on, the gauge responded by pegging itself to about the 3:00 position. I tried a few more tests as outlined by Willcox tech instructions. Still no obvious issues. However when gauge and sending unit were re-connected and ignition turned on, I would get smoke every time.

For the heck of it, I decided to add some more fuel to the tank so as to change the resistance reading from the "signal" wire to the fuel gauge. With an ohm meter back at the connector for the fuel gauge with it unplugged from the gauge, I got 15.7 ohms this time. Again for the heck of it, I plugged the connector back on the fuel gauge....AND NO SMOKE!

Again, trying for anything and doing a little experimenting, I used a long screw driver to push down the float in the tank. Verified 0.8 ohms at the tan signal wire at the fuel gauge connector. Hooked the connector back up, turned ignition back on....And Again SMOKE! I then turned ignition off, went and pulled the screwdriver that was holding down the float in the tank which allowed the float to come back up per the amount of fuel in the tank. Verified 15.7 ohms as before. Turned ignition back on, and No SMOKE.

So it seems that if the tank is empty or there is very low resistance at the signal wire to the fuel gauge, smoke will start coming from the back of the gauge. But if there is higher resistance, No Smoke. I'm stumped now and not sure what else to check. Could this be an issue with the sending unit or the fuel gauge itself? Pretty confident it's not the wiring in between...

Well, sorry for being long winded. Any thoughts would be appreciated!

'75

It sounds like you've been pretty thorough in your testing. Is this a new fuel gauge or the original? Is there a new resister on the back or the gauge causing the smoke?

spdrcr29

Hi '75. The fuel gauge is the original as is the resistor that came with it(reads 90 ohms as it should). Prior to restoration, the fuel gauge seemed to work fine. The sending unit was replaced with a GM NOS part from eBay. Its been bugging the &^%$ out of me today, so went to my 72 parts car and pulled the fuel gauge out of it and installed the 71. With the sending unit float arm held down in the tank with a screwdriver, Guess what....MORE SMOKE! As soon as the screwdriver was removed and the float arm moved back up to the level of fuel in the tank (higher ohms), No more smoke. So basically no change when swapping gauges. Not 100% convinced its the sending unit itself, but at this point I'm willing to try another one unless you or someone has some other ideas??

'75

You should be able to ground the Signal terminal on the gauge and make the needle read empty. The smoke makes me wonder if you have gauge wired wrong or missing an insulating washer.

'75

Just had another thought, is the small gauge panel grounded, The temp gauge has a tab from it's ground terminal to the metal gauges housing and gets the ground for all the gauges from the temp gauge connector.

spdrcr29

I will try grounding the signal terminal and see what happens. I know when I pulled the signal wire off the sending unit, the gauge pegged at the 3:00 position. So the gauge seems to be responding. As far as wiring to the gauge, its only 2 wires. Matched it up to Willcox tech instructions and its dead on.

spdrcr29

Yes, the panel is grounded. Even had the meter ground connected to this point at the temp gauge, so I know its good. I have been thinking about the sending unit causing this, but don't really see how it could cause the gauge to smoke. Gotta be something else somewhere. Probably something stupid that I just haven't found yet!

Thanks for your help so far!

spdrcr29

Ok guys, it appears to be the fuel gauge itself that is the issue. Pulled the gauge from the housing and reassembled it on the bench using the insulator, terminals, and resistor. Following Willcox tech instructions again, I provided B+ and ground where instructed and then connected another ground to where the signal wire would normally connect. I didn't have any resistors or an extra sending unit to be able to sweep the signal wire from 0 ohms to a higher resistance though. But with everything connected properly, sure enough...Smoke!

When removing either of the two grounds, the gauge did still operate as it should by either going to the 3:00 position or to empty position depending on which ground was removed. While testing, I noticed that the gauge continued to smoke for a bit, but it eventually stopped. Just a real light and barely noticeable smoke is all I ever have seen to begin with. I actually had to hold a flashlight near the back of the gauge to even see the trace of smoke that I have been describing. Kind of hard to see the smoke otherwise. Anyway, after the smoke seemed to stop, I continued testing the gauge and it seemed to work just fine. I even checked the resistor and it tested good as well.

What's weird is that I also bench tested the fuel gauge from my 72 parts car and got the same initial light smoke from the back of this gauge while testing as well, but it too eventually stopped. I again continued to test this gauge and it also seemed to respond as expected when either ground was removed.

So question for you guys. I believe that I had seen in one of Willcox videos that Ernie said there was some sort of oil inside the fuel gauge assembly? Could it be that due to age and/or amount of time that these gauges have sat without being used has caused them to smoke some after finally hooking them back up? The gauge from my 71 hasn't operated for about 7yrs and the gauge from my 72 parts car probably hasn't been connected for at least the last 15-20yrs.

Any thoughts??

7T1vette

Somehow, that 90 ohm resistor is not connecting with the system/gauge wiring like it should. That resistor is what prevents excessive current from flowing into the gauge. Since it is smoking as the tank sender resistance approaches zero ohms, it's pretty apparent that the 90 ohm resistor is NOT hooked up correctly. It could be a case of having insulator washers in the wrong location...or the resistor across the wrong terminals....something of that nature.

spdrcr29

Hi 71. Thanks for your input. I get what you are saying, but I've tested the resistor for both of the fuel gauges I have and both test right at about 90 ohms. After repeated testing, the smoke seems to have subsided on both gauges, but yet the gauges both still work as they should when removing either of the grounds. The resistors still test out fine as well. As far as resistor installation, there are no insulator washers. Just a one-piece hard plastic insulator that installs over 2 of the poles on the back of the gauge. I've read and re-read the tech instructions on Willcox website several times. Installing the pieces on the back of the gauge is pretty cut and dry. I don't think I'm at liberty to repost the info from Ernie's website, but below are some photos of the gauge, insulator, terminals, and resistor layout and installed on the gauge in order. This layout matches the tech instructions on Willcox website to a T. I'm still not ruling out that I'm causing this issue somehow, but for now, not sure what it could be. Also, not sure how I have 2 fuel gauges doing the same exact thing. Maybe you or '75 or anyone else can confirm that everything is proper per my photos below? Thanks again!

cagotzmann

I think the problem is with the 90 ohm resister. Test by applying the voltage across the resister only.

With a amp meter is series with one of the terminals.

With 12 V this should produce ~ 133ma. = ~ 1.6 Watts. If it produces smoke then the resister can no longer dissipate enough heat and starts to burn the insulator material. My unit doesn't have exposed wires and has a much thicker insulation layer.

With the sending unit in place with a near empty tank the sending unit will be near 0 ohms.

spdrcr29

Ok, so just tried this and I got 136.7ma which translates to about 1.64 Watts. The resistor did not produce any smoke, but it did seem to get very hot and I could smell it. I then got out my infrared temp gun and measured the temp several times across the grid of the resistor which ranged from about 177 to 202 degrees F. Not sure if this is a normal temp reading or not, but it kind of worries me since the restoration on my Corvette was due to a fire. I don't want to start another one!

One of my co-workers suggested that the original smoke that came from the resistor could have possibly been the result of several years of not being used, combined with possible oily/greasy finger prints from me handling it?? Sort of like when you turn your household heater on for the first time in the fall. As mentioned earlier, the smoke has subsided and the gauge still seems to be operating as it should.

Based on the readings I took above, any thoughts?

cagotzmann

Ok do the same test connected to the gauge. using the same terminals.

Then run the same test with the gauge grounded also. This should simulate a empty tank. If you look at the gauge is should read empty.

If you get more than the previous ma then there is a short somewhere that should be insulated and current is traveling down the wrong path.

If this test all works then you need to look at the wiring leading to the gauge. (Printed circuit board ) or terminal plug in the center of the gauge console where something is shorting the circuit.

spdrcr29

[QUOTE=cagotzmann;1594189461]Ok do the same test connected to the gauge. using the same terminals.

Then run the same test with the gauge grounded also. This should simulate a empty tank. If you look at the gauge is should read empty.

If you get more than the previous ma then there is a short somewhere that should be insulated and current is traveling down the wrong path.

If this test all works then you need to look at the wiring leading to the gauge. (Printed circuit board ) or terminal plug in the center of the gauge console where something is shorting the circuit.[/QUOTE


Thanks for trying to help here. I may have lost you a bit on exactly what you wanted me to do, but basically did the same test with the amp meter connected to the gauge without the resistor and it read about 139ma. I did have the gauge grounded and it did show empty. I then did the same reading with the resistor reinstalled on the gauge and got about 274ma. Still no smoke from the resistor during either test, but it did get very hot.

Also as a reminder, when I originally started bench testing the gauge/resistor it did smoke for a while on the bench. So this leads me to believe it is not due to something in the car and more so with the gauge? As for the car, all new Lectric Limited Harnesses (front, back, dash, etc.). I've also checked the sending unit wire every which way to Sunday and find no issues. Same with the ground feed that comes from the temp gauge and grounds the gauge cluster housing.

Thoughts?

0Willcox Corvette

I sent you a message today... are you sure the resistor is clean and making good contact with the terminals?

Willcox

spdrcr29

Thanks for the message Ernie! The contact with the terminals seem to be okay. Something interesting I came up with last night though... Realized that I had a box of old gauges that I got several years ago. Anyway, dug those out and pulled the resistors off each one of them (temp and fuel) and hooked them up per direction of Cagotzmann. Sure enough...more Smoke! As a couple of minutes of testing went by on each of these resistors, the smoke eventually subsided just like the resistors for my 71 and 72 did.

So, can it be quite possible that the smoke that initially came off each of these resistors was a result of several years of non use where they simply collected some dust, a little fine debris, or possibly greasy/oily finger prints? Kind of like the smell you get when you turn your furnace on for the first time in the fall. Given that I now have 4 resistors that have done the same exact thing, leads me to believe this may be the case??

cagotzmann

Anyone have a picture of the resistors from a newer model. I remember my 78 resistor is made from different materials and may not suffer similar symptoms. This may be a option if they are available.

I would leave it connected outside of the car for 30 minutes to see the final status. If it is going to fail have it fail outside of the car.

But if they all do the same thing I would move to the next step to check the reading is reasonable based on the fuel sending position. Full vs 1/2 full vs empty.

7T1vette

Sounds like your gauge and resistor are acting as intended. Very possible that resistor wire had some petroleum-based contaminants on it. That wire does get hot; that's why it is wrapped around an insulator.

Just don't run any electrical wires up against that resistor...or it could catch plastic wiring insulation on fire.