Fuel gauge literally burned

If your instrument cluster isn't properly grounded this can happen. There's a voltagr regulator for the instruments thats inside the fuel gauge- it normally limits voltage to the gauges/senders to 5 volts, without the ground full battery voltage is applied which is more than the gauges can handle.

H.

 

Are you sure about that Walt? I can see where it would affect the output of the regulator but that would also affect the temp gauge as it's fed from that regulator as well. It seems to me that you should use that screw to set the regulator to the correct voltage output and then adjust the gauge position by moving the movement mounting arm using a small screwdriver in the the 'L" shaped slot to move the arm up/down & left/right.

Enquiring Minds Want to Know...

H.

 

Yes.



Yes, I Am.








Honestly, with those symptoms there wasn't much else it could be.

It's a crap shoot on re-using the gauge, the winding may still be good but with that much heat the strip may have permanently bent so much you'll never be able to properly adjust it.


FYI for those that don't know, the way these things work is that they rely on something called a "bi-metallic strip" which is simply two different types of metal bonded together- the metals are chosen for how much they expand when heated. This difference in rates will make the strip bend when heated, the hotter it gets the more it bends. They wrap some heater wire around the strip, connect one end to the instrument voltage regulator & the other to the sensor- i.e. temp sender or fuel sender. The senders have varying resistance, allowing more or less current to flow in the circuit, which affects how hot the heater wire gets, which determines how much the bimetallic strip bends, which determines where the needle points. Fiendishly complex in it's operation but simple to stare at & use.

The instrument regulator also relies on a bi-metallic strip & heater wire- in this case the battery power to the wire is routed through a set of breaker points (connected to the strip) through the heater wire to ground. The contacts are normally closed. When power is applied the resistance wire heats up which bends the strip which opens the contact which stops the current flowing through the wire which allows it to cool off which causes the strip to bend back the other way which closes the contacts which allows current to flow which makes the wire heat up again which bends the strip which opens the contacts which stops the current flowing through the wire which allows it to cool off which causes the strip to bend back the other way which closes the contacts which allows current to flow which makes the wire heat up again which bends the strip which opens the contacts which ... well, you get the idea. So what happens is that at the point where the heater wire connects to the contact you alternately get battery voltage or nothing, this actually occurs multiple times per second. The strip is adjusted so that the average of the time it's "on" compared to the time its "off" in effect causes an average voltage of about 5 volts- this point is where they take the power to feed the gauges. If you don't have a good ground connection for the regulator then no current will flow through the heater wire, which means the strip doesn't bend, which means the contacts never open, which means full battery voltage is applied to the gauge wire all the time which causes it to get too hot, pinning the meters & after a few seconds letting the magic smoke out.


BTDT

You can see all this very clearly in the photo in the post above, the regulator & gas gauge are housed in the same can (note- pre '57 the regulator is a separate unit on the back of the speedo cluster).

H.

 

Well if it's a standard instrument cluster then yes the regulator also supplies power to the temp gauge.

Now, I don't for one moment expect that the knock-off guages suppled by the aftermarket are built to the same tolerences that S-W held itself to back in the day, & I rather suspect even factory stock wasn't all that precise to begin with.* That being said these things are at least in theory designed to work correctly under a certain set of circumstances & in my mind the voltage supplied by the regulator needs to be within a certain tolerance of what the gauge system was originally designed for, if you move away from that all bets are off. My recommendation is to not mess with that setting.

For a bit more on the gauges see this from member (?) 1965CJ5

http://home.comcast.net/~sday77/binky/images/gauge_info.gif

H.

* To the point about accuracy- for anyone who's ever been in a small aircraft of 50's-60's-70's vintage the fuel, temp & oil pressure gauges may look startlingly reminiscent. In fact they often used standard automotive parts for this. You would expect that the aviation regulatory agencies would require that these be adjusted to be perfectly accurate- in actuality all the regulations have ever required is that the fuel gauges be accurate in one position only-

"empty"

YMMV but in a plane you better damn well know what it is.

 

Note to all- the above explanation I posted applies *only* to the stock gauges as used in the Stewart Warner & King-Seely speedo clusters. The standalone gauges used in MBs, 2s, early 3's & M38s operate on a completely different principle- see Dr. Vern's explanation here-

http://www.vernco.com/Sparks/id697.htm

Likewise aftermarket gauges usually use a D'Arsonval meter movement.


Neither of these is dependent on an instrument regulator and generally will run off straight battery voltage without going "poof".



H.

 

I do do electricity for a living but not automotively- just passing on what I've learned here (a lot from Walt ) & from a few other sites.

H.