1951 Buick gas gauge conversion to 12 volts

[Outbdnut]

I converted my 1951 Special to 12 volts.   I have not hooked up  the gas gauge yet.  The sending unit is a new one from CARS (oldbuickparts.com) that is correct for the 6 volt system, with a range of 0 to 26 ohms.   What should I use for a resistor, or network of resistors to make the gauge read right with 12 volts? 

Thx,

Dave

[Bloo]

Buick gas gauge sending units prior to the mid 60s are 0-30 ohms. That is for both 6v and 12v systems. 0-26 ohms won't swing it all the way from empty to full at any voltage.

 

The gauge itself is what changes from 6v to 12v. It is designed so that the reading does not change much with voltage. I have heard of people just running it on 12 volts, but the current would be too high (double) and I would be afraid of burning it out. Chevrolet gauges are electrically the same. Some posters on the VCCA forums have used a "Runtz" voltage reducer. It looks like about a 50% success rate over there with half of them saying it worked great and the other half never getting it working right. Good luck.

 

Edited June 16, 2020 by Bloo (see edit history)

[Ben Bruce aka First Born]

I used a runtz. Still ok ten years later

 

  Ben

[Outbdnut]

Thanks for the info.  I found two types of Runtz reducers online.  It appears to be nothing more than a 6 or 7 volt zener diode  to put in series with the gauge to soak up half the voltage.  One I can buy for $19  that has a built-in circuit breaker to protect the gauge in case the  zener diode  or other wiring shorts, and attaches nicely to the gauge with a nut.  I found a Runtz without the circuit breaker online for $3.25, that appears to be just a zener diode with connectors on the leads.  I'm a retired electronic circuit designer and I have a bunch of the proper voltage zener diodes, but none with the power rating to take the 1/4 amp max  that goes through the gauge when the tank is empty, so I think I'll buy the one with the circuit breaker to add a margin of safety rather than spend a lot of time building my own.  I calculate 1.75 Watts dissipated by the diode, so would need a 2 watt or higher zener diode.   If it doesn't give me a full gauge swing,  I may try to design my own circuit....or just live with gauge,  once I know how far off it is.

 

Incidentally, I measured the 0 to 26 ohms on the new sender with a Fluke, lab-quality digital meter before putting it in the tank, so not being a full 30 ohms must mean the manufacturer's quality control of resistance tolerance is poor!

 

Thank you very much!

Dave

Below is what I found:

https://www.ebay.com/itm/Dash-Gauge-Voltage-Reducer-12-6-Volt-Runtz-Diode-to-resistor-a-/301781706995

 

and

https://www.speedwaymotors.com/Speedway-Runtz-12-Volt-to-6-Volt-Voltage-Reducer,2374.html

 

 

Edited June 16, 2020 by Outbdnut (see edit history)

[old-tank]

Search "ford constant voltage regulator".  Buy new or used.  Ford switched to 12V in 56 but supplied 6V gauges throughout the 60's.

The most dependable that I have used.  Case must be grounded, positive to switched wire, negative to gauge.

[Bloo]

But the Ford thing (also used on Mopar) doesn't reduce voltage, it just turns the power on and off for a 50% duty cycle. The Ford gauges are thermal, so do not respond very fast. It is tough to imagine that working out on a GM gauge. It seems like it would just wave around a lot.

[Rusty Heaps]

Runts on both my ‘46 and ‘50 Chevrolet’s, and will install one on my ‘52 Special Riviera 

[old-tank]

1 hour ago, Bloo said:

But the Ford thing (also used on Mopar) doesn't reduce voltage, it just turns the power on and off for a 50% duty cycle. The Ford gauges are thermal, so do not respond very fast. It is tough to imagine that working out on a GM gauge. It seems like it would just wave around a lot.

I must be mistaken.

1 hour ago, Rusty Heaps said:

Runts on both my ‘46 and ‘50 Chevrolet’s, and will install one on my ‘52 Special Riviera 

Wait, I guess not.

[critterpainter]

Be aware that Buick often used the power terminal on the fuel gauge as a junction block and the power wire from the ign switch goes to the fuel gauge.   If you don't disconnect this wire from the fuel gauge BEFORE installing the 12V battery you will kill the fuel gauge even if the sending unit is not connected.      

[d2_willys]

Converting gauges to 12 volts is relatively simple. Runtz regulators are a simple way of doing the job, however the diodes they use will probably get warm. Especially when the gauge sender is at zero ohms (full on gauge). You can use resistors to drop the voltage, but they will also need to be 5 watt resistors or higher. And the gauge will not be very accurate.

The Ford constant voltage regulator does, in fact, send out a 50% duty cycle, which averages 6 volts from the 12 volt input, but with GM balancing coil type gauges the Ford regulator will cause the needle to oscillate. You could put a capacitor across the output of the regulator to ground to filter the oscillations out.

 

My preference is using a voltage regulator such as a 7806 or a LM340-6. These are very simple to use and are available from Digikey.. To install, connect the +12 volt ignition to the leftmost pin, the middle pin or tab is grounded, and the rightmost pin is the output to the power pin of the gas gauge. Install 1uf non polarized capacitor from ground to input pin, and one (same value) capacitor from ground to output pin. You won't be disappointed!