VCCA Home Forums TECHNICAL - GENERAL Electrical - 12 volt Gauge conversion

Does anyone have personal experience with a gauge conversion from six volts to 12 volts? I'm aware of resisters being available but my question is about converting the gauge itself to 12 volts. There are tons of companies who advertise but I'm looking for someone who's actually had experience with having it done and can provide first hand feedback. Thanks.

I have not given this conversion much thought.

My thinking is that the gauge must be modified to either decrease the internal resistance to 25% of the value it has as a 6 volt gauge or be capable of handling 4 times the power it did as a 6 volt gauge.

For example, consider a 6 volt fuel level gauge that has a fixed resistance of 30 ohms. (Note this value matches the value of the sending unit when it is full.) The current passing through the gauge is 0.2 amps. It consumes 1.2 watts.

If you pass 12 volts through that gauge, the current passing through it is 0.4 amps and the power is 4.8 watts.

Knowing that most gauges use variable resistance to create the reading, I am not sure how the conversion process could maintain the reading output while reducing the internal resistance.

The power is essentially dissipated as heat. So somehow the gauge needs to handle all that heat.

Of course any bulbs need to be replaced with comparable 12 volt bulbs.

The gauge is a balanced coil design, and is insensitive to voltage within reason. At 12 volts the gauge will probably work or mostly work but draw too much current (until it burns out). The sending unit is 0-30 ohms, and that was the same after the change to 12 volts until about 1965.

I have never heard of someone actually rewinding the gauge but I'll bet it's possible. Having seen inside those gauges, I'll bet it is not easy. Also, a lot of older GM gauges are about the same physical shape and size so it might be possible to rivet the scale from on older 6 volt gauge onto a post-1954 one, or maybe a post-1952 one from a V8 car from some other GM division. I would pay careful attention to whether the needle swings the right way. I do not know if they are all the same.

I think a voltage regulator device would do better than a resistor because the current changes as the float goes up and down, and the voltage will not be stable. Some use a Runtz, which I imagine you have heard of. Reviews of those have been mixed, but I would expect better results than with a resistor. For what it's worth Ben Bruce on the AACA forums has been using one on a 50 Buick for years now and he says it works fine.

I dont think fuel level guages will have a problem with 12v as they work on a balanced resistance system which will remain the same even with 240volt.
Tony

I put a Runtz on it and it's DOA. Doing some research on the Runtz (of course after I bought and installed one) I discovered they have a high failure rate out of the box. The info above about how the gauge actually works is what I'm needing. It seems any conversion of the actual gauge would be a challenge. I did, some years ago, own a 55.1 panel that I converted to 12v. It never occurred to me that I might need to protect the gas gauge and I put over 100K on the truck after the conversion with the gas gauge working fine the whole time. Since I now have a much harder time crawling under the dash than I did in 1970 and having read tales of woe from folks who say they've destroyed their gas gauge on 12v, I'm being a bit more cautious about possibly killing mine. I think what I'll do is put a buck converter on the gas gauge, similar to the one I put on the heater motor. They are inexpensive and easy to install. The one on the heater seems to be working great. The one I put on the heater is good up to 10 amps output and the one I ordered for the gas gauge is 2.5 amps which should be more than adequate. Buck converter

Hi Tiny,

I like your plan to use the converter rather than a resistor.

Also, I am glad to hear that things seem to be working with the blower motor. I did not realize that you were converting the ”˜53 to 12 volts. I assumed that you were just installing a 6 volt alternator like in your ”˜38.

Howdy Rusty, I did have a 6v alternator on the car. I had to buy and have wired in a step up to run the GPS and another to run the new radio that looks like the old radio. Having to continually jump through hoops and having to special order simple things like bulbs just kind of pushed me over the edge to just doing the conversion. Except for the gas gauge I got the car "finished" this morning and took it for a shake down. I forgot the starter solenoid is voltage sensitive so the six volt solenoid isn't happy with me right now. If there's a nice part to having to replace the solenoid, it's a stock item at the parts house instead of having to find somewhere to order one and pay three times the price. I'll pick it up my next trip to town. Everything will be easily convertible back if the next owner (read my son) decides to return to 6v. The 38 will remain 6v.

Hi Tiny,

Thanks for the additional information. I understand your motivation for the change. I keep debating that mainly to get better headlights for the '37. It had been converted to sealed beams before I got it. As 6 volt headlamps they are definitely better than the old reflector set-up.. Except the stock generator cannot keep up with the demand even on low beam. At night the battery is being slightly discharged even at road speeds.

Going to later model 6 volt generator or alternator would help. Even with that upgrade I am concerned that the high beams will pull more amps than the wiring can handle.

I assume you did put a step down resistor in the wire to the coil and points. Even if you installed a12 volt coil the points will last longer at the lower voltage. Think of it this way. The ignition system worked fine at 6 volts.

I put a 12v coil on it and the points are the same whether 6 or 12 are they not? Any time I bought points I'd just tell them six cylinder Chevy points.

Yes, the points are the same. The key is that even in 12 volt cars there was almost always a step down resistor to run the points at about 6 or 7 volts. They lasted a lot longer.

Typically the starter switch would send 12 volts to the coil and points in the start position. In the run position the ignition was through the resistor.

Rusty is correct a lot of 12v cars do feed battery volts (often at cranking is reduced to 9v) to the coil while cranking but then through a reducer in running position, though the in between systems are battery direct connection at all times. A lot of coils are marked "12v use with resistor" for later models.
Tony

I learned something today but the lesson isn't done yet. If I put a step down resistor between the switch and the coil would that not necessitate using a 6v coil instead of 12v? Would it not have an input of 6v rather than 12v? If 12v coils are running at 6v why have a 12v coil? Inquiring minds want to know.

Edit: Secondary question. How would I wire that resistor in? I've been looking at diagrams & they show two wires from the ignition switch to the coil. I only have one. Is it as simple as running another wire from my ignition switch "on" terminal to the coil with the resistor in the circuit? My switch is only lock/off/on/start.

Hi Tiny,

Yes, this is a little crazy. I first learned about this on our ”˜72 Chevy van. Until then I was not aware of the drop-down in voltage to prolong point and coil life.

The 12 volt or 6 volt coil thing can be confusing. The coil design is essentially the same. The winding’s ratio is the major difference. The concern is the same as with all electrical devices. Will it withstand the heat?

The resistance of the coil is a key factor. Higher resistance means more heat. Typically what we call a 12 volt coil has more primary windings (more wire) and more resistance. More windings give a hotter secondary voltage at any input voltage compared to a 6 volt coil.

The limitation is that the higher resistance creates more heat, especially at slower engine speeds when the points are closed for a longer period of time (time, not dwell or gap). So the manufacturers install a step down or ballast resistor to reduce the voltage going through the windings. Plus this improves point life. Less voltage at the points creates a smaller arc when the point open and close. Note: Look on the casing of the 12 volt coil you bought. It might have something stamped on it about using a ballast resistor.

You can do lots of Google searches on 6 volt and 12 volt coils especially on these older cars. As always, lots of different experiences and results.

I would be cautious about pushing 12 volts through a 6 volt coil. Things can get pretty hot fairly fast. On the other hand, I think that you could run your original 6 volt coil with a ballast resistor that has a value about equal to what you measure across the primary circuit in that coil. I expect that should be around 1.5 ohms.

With respect to wiring, I would start by trying to keep things simple. Put the resistor in the circuit before the coil. Measure the voltage at the input side of the coil.

If you want to be fancy and run a 12 volt coil, you can add an extra wire that ties into the start position on the ignition switch. Run that directly to the input side of the coil. That will give you 12 volts through the coil for starting. When you let the key go back to the run position the 12 volt current to the coil goes through the resistor.

My friend wanted to convert his 49 3100 pickup to 12 volts which we did. He did install what was sold as a 12 volt coil. We have a ballast resistor in the wire running to the input side of the coil. We do not have a separate circuit to feed 12 volts to the coil when starting. The truck has the floor pedal to engage the starter and a simple on-off key switch. We are not having any problems.

The next time I am with the truck I should measure the actual voltage to the coil and the resistance of the coil itself as well as the ballast resistor.

Sorry for the longer post but hopefully this will help.

Thank you Rusty. School is in session. Since I have the 12v coil installed already I plan to use it. The wiring is currently stock in reference to the routing of the wires. My manual does not show the ignition wiring schematic. On a stock 53 would the existing single wire to the coil be attached to the "start" lug on the switch or the "run" lug? I can't get under the dash to look. IF it's on the "run" lug, I'll add a resistor to that wire at the coil and run another wire to the coil from the "start" lug if I understand you correctly.

Hi Tiny,

I just reviewed the '53 wiring diagram in the Online Manuals info. Not a great picture but usable. Your thinking is correct.

The single coil wire you see is connected to the "run" terminal on the ignition switch. You should put the resistor in that line ahead of the before the coil. It is energized when the switch is in both the "run" and "start" positions.

If you want to feed 12 volts for starting your are also correct about adding a wire from the energizing terminal on the solenoid to the input side of the coil. That wire coming from the ignition switch is only hot in the "start" position.

I suggest that you wait to go to the trouble of running that second wire until after you try things with the resistor in the "run" circuit. As I noted, my friend's '49 3100 starts immediately on the "run" circuit without 12 volts to the coil.

Thanks Rusty. I appreciate it. See you in Canon City in September?

Still thinking about that. It’s a pretty long trip for me.

Roger that. Kind of why we're not going to Louisiana, way too far. I picked up a Standard RU10 resister. I put it on the meter and it shows 2.6 ohms resistance while allowing 12 volts through. Now to find a good place to mount it.

Guys all:
Not to hijack but, please let me; Thank you!
In my opinion this is the MOST useful, helpful thread I recall seeing here in Chat or another website I frequent, in a very very long time!!
And ALL done in a spirit of helping a brother car enthusiast work through a learning journey while helping a bunch of others as well, no doubt.
Cheers!

Done. I tapped a 10-32 thread into an existing hole on the coil bracket and attached the ballast resistor there. I then attached the existing ignition to coil wire to one end of the resistor and a new wire I made up from the other end of the resistor to the positive terminal on the coil. She started right up and ran well in the garage. It's a tad damp to take her out for a road test. That may happen this afternoon. A big THANK YOU to everyone who helped, you too Rusty......

Excellent. Glad we could help!

This morning we were at Cars & Coffee on the levee on the Mississippi River in LeClaire. My friend had his '49 3100 there. He says it is starting great without a separate 12 volt feed to the coil.

Of course it helps that he is still using the 6 volt starter so it turns over pretty quickly.

If you get a chance someday would you measure the voltage going to the coil and points? Another inquiring minds thing.

Both taken with the car on but not running. Voltage to coil 12.6. Voltage at the - terminal on the coil to the distributor 12.6. Is that what you're needing Rusty?

Hi Tiny,

Let me think about that. I was expecting a drop across the resistor.

Let me try a few things on the ”˜49 truck and get back to you. It might be a few days.

I was too initially Rusty but then I did this. ^

If little current is flowing the meter might not show much difference in voltage. Just a thought. Don't know if that is what is happening or not.