12 Volt 40 Coupe

[47Merc]

I am converting my 40 to 12 volt and have read that I need a runtz reducer on each of the oil, fuel and temp gauges but cannot find any information on what I need to do for the battery gauge.
Can anyone advise what I should do.
Thanks

[39portlander]

Leave your amp meter as is, hook up a volt meter. I tucked mine to the right of my steering column with my oil psi gauge.

I would go on the Ron Francis wiring website, www.RonFrancis.com a wealth of knowledge for 6 & 12v systems. The Bare Bonz fuse panel package fit my needs.

[Old Ford Addict]

I would think you only need one drop to supply the interments that require staying 6v

[TJ]

You only need to use a CVR for all the gauges except the amp gauge. CVR's were used by Ford for years. You can get them at your local auto parts store or from anyone that sells Mustang parts.

[AnthonyG]

When measuring Amps as the stock gauge does, it works fine as long as you feed the wire through the gauge the right direction. If it is fed through the wrong way it will register in reverse and go minus when charging. The original works as it's amps it's measuring and the doesn't matter whether it's 6V or 12V. The only issue on mine ( I have a 100 Amp alternator ) is the gauge only goes to 30 Amps so when the engine rpm is high it pegs and I know it's charging at much high Amps. At normal driving speeds 30 mph to 55 mph it registers correct.
There are kits on the Internet to convert 6V systems to 12V which will include all the voltage reducers as well as all the light bulbs which need replacing. Also Amazon has a number of higher capacity reducers that you can spice into the harness to reduce back to 6V for all the gauges and lights. I went with the kit that puts a resister at each gauge that needs it and all new kight bulbs. You didn't ask but I found leaving the 6V starter also works fine ( researching the threads seems most agree ). Spins faster and starts my Flatty great.

[Flat Ernie]

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The '40 does not have an AMP gauge, it has a battery gauge. It is unique from the rest of the early Fords in this regard.

I do not recommend using Runtz resistors and I don't like the OEM Ford choppers used from '56 on up (while Ford may call them CVRs, they are not - they are choppers). I prefer true electronic CVRs for your gauges. I've done my '40 and have helped others do their '40s (and other early Fords). Google L7806 - it's a tiny, solid-state CVR (constant voltage regulator) that takes pretty much any input from 6V-18V and outputs a steady 6V. You can probably run all the gauges with one, but they're so cheap (<$0.25 ea), that I run an individual one to each gauge (except battery).

I prefer the CVR because they always put out 6V regardless of input. With dropping resistors or choppers, the input varies a little as the output of the alternator/generator varies. This means your gauges vary. This is not abnormal, but if the purpose of the gauge is to be accurate, I like it to be as accurate as it can be. This is why a lot of the early gauges, particularly the gas & oil pressure, read much lower at idle than above generator/alternator kick-in. It's tricky to tell with the oil pressure gauge as it will increase with RPM anyway, but the drop of voltage once the generator/alternator drops offline exacerbates those low idle readings. It's easier to see on the gas gauge. With a CVR, this is totally eliminated.

Regardless of how you decide to drop the voltage for the rest of your gauges (all will work, I just think the CVR is far superior & cheaper), for the battery gauge, you need to find a precision 60 ohm resistor and wire it in series with your 12V input to the BAT gauge. I like to use a 1/2 watt wire-wound potentiometer to dial in the 'green zone', but if you get an accurate 60-ohm resistor, your gauge will be pretty darn close. If you do use a potentiometer, check the voltage on your battery and if it's exactly 12.0 volts, adjust the potentiometer so the needle sits at the very bottom of the green zone to a needle-width below the line. If you have a battery charger - measure that voltage and assuming it's about 14.0V, adjust that to the top of the green zone. Remember to give the gauge about 10 seconds or so to stabilize as the bi-metallic strip heats up.

Here's a listing for 5 on eBay. eBay prices are higher than if you go to an electronics outlet like Mouser, but more convenient for most folks:
http://www.ebay.com/itm/5-x-L7806-Vo...item338e72045b

[Juergen]

Flat Ernie, Can you explain how the 60 ohm resistor cuts the voltage in half as there is no 60 ohm impedance in the gauge? I would think that two 60 ohm resistors in series with one end going to the battery and the other to ground with the battery gauge tied to the center tap would give you a half voltage reading and only draw 0.1 amps continuous.

[19Fordy]

With the help of Flat Ernie , several years ago, I reduced the voltage input from 12V to 6 V to my 1940 Ford BATT gauge so it would read accurately . It is a volt meter.
To do this you must isolate the BATT gauge from the other gauges so that is supplied separately with 6V using the 60 ohm resistor. It still works fine and dandy. Here's how. Be sure and read ALL of threads 6 and 7.
https://www.fordbarn.com/forum/showth...ord+batt+gauge
Here's where I bought the 60 ohm resistor. Buy several as they are cheap.
http://www.westfloridacomponents.com...vc?Screen=SRCH

[Flat Ernie]

Originally Posted by Juergen

Flat Ernie, Can you explain how the 60 ohm resistor cuts the voltage in half as there is no 60 ohm impedance in the gauge? I would think that two 60 ohm resistors in series with one end going to the battery and the other to ground with the battery gauge tied to the center tap would give you a half voltage reading and only draw 0.1 amps continuous.

The Ford service bulletin lists the resistance of the gauge itself as 60 ohms. Doubling the resistance when doubling the voltage means current remains the same - these gauges all measure current.

[gearhead1952]

Pretty sure the Runtz is a 7806 all packaged up and ready to go. It is not a resistor. Just bought some and have used them before.

[Juergen]

Originally Posted by Flat Ernie

The Ford service bulletin lists the resistance of the gauge itself as 60 ohms. Doubling the resistance when doubling the voltage means current remains the same - these gauges all measure current.

Thanks for the explanation. I mistakenly thought the early Ford voltmeter was a high impedance device like modern car and aftermarket voltmeters.

[47Merc]

Thanks for the advise guys. Much appreciated

[Flat Ernie]

Originally Posted by gearhead1952

Pretty sure the Runtz is a 7806 all packaged up and ready to go. It is not a resistor. Just bought some and have used them before.

There are some that are now, apparently, but there are Runtz resistors as well. And they're $15-20 for a $0.25 part and some soldering...a bit over the top.