Note that ALL the gauges except the ammeter work on this same principle.

 

Ross, I don't have a copy of the manual but can you tell me about the configuration of the water temp sensor? My thought is that the sensor for the water temp is not as complex as the fuel sender but if the gauge works on the same principle then there may be a solution there.

 

On the flat V8's, there's a wrinkle, but otherwise identical. Since the V8's have essentially two separate cooling systems on a common radiator, one bank has a temperature switch ("excessive heat unit") that sends the gauge to Full Hot if it overheats. The other bank has a heated bimetallic element that drives the gauge. Oil pressure thrown in for good measure.

 

The gauge doesn't make/break, the sender does, so the gauge will work with variable resistance type senders. You're just controlling the current (heat) through the gauge.

And a 1940 passenger car BATT gauge (not sure about the trucks - did they have a BATT gauge or an ammeter?) - that's an odd one year only deal. It was the same gauge, but fed current directly through a 60-ohm resistor. This one gets converted to 12V by adding another 60-ohm resistor in series. Working a treat on my '40 coupe (although I actually used a small precision wire-wound potentiometer and calibrated the gauge).

There likely is a solution - variable resistance will work, the trick is calibrating it all. Same with oil press.

 

What concerns me is the make/break in the circuit. The effect of this action is that there becomes a duty cycle at the gauge. The manual states that the bi-metal/heater in the gauge does not cool quickly so the needle on the gauge remains steady. If there is a continuous resistance presented to the gauge would it damage the bi-metal/heater? As an example...in LED circuits the activating voltage is pulsed to extend the life of the device. Our eyes cannot differentiate between to on/off cycles so we think that the LED is continuously lit.

 

It's a tough nut to crack, Charlie. Ford used this kind of gauge well into the 60's, when they went to 12v in '57 (*) they added a CVR (constant voltage regulator) to cut the voltage down to 6v so they could continue using the same gauges (cheap). And how do CVR's work? They're a thermal cycling regulator. So you had the CVR cycling, the senders cycling... on top of the generator cutting in and out, with a step up/down in system voltage of a volt or two at random intervals.

The key as you say is, the needle is slow to respond. I expect Ford didn't want people fretting over gauges that jumped around. They weren't concerned about them being real accurate, except for reading Empty when there was no gas.

Before I'd put much effort into this particular situation, I'd run a dedicated under-dash gauge that matched the Mustang's sender, or adapt a later dash gauge into the stock cluster (a la Julie and others).

(*) I believe 56 gauges were true 12v gauges?

 

CVRs...we used to call them "DC transformers". I would think that 55 and 56 would have 12V gauges since they have 12V systems. I have some in a box someplace maybe I'll drag one out and try some experiments with various resistances and voltages to see how they react.

 

I believe 56 was an orphan year with true 12v gauges, no CVR. Just from what I've seen on here. I assume they were still King-Seeley type.

 

Keep in mind that the duty cycle is based on a full 6V on/off. What you are doing is heating the bimetallic strip with "doses" of (relatively) high current. The variable resistor, when sized correctly, will regulate the overall current to be quite low, but ample heat to move the needle.

My '40 gauges have been converted for several years. My '36 pickup was converted as well and is still working (as far as I know).



Thermal cycling is code for chopper. It's an on/off full 12V to bring the pulsed DC down to 6V average.


The Ford CVRs weren't really CVRs, they were "choppers". They pulsed the 12V on/off at a rate so the average voltage was 6V, but if you looked at it on an oscilloscope, it would be very dirty. The "dirty" was filtered out by the gauge.

Providing a constant true 6V to the gauge with an electronic CVR like the L7806, and using a 'modern' style variable resistance sender to control the heating of the gauge bimetallic strip works just fine. Thousands, if not tens of thousands of KS gauges have been converted this way. I've not heard of a single failure. I wouldn't overthink it too much.

 

FWIW, I am using an electronic regular for my gauges, and it's adjustable. The gauges are not particularly sensitive to supply voltage. They read the same at anywhere from 6.3 to 7.2 volts. I think that is one of the "advantages" of the K-S type gauges, in the context of crappy old generating systems in the '30's and '40's.

 

I think this would work. it's from a 65 mustang. Just use your original face plate. and JB weld the gauge to the cluster. edit* I cant get the picture to load..

 

I'm not sure you guys haven't confused the OP by now on this one! I found this thread really interesting because I worked as a line mechanic at a Ford dealership in the 60's and did a lot of the electrical troubleshooting. My "cure" for using the later sending unit with the early gauge on 12v, would be to put the "constant voltage regulator" unit in the wire feeding the dash units. I believe the fuel gauge would have read "about right" if the system hadn't been converted to 12v. If the dash units for the oil pressure and temperature haven't already been damaged, I believe this should make them work "about right" too. The ammeter is the inductive type and isn't affected. I still have an old fuel tank unit with just a short piece of the float lever still intact that I use for testing Ford gauges. I disconnect the sending unit wire and place my "tester" in series between the disconnected wire and a good ground. With the ignition turned on, moving the lever on the tester will give low, mid, or high readings on the gauge as you move it (the lever) to those (3) positions. This will help determine if your problem is in the dash unit or sending unit. On the Ford 12v systems, If you have oil, temp, and fuel gauges all acting up at the same time, the problem is most always the constant voltage regulator, and that can be determined with a volt meter, or by using a test light to see if the output is pulsing as it should. - Bob

 

I really hate the fact Ford uses the term "constant voltage regulator" for their pulsing/chopper unit. There's nothing constant about it and it's misleading. Depending on the gague style, it can cause pulsing of gauges too (generally not a problem with Ford gauges).

A true CVR does what the name implies; it provides constant voltage via a regulator. The only true CVRs are the electronic ones like the L7806 and other similar ones.

The price for a Ford-style 'cvr' seems to be around $30 new. Of course, if you know what you're looking for, you can find them in just about any junkyard for nothing.

The price for an L7806 is about $0.50 brand new...I generally buy about a dozen at a time, use a few, then store the others where I can't find them the next time I need them, and buy more!

 

Well it's constant enough with respect to the gauges, since they are so sluggish. And, these were made back in the age of stone knives and bearskins, man!

The reproduction senders have quite a bit of adjustment by means of the set screw attaching the float arm and that may be enough to get the fuel gauge calibrated right there.


Sent from a pay phone

 

Fair enough, but the Ford-style CVR, as sold by NAPA and others, are about $30, whereas the L7806 are about $0.50.

The other way the Ford-style choppers are inaccurate is in that they chop incoming voltage. So at idle, below alternator/generator cut-in RPM, you'll get battery voltage chopped down to approximately 6V. However, once the RPM is above cut-in voltage, you get whatever that charging voltage is chopped down to a similar percentage, so roughly 7V or so.

This is a small change, but can be disconcerting if you're running down the highway with your gas gauge needle on E and when you pull off to get gas and are waiting at the light with the engine idling, your needle is now below E.

This is where true electronic CVR have a big advantage - they take any input voltage from about 9-25V and spit out a steady 6V. Your gauges read accurately and stable regardless of RPM and system voltage.

So, if you use a chopper, calibrate your gauge with a batter charger hooked up to replicate the truck's charging system above cut-in voltage. It'll give you peace of mind that it's accurate then, so when your needle dips below E, you'll know it's voltage related and not fuel level related.


And be careful with those vacuum tubes - somebody could get hurt!!