Make a tool to test your gauges - easy

[SA69mach 39]

I have refurbished the instrument cluster on my 70 coupe and my 69 Mach 1

Both cars had weak and incorrect fuel gauges, which made me leery of the accuracy of the other gauges.

 

I knew very little about electrics. I bought a decent multimeter and started searching here and VMF on how to diagnose and fix problems.

VMF is mixed year cars, and the 69 and 70 are a bit different to the earlier cars.

 

I installed new circuit boards (flexible plastic reproductions) and solid state IVR/CVR (constant voltage regulator). The CVR turns 12 volts into 5 volts to power the gauges and sending units.

 

The gauges consist of three parts, in simple terms.

Sending unit - installed in the engine or the fuel tank

Wiring - clip connecter to the sending unit, then to the wiring plug that fits in the slot of 69.70 instrument clusters

Gauge.

 

The fuel, oil and temp gauges are pretty much identical, and operate on the same resistance values for most all Ford cars of that era. 10 ohms to 78 ohms

10 ohms is Full, Hot, or High

78 ohms is Empty, Cold, or Low

 

33 ohms is Half.

 

All the gauges have two terminals on the back. One is connected to the 5V power via the circuit board, and travels through the gauge and out the other terminal, and there to the sending unit. The sending units are simply a variable resistor, nothing more than that.....the range of their resistance is, as expected, 10 ohms to 78 ohms.....

As the engine warms, it warms the temp sender body, and the bi-metal resistor bends and reduces the resistance, according to the heat it receives.

Gas sender is a rotary type - as the fuel float raises and lowers, it alters the resistance.

Oil pressure I don't know exactly how it works.

 

All of the sending units are grounded by the engine or the chassis. They must be to make the circuit from the gauge.

 

I read, and tried the grounding test, to see if the gauge will peg out "high".

That shows the wiring is good and the gauge is not broken.

 

What I found frustrating is you cant tell if the gauge is accurate through the range by grounding it out, or with a multimeter.

You need a way to apply varying and exact resistance values to check each gauge.

 

I went to a local electronics store and got a 100 ohm potentiometer, two plug terminals, and a plastic 3" x 2" box, and made my own variable resistance test unit. Less than 10 dollars in parts, and 45 minutes later I had soldered the wires from the pot to the plugs, and fixed them all in the box. I used my ohm meter to mark on the box where 78 ohms, 33 ohms and 10 ohms were.

It is a rotary dial potentiometer, so you can mark as many levels as you wish on the body of the box.

 

I tested the temp, oil and fuel gauges in about 15 minutes, and verified the oil and temp are good, the fuel gauge reads slightly low as 10 ohms. Reads correct at 7 ohms (full) So that is a small fault in the gauge.

Since my fuel gauge shows just over half full when it is topped up, I now know for sure that the fault is in the sender. I can live with a slight inaccuracy in the full level of the fuel gauge.

 

Well worth ten bucks to me, and something I wanted to avoid wasting time with gauges. They can be very frustrating to fault find, at least to an amateur...and expensive at an auto electricians shop.

 

If there is any interest, I can take some pics of my box, but really, any electronics store will set you straight on parts and assembly. Simple and useful tool for my Mustangs and other Fords.

[manley 13]

Sounds like a great project. I would love to see your pics.

[1969_Mach1 333]

When the gauge is inaccurate, how can it be calibrated?

[SA69mach 39]

Sounds like a great project. I would love to see your pics.

 

OK. I will be getting the sender out of the fuel tank next, so, after that I will take some pics of the box and the parts. Probably tomorrow

[SA69mach 39]

When the gauge is inaccurate, how can it be calibrated?

 

Good question. I am not sure how they are calibrated or manufactured, or even if they can be adjusted.

 

I will see what I can find out. Might dismantle one in the near future. I have a repo for the 69 if I ant get it accurate enough. I am hoping a new sender will be the main fix. If not I will try the new gauge and pull the other apart.

[miketyler 15]

Good stuff. The gauges have very delicate adjustments that will influence light "spring pressure" and deflect the needle in either direction. My guess would be that an adjustment made in one direction might have an affect on the full sweep point in the opposite direction. I think I have pics of these somewhere when I restored mine.

[Midlife 814]

Here's a tip: the fuel/oil/temperature gauges can be adjusted together with the CVR. On the back of the mechanical CVR is a small potentiometer (pot) that if you break off the epoxy, you can access a small post. Rotate the post to adjust your three gauges.

 

I adjust mine so that when my fuel tank is 1/4 full, the gauge reads 1/4 full. Beyond that, the gauges are really only useful for changes relative to their normal conditions.

[SA69mach 39]

Here are the pics of what I made yesterday. A bit cheesy, but it works, and it does something my multi-meter cannot - it allows me to apply resistance into a line, at the exact value I want to test.

 

[ashleesmach1 28]

Outstanding!

[ashleesmach1 28]

Can you show how u use it on the gauge?

[Guillaume69 150]

Good practical tool right there. Thanks for sharing!

 

I replaced my fuel sending unit while I restored my car. And now, it does just like yours: shows just above 1/2 when the tank is full. I really hate these faulty new parts. I am thinking about putting the old one back in as it worked better. Same with the new Temp sending unit. Never worked. Had to put the old one back in. The Amp gauge, new as well, is as useless as its predecessor. Even when my mechanical VR got stuck, allowing 17v to the battery, that gauge didn't give any indication that the battery was getting fried. I'd rather have a volt gauge showing a range around 10-16v, with the "green" sector being 12-14v. That would be a lot more useful, IMO.

 

There seem to be a bunch of crappy electronic repos out there on the market. I just find it, well, a little pathetic. Some people are trying to make too much profit and that has become detrimental to quality. I am glad Neil Armstrong and his crew could rely on better electronics for their 69 ride to the Moon than we do now for our 69 cars.

 

G

Edited December 12, 2014 by Guillaume69

[SA69mach 39]

easy - whatever sending unit you are testing (and gauge), just unplug the wire to the sending unit, connect with short line and alligator clips to one terminal of the test unit, and alligator clip and line to ground. I go direct to negative battery terminal with the temp and oil pressure) The unit is now taking over from the sender.

 

Turn on the ignition and your selected gauge should respond with the level you have input on the test unit. Ford is 10 ohms full, and 78 ohms empty. 33 ohms is half.

 

Really simple. At the gas tank just ground to the chassis (find a clean bolt) and line in from the wire to the sending unit.

 

You are bypassing or replacing the sending unit and artificially setting a resistance value of your choice. The gauge should check out as you have selected.

 

On closer checking of my fuel gauge today I found it was not responding correctly, so I rigged the test unit to check the new fuel/temp gauge. I found this gauge to be accurate, so I will be installing it tomorrow. ( I rigged a 12 volt feed to a spare CVR I had, checked I had 5 volts out, then connected 5 volts in to the gauge terminal, and wire to test unit - then to ground. I could test the gauge on the bench, just like it was in the car.

CAUTION - bench testing gauges needs 5 volts, not direct 12 volts. You will need a DC power supply or a spare CVR .

 

I have found a use for this unit twice already. Unfortunately the markings I made for the values on my plastic box are not accurate enough, I have to check the resistance carefully with the ohm-meter each time I set the value. Simply stick the electrodes of the ohm meter into the terminals of the test unit and turn the rotary dial until it is exactly the value you want. then re-connect the test lead to the sending wire and earth wire. ....

 

I have found the gauges and electrics generally to be challenging on the Mustang. I know they are simple, but you must still have diagnostic tools. I think this is a handy tool for the amateur - like me.

 

example - my fuel gauge will peg out over full when grounded, but as I have said before - that does not mean the gauge is good. And it looks certain that it is actually BAD, not GOOD like the peg test indicated. This tool allows you to really check exactly the workings and accuracy of the gauge.

[SA69mach 39]

Good practical tool right there. Thanks for sharing!

 

I replaced my fuel sending unit while I restored my car. And now, it does just like yours: shows just above 1/2 when the tank is full. I really hate these faulty new parts. I am thinking about putting the old one back in as it worked better. Same with the new Temp sending unit. Never worked. Had to put the old one back in. The Amp gauge, new as well, is as useless as its predecessor. Even when my mechanical VR got stuck, allowing 17v to the battery, that gauge didn't give any indication that the battery was getting fried. I'd rather have a volt gauge showing a range around 10-16v, with the "green" sector being 12-14v. That would be a lot more useful, IMO.

 

There seem to be a bunch of crappy electronic repos out there on the market. I just find it, well, a little pathetic. Some people are trying to make too much profit and that has become detrimental to quality. I am glad Neil Armstrong and his crew could rely on better electronics for their 69 ride to the Moon than we do now for our 69 cars.

 

G

 

Agree 100%. In light of the many reports of faulty reproduction electrical items I really wanted to understand and be able to test the items. Fuel sending units in particular. My 70 confounded me as nothing seemed to work how it should, and fixes did not really make a fix!

Multimeter will test many things. It helps to understand how the gauges and wiring and senders actually work. Once I understood that, it made it a lot easier to start testing things. Before that I was just replacing things that might be bad, and hope for the best.

 

I managed to get a few spares - circuit boards, CVR's. sending units. Now I test them and have a better understanding of how things work, and how to fix them.

 

I am also disappointed at the poor quality of some items. How disheartening to make the effort to change out a fuel sending unit, - and then it does not work.....

 

I will be testing anything new from now on. Before installation.

[Guillaume69 150]

I will be testing anything new from now on. Before installation.

 

Exactly. And we should all probably do this. ;-)

[MAD IN NC 21]

 

I am also disappointed at the poor quality of some items. How disheartening to make the effort to change out a fuel sending unit, - and then it does not work.....

 

I will be testing anything new from now on. Before installation.

 

 

I'm with ya..... took a gas bath recently as the replacement sending unit did not seat in old tank. Seems lately you buy 2 parts to get one working one

[SA69mach 39]

Took the gauge cluster out. The fuel gauge was testing bad occasionally, and I had a new set fuel&temp, which tested correct, so put that together. With a bit of care, I tested all the gauges in fully assembled form, on my tool bench. All tested good and in range.

Installed the instrument pod, speedo cable and main plug. Danged fuel gauge showed just over half with a full tank of gas............not happy at that point.

 

Siphoned all the gas out- pulled the sender out. An ancient and original part for this car. The sender was so out of range I wonder how it even registered.

Normal range is 10 - 78 ohms. 78 is empty, and 10 ohms is full

This one showed 211 ohms en 'empty' position, and 55 ohms "full'

 

So, mystery solved for me. I installed the repro sender, filled the tank with gas and it is reading perfectly - along with all the other gauges and lights.

 

No leaks so far. I used some marine grease to hold the rubber (?) seal in place when installing the sender. The tabs on the sender that go into two small cutouts in the tank were not aligned properly and frustrated the effort to seat the unit. After some careful adjustment of the tabs it went on. I figure it will leak immediately or not at all with a full tank of gas.

 

In comparing the original sender to the reproduction, I have to admit the repro is better in a number of areas, than the original. (Pivot and 'stops' for arm travel)

Worse in comparison to the original tabs.

 

Thanks for reading and I hope to be able to help others with 69 and 70 gauge issues in future. I think I am 90% up to speed now.

[miketyler 15]

Was going thru pics on my PC and thought I would add this illustration for reference. I tool these during a recent restoration. Here you can see the adjustment levers that can be carefully moved with a slot screwdriver. A little movement of these can ease or add spring tension at either end of the gauge reading. I would assume making any changes here might affect behavior over the full range. If you find one gauge that is off more than the other two you can go here but as always, take care not to damage or bend any of the very delicate parts inside. This pic is the inside of the temp gauge.

[SA69mach 39]

thanks miketyler. Very interesting pics and not what I expected to see as far as the working parts and the arrangement.

 

I can see how the gauges could be adjusted for full and empty on a bench, with 5 volts powering the cluster. By applying the exact resistance, the 'empty' and 'full' positions could be adjusted.

 

I will remember that. Thanks again.

[rangerdoc 22]

I made a tester thanks to the instructions above. I still need to get a permanent knob and put on proper markings. Verified all the gauges work and made some minor adjustments to them to get them more accurate. I'm also ditching my repop fuel sending unit after finding a reconditioned OEM one.