Bench Testing Fuel/Gas Gauge

You just got some gauges at a swap-meet but you don’t know if they work? What do you do next? Test them!

The fuel gauge in most pre-digital cars work by pointing a needle within a sweep range on a gauge based on variable resistance through  variable resister in the fuel tank.  This resistance measured in ohms (Ω) varies for each car and manufacturer. The basic principle applies, as the resistance decreases, the gauge moves to EMPTY and as resistance increases the gauge moves toward FULL.  To test, you will need a variable resister (potentiometer), testing wire (with alligator clips preferred), a 12V source and a volt meter that can measure ohms (resistance).

The following instructions are specifically for a 1970-1981 Firebird and Trans Am fuel gauge. The same steps may be followed for your gauges, just keep in mind that the ohm range between FULL and EMPTY may be different. You will also need to identify which leads in your gauge are positive (power source), negative ground and lead to fuel tank sender unit.

Tools Needed

  • 12 Volt power source – I used two 6 Volt lantern batteries (Amazon: Lantern Battery, 6 Volt 2 needed), they are light, portable and are generally safer to use for this type of testing. You will need to link them together in series in order to get 12 volts. You do this by connecting the + positive terminal of one battery to the – negative ground of the other.
  • Various test lead alligator clips – To make testing connections (Amazon: SE Clip Test Lead Set (10 Piece))
  • Variable resister (potentiometer) – To test different ohm resistances (Amazon: 100 Ohm Potentiometer) A 500 ohm potentiometer may be necessary depending on the ohm range of your gauge. The Fuel gauge in our Trans Am reads full at 90 ohm, so a 100 ohm potentiometer is ideal for this gauge.
  • Volt Meter -that can test resistance – To test ohm resistance in potentiometer (Amazon: Digital Multimeter)

Test for Empty Tank (No Resistance, 0 Ohms)

To test for zero resistance, connect the ground lead of the gauge to the negative ground terminal on the battery with a black test lead wire and the positive lead to the positive terminal on the battery with a red test lead wire . By not connecting the lead that connects to the fuel tank sender, we are telling the gauge that there is no resistance to the tank which should make the needle move to EMPTY.

Test for Full Tank (90 Ohms)

To test for a full tank, we now need to introduce the potentiometer into the equation. The potentiometer will have three leads, the center is the ground and the left or right lead should connect to the gauge lead that would go to the fuel tank sender. Note, the other lead on the potentiometer should not be used. It does not matter which lead you use on the potentiometer, picking one side over the other just changes the direction you should turn the dial to increase/decrease resistance.

Before wiring in the potentiometer, we need to set it to about 90 ohms of resistance. To do this, turn on your volt meter in ohm reading mode and connect the center ground terminal to the ground wire on your volt meter, and the selected side terminal to the positive lead of your volt meter. Slowly turn the potentiometer until you read 90 on your volt meter. Now your resistance is setup so your gauge will read FULL.

Now connect the center ground lead terminal of the potentiometer to the negative terminal on your battery, and the previously used side terminal to the fuel tank sender lead on your gauge. Your ground and positive connections made when you tested for Zero Resistance should be in place as well. Once all connected, your gauge should now read FULL.

We also connected the positive terminal of the volt meter to the positive terminal of the battery to test the volt meter at the same time. You can see both the fuel gauge and volt meter are working.

Test for 1/2 Tank (45 Ohms)

To test for 1/2 tank of fuel, repeat the last steps, except set the potentiometer to about 45 ohms of resistance.

As you can see the gauge is very close to 1/2 full. This is good enough for a $5 swap-meet find! This time we did not test the volt meter.

A similar approach may be used to test the water temperature and oil pressure gauges. We will cover these in future posts.

Body Bushings and Sub Frame Connectors for Firebirds & Camaros

BearingIf you own an f-body 1st or 2nd generation car from 1967-1981 (F Body chassis by GM found in Pontiac Firebird, Trans Am and Chevrolet Camaro first and second gen vehicles from 67-81) and are getting ready to replace your body to sub-frame bushings, you have three options: stock rubber, polyurethane and solid. If you plan on adding or currently have sub frame connectors connecting your front and rear sub-frames together, then you will most likely be required to use solid body bushings (check your sub-frame connector documentation).

Rubber Body Bushings – This is the most expensive option and was how the car original came from the factory. This is your option if you are doing a concourse restoration and/or want to keep your vehicle stock. These bushings will deteriorate over time and will need to be replaced after a period of years.

Polyurethane Body Bushings – This is the most popular alternative to rubber body bushings. They are much harder and will last longer than rubber. Polyurethane body bushings will not squeak as they age like some polyurethane A-arm bushings for suspension components sometimes do, so noise for this application is not a factor. Polyurethane may transfer more vibrations from the sub-frame to the body.

Solid Body Bushings – This is a more permanent solution using bushings typically made out of aluminum. They are solid and should last forever. Solid body bushings will transfer vibrations from the sub-frame to the body. They are required by most sub-frame connectors so body movement does not add stress to sub-frame connections.

We decided to use solid body bushings for Project Trans Am since we plan on installing sub-frame connectors in the future.

Fact: Did you know the 3rd generation F-body chassis did not use body bushings? Instead, the front frame was welded directly to the body.

Installation is straight forward and is typically performed during restoration, when adding sub-frame connectors, or when addressing body flex issues with the front suspension. Most kits come with instructions, new fasteners and torque values. For rusty vehicles, 1-2 weeks of spraying the body bushing fasteners daily with penetrating oil is recommended. Use a breaker bar (do not use am impact wrench) with a 6 point socket to remove these bolts to prevent excessive torquing and damage to the bolt heads.

Snap Retaining Ring (Lock Ring) for GM/Chrysler Steering Column

During the disassembly of the steering column from the Trans Am the retaining snap ring that holds the lock plate got damaged. One option is to bend it back into shape. The other option is to replace it. Unfortunately, you’re not going to find an auto parts store that knows what a continuous retaining snap-ring is, let alone have one in stock. Most snap rings today are either internal, external or have a specific shape for the application. This simple snap-ring used for the lock-plate is one of those “caught in limbo” fasteners that no one seems to stock.

The solution?

Fabricate yourself one out of a spring!

The diameter of this retaining snap-ring is 3/4″  (0.75″) with 1/16″ (0.0625″) thick round steel. The gap at the end is approximately 3/32 to 1/8″ (0.09375″ to 0.125″).

Find a quality 3/4″ diameter spring with 0.062″ thick steel at to your favorite hardware or home improvement store. Then use either a saw, dremmel cutting wheel or wire cutters (if you’re really in a hurry) and cut one coil out of the spring. If your cutting wheel is 1/8″ thick, you can simply make a straight cut across multiple coils and you should have the exact ring with the gap you need as the result. Depending on the coil of your spring, you may need to bend it slightly to straighten it. A quick press in a vice between blocks of wood should do the trick.

The following solution should work for all Saginaw steering columns from the 60’s through the 90’s that are commonly found in GM and Chrysler cars and trucks.