Both my Fuel and Temperature gauges were dead when I purchased my 1966 C10. Many of these trucks had dash indicator lights (idiot lights); for Temperature, Oil, and Gen; my truck has the gauges. When refurbishing the engine, I installed a new temperature sender in the block. With a new fuel tank, came a new sender. Still gauge deadness persisted. The ammeter (Charge/Discharge) worked as did the Oil gauge; the oil gauge is driven by an oil tube that comes out of the passenger side of the 292 block. The wiring from the senders to the gauges tested good, so the gauges became prime suspects. Between the posts, both measure open circuit, which didn’t seem correct. After purchasing new gauges from Classic Parts, I measured the resistance of both; the Fuel gauges is 93 Ohms and the Temperature gauge is 75 Ohms. I don’t know if OEM gauges measure the same, as my dead gauges had infinite resistance (open circuit).
Dash Removal
So, I pulled out the dash. If that sounds easy, I have mislead you. First, disconnect everything from the bottom edge of the dash assembly, including (left to right) headlight switch, wiper switch, choke, speedometer, ignition, lighter (if so equipped). There are a variety of knob and shaft retaining techniques for you to navigate. A zip-tie around the speedometer cable a short distance from the thumb-nut will prevent that nut from fall down the cable all the way to the firewall. Around the dash perimeter, there are five screws to remove. Unlike a modern dash, all screws are all visible and accessible. Now it is time to get under the dash; there is no comfortable way to do it. Unless you have the seat out or the steering wheel removed, that brake pedal will be in your right ear. Bring light; it is dark under there. You will need to remove the dash with its wiring sub-harness; find two large connectors from the dash sub-harness to the main wiring harness. You will likely need to pry. Be careful and work the connectors incrementally and alternately from opposite sides. All that remains is that Oil gauge; get a short 5/16” open end wrench (8 mm fits too). To improve wrench access to the Oil gauge nut, which is at the bottom of a carelessly engineered depression in the back of the dash, carefully pry the connector free from the Fuel gauge connector. Though difficult to find, a 5/16” crow’s foot wrench can be purchased. But, I don’t think there is sufficient clearance to engage the nut and turn it. If I had to do this again, I would modify a 5/16” socket wrench by slitting the side top to bottom so it could pass over the oil line, and grind two flats for a 3/8” open end wrench. Carefully work the nut loose until the oil line pops free of the gauge. Now the dash is free, but still in the hole. I was able to wrestle it out over the steering column, scraping the top of the column a little bit. For additional clearance, you can loosen two screws and lower the steering column.
When my dash came free, many things were rattling inside. When I took out the gauges, nuts and shells fell free and others had to be fished out. Note to self: Securely cover unused light socket holes to prevent mice insurgency. It seems they disturbed the delicate wires of the gauge coils.
Power Testing the Gauges
A 12 Volt car battery of 12 Volt DC power supply can be used. To prevent an accidental over-current, I used 2 Amp fuse in an inline fuse holder between Positive power and the gauge power post (pink wire), which is on the right side when viewed from the back of the dash. Connect the Ground lead from your power source to the bare metal on the dash or to the gauge (if not mounted in the dash).
Fuel Gauge
When you attach the ground lead to the bare metal of the gauge or dash, the gauge needle will swing to Full (and beyond). Connect a lead from the gauge sender post (left side/tan wire) to Ground; the gauge needle will swing to Empty. If you add a 15 Ohm resistor to the connection between the sender post and Ground, the gauge needle will swing to Half.
Temperature Gauge
Move the Positive power lead to the Temperature gauge power post (right side/pink wire). Connect a lead from the gauge sender post (left side/dark green wire) through a 50 Ohm resistor to Ground; the gauge needle will swing to Hot. If you use an 80 Ohm resistor, the gauge needle will swing to Center.
Dash Installation
The dash is almost ready to go back in. Test each dash lamp for continuity, and replace any dead or darkening bulbs; the lamp number is 1895. Remember to plug or cover unused holes in the dash housing. I recommend using a contact cleaner on all of the connector contacts; spray them down and orient them to drain and dry. Reverse the order of operations to install, with one exception. To reattach the tube to the Oil pressure gauge, I removed that gauge pair (Oil & Fuel) from the dash. This afforded me additional clearance for getting the nut started. Be careful not to cross thread that little brass nut; put some light oil on the threads and proceed with caution. I used the 5/16” open end wrench to apply some pressure to get the nut to engage the mating threads on the gauge back. Don’t fully tighten this nut just yet; the oil tube will need to rotate a little. Insert the dash into its hole, as you tip this gauge pair carefully into its hole in the back of the dash. Don’t damage the delicate gauge needles! Add the three sheet metal screws to secure the gauge pair; you will need a ¼” open end or box end wrench, as nut-driver clearance is lacking on two of three screws. Now you can install the five dash retaining screws from the front. Back under the dash, finish tightening that Oil gauge nut, mate the connector on the Fuel gauge, and mate the two large connectors from the dash sub-harness to the main wiring harness.
On my C10, the Oil pressure and BATT gauges survived the years and the rodents. I added a little color to the needles from an orange paint pen. Oil pressure is higher when temperature is lower and RPMs are higher; if you observe that, the gauge is working. For the BATT, the needle should move slightly towards charge as you rev the engine from idle.
Get it all working and enjoy all the information a fully functional dash can provide.
