Hi, I have a 68 chevy p/u and need some advice about getting my guages to work. I dont know an incredible amount about this sort of thing yet, but will definatly learn. Does the speedo cable run directly from tranny to guage? Does the oil pressure run directly to guages as well? None of mine work yet. I'm also trying desperatly to get the fuel guage to work. It is hard knowing how far I can go without that thing. Also, anyway to restore the function of the trip?

your speedometer runs trans to gauge via a cable.and your oil pressure guage runs from the sending unit to guage via a small metal or sometimes plastic tube.if you un hook the speedo cable on the trans side and attach a cordless drill to the "runner" cable inside and spin it your speedo should move if not dead guage,if so the plastic gear in the trasns might be bad. i think you can remove that out the side of the trans. to test the oil pressure you need a test guage. screw it into the spot that the sending unit goes and see what it says upon start up.under 20lbs at idle is not good. hope this helps.

Doc here,

There are TWO types of Gauges. Electrical, and Mechanical.

Electrical is Gauges that use remote "Senders" that relay on Wires to send data to an Electrical meter movement that is a Calibrated Resistive Value with regard to the sender Resistance.

TEMP GAUGE:

The sender is a Device that has a Bimetallic Strip inside it, which is sensitive to heat (for a temp sender) as the strip heats up, it moves and drives a Potentiometer, (A Resistive load Somewhat like a Volume Control knob on a non Digital radio..) This resistance is reported to the gauge..

The Gauge has a Bimetallic Strip inside it, The index pointer is Mechanically connected to it. ( the face plate needle) As the resistance changes with the movement of the sender value, the Strip in the gauge heats up and drives the index pointer..(the HIGHER the resistance the lower the Reading on the gauge)

The Gauge has 6 or 12 volts power and ground wired through the Strip..at or with no sender resistance, the value is "Calibrated" to indicate FULL on the scale. (remove the sender wire and the gauge should peg full..Ground the sender wire and the gauge should peg to empty. a good way to test the gauge and wires from the sender.)

The Sender RESISTANCE is tapped into the strip..It's resistance determines the value across the strip and how "Hot" or "Cool" it gets by turning Electricity To Heat...(IE: how much movement, and which direction it moves)

So the typical Connections are :

• POWER = 6 or 12 volts DC ( "I" terminal on some gauges)
  
• GROUND = hard Ground ("G" terminal on some gauges)
  
• SENDER = Sender wire to the engine bay or tank ("S" terminal on some Gauges)

FUEL GAUGE:

Exactly the same as The Temp Gauge EXCEPT:

The Sender is NOT a bimetallic Strip, But a potentiometer Mounted Directly onto a Float that is immersed in the media..as the media is reduced or increased the float follows suit..as the arm moves the pot the resistive value changes, and sends that data to the Bimetallic Strip in the gauge. (The SAME test , wire off, wire to ground can be used here also to check if the gauge is working)

OIL PRESSURE GAUGE:

Exactly The same as all above..EXCEPT:

The sender is a small resistive element that is moved by the calibrated pressure entering the orifice of the sender that moves a mechanical arm which causes the movement of the pot or resistive element.

There are ALSO two types of Electrical Gauges..ANALOG and DIGITAL.

Analog is as described as above.

DIGITAL is a system that uses SENSORS that operate much the same as above with minor differences.

For instance a Piezo weight detector may be used as a fuel gauge sender..It determines the Volume of fuel by It's weight..the more weight, the more fuel. This type of detector is found in higher end late model cars like Vettes..

Instead of reporting a Resistive Ground back to a gauge, a system of this type usually reports a "Low" referenced pulse back to a computer. The difference in the voltage is the difference in the readout.

Instead of a Bimetallic strip in the gauge, It uses an item called a Whetstone bridge. This is Four Precisely Balanced Resistors in a "Diamond" Shape and hooks across the windings of a motor meter movement. As the Value of the bridge changes the meter moves up or down accordingly with the output of the computer or decoder. This type of system CAN NOT be tested by grounding the sensors..IN FACT you will damage it by doing so.

MECHANICAL GAUGES:

Simple System, a Temp gauge has a Bourdon tube full of Mercury that when heated expands, as the engine end expands from heat, it pushes the media up to the gauge through a tube, it in turn coils or uncoils a spring that moves the index pointer.

OIL GAUGE :

Same as above EXCEPT it uses the oil sent up through the tube..to the gauge.

FUEL GAUGE:

Almost ALWAYS Electrical.

SPEEDOMETER:

This uses a Calibrated gear in the transmission, which drives a spring like cable up to the gauge..

The Gauge has a Magnetic motor inside of it..this is a set of magnets rotating around a metal shaft..a certain amount of "Calibrated" slippage is involved since there is no direct connection between the two. The faster the magnet spins the more the shaft moves, the more the gearing inside the speedometer moves. As the gearing moves, it drives the index pointer (MPH) and the odometer as well as the trip gauge through calibrated gearing. This changes the digits on the faceplate.

DIGITAL:

Works pretty much the same way except instead of mechanical movement, it uses a transducer at the transmission that sends signals to the readout in the dash.

Being as your truck is a 1968, the gauges could be either Electric or Mechanical, you need to look to see if it has senders or Tubes for the dash units.

To test electrical, do as was said above , remove the sender wire, monitor the gauge, it should peg top, then ground it, it should peg over to low..If you have that the system is working, the sender needs replacement.

For mechanical, a temp gauge can be tested with heated water and a thermometer, verify movement and calibration if it's working, if no movement the gauge needs replacement.

Oil pressure, just insert about 20 psi of metered air into the sender, if the gauge does not move or is in error, replace it.

Speedometer, little user serviceable parts here..The cable usually snaps..simple replacement. If the odometer or trip gauge is stuck, this is usually rust. use electrical cleaner on the gears, (don't stain the faceplate or the numbers) then gently rock back and forth. After it dries, coat with some graphite lube. If that doesn't fix it, replace it or have a shop rebuild it.

Doc

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thanks, my speedo isnt connected at all. i guess i need to go get new speedo cable anyways. i also dont believe i have the oil hooked up either. is the temp guage wire supposed to be a very smal guage (smaller in diameter)? i beleive know which one that is too.

with the sending unit for the fuel tank, the only thing that could stop it from working would be either the unit itself or the guages right?






?

Doc here,

The Temp gauge wire would be about 18 gauge, probably green in color.

The Fuel gauge is tested by removing the sender wire, it should then peg full, if it does, it's working Ground the wire and it should peg empty..Then the sender GROUND at the tank could be a problem, OR the sender itself may be bad.

Doc

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