Gauges
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In this page we well explain how the gauges in your car and truck work, as you know these gauges are usually mounted in the instrument cluster, some makes an models may have a couple of gauges mounted in the center console, regardless of their location their purpose remains the same.

- The main purpose of the gauges in your vehicle is to let you know what's happening inside the engine compartment, without these gauges you would be completely unaware of your  engine's temperature, oil pressure or lack of it, speed, engine RPM's and so forth.

- The most common gauges mounted in the instrument cluster in most vehicles are: speedometer, tachometer, fuel level, water temperature, and oil pressure. Some vehicles may use warning lights instead of gauges for the water temperature and oil pressure, they may use a combination of both.

- There are other gauges that are found in high performance vehicles or some enthusiasts choose to install aftermarket gauges that monitor engine oil temperature, transmission temperature, fuel pressure, automatic transmission fluid pressure among others.

- Our goal is to show you how each of these gauges work to allow you to diagnose and repair any problems related to these gauges.  



Speedometer: The speedometer in older vehicles is composed of 3 different parts: speedometer gauge, speedometer cable and speedometer drive gears.  The speedometer gauge as you know has a needle that moves up as the vehicle's speed increases, this older speedometer gauge is a mechanical version, it has moving parts inside that turn, the circular movement is transmitted by the speedometer cable, this cable gets the circular motion from the speedometer drive gears, these gears are usually mounted in the transmission's tail shaft or the transfer case if the vehicle is 4wd . The odometer and trip meter are usually part of the same gauge, specially because they use the same signal to keep track of the miles traveled.

The speedometer gauge has counterweights that regulate the movement of the needle, without these counter weights the needle would spin out of control , each gauge has been preset by the manufacturer to have an accurate reading depending on differential ratio and tire size, if any of these two items is changed ( installing larger tires for instance ) it will  cause an inaccurate speedometer reading . There are two ways to restore the speedometer reading after a tire size or gear ratio change in a mechanical speedometer gauge, one is to install a gear reduction between the speedometer drive gears and the speedometer cable, the second option is to have the speedometer gauge re-calibrated by a specialized shop, most owners choose the first option because is much cheaper and easier to do.

Newer vehicles equipped with electronic fuel injection don't have a speedometer cable anymore, these newer vehicles use a speedometer sensor mounted on the transmission tail housing or the transfer case to convert the circular motion signal received from the speedometer drive gears in to an electric signal that can be read  by the ECM ( engine control computer ), the computer sends this signal to the speedometer gauge, the needle will move accordingly depending on the electrical pulses being received.



   
  Click on image to learn how to replace the speedometer cable in a Nissan pick-up and Nissan Pathfinder (1986-1996 models)




 Oil pressure gauge: The oil pressure gauge as its name implies, monitors the amount of oil pressure inside the engine, this information is very crucial specially because without normal oil pressure, severe engine damage will occur.

You may wonder, how is the oil pressure generated inside the engine to begin with? well.....every engine regardless of its size is equipped with an oil pump, the oil pressure in a gasoline engine is usually between 20 and 60 psi, oil pressure is required to make sure that all moving parts get adequate amounts of oil for lubrication, and most engines are equipped with hydraulic parts that require oil pressure for their normal operation like: hydraulic lifters, hydraulic lash adjusters, VCT solenoids, cam phasers, among others.

It is very important to have a working oil pressure gauge in every vehicle to monitor the oil pressure inside the engine, without an oil pressure gauge, you won't know if your engine has oil pressure.
 

So, how does the oil pressure gauge work?

- There are two types of oil pressure gauges, mechanical and electric/electronic; the mechanical oil pressure gauge was used in older vehicles, today is used in race cars and performance oriented vehicles, the way this gauge works, there is an oil line connected from an oil passage in the engine directly to the mechanical oil pressure gauge, as the oil pressure increases the needle in the oil pressure gauge moves.

The electric/electronic oil pressure gauge has a sending unit ( oil pressure switch) mounted on one of the oil passages in the engine, this oil pressure switch varies the resistant as the oil pressure moves from high to low, the electric gauge is designed to move accordingly, in older electric gauges, the signal is sent directly from the switch to the gauge, but in modern vehicles equipped with  digital gauges, the signal is sent to the computer, and the computer sends the signal to the gauge
  
  Click on image to learn how to repair the oil pressure gauge in a Ford Bronco II

 
   - Click on image to learn how to repair a malfunctioning coolant temperature gauge in a 1993-2005 VW Jetta, Golf, GTI with a 2.0 L engine