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Oil Pressure Gauge ... Or Is It?Posted 3/17/1998By Jamie MacFarland
I have always viewed oil pressure, voltage and temperature gauges as nice features for monitoring the underhood conditions and trends of an automobile. The downfall of warning lights (also known as "idiot lights") is that they only alert you to a severe condition after the fact - the engine is extremely hot, or the oil pressure is extremely low - but nothing in between! When you see an oil pressure gauge, you just assume that it is indicating engine oil pressure ... right? In recent years, Ford Motor Company has been doing something I view as a bit sneaky. They have taken the oil pressure gauge and turned it into a glorified warning light. It says it's an oil pressure gauge, it looks like an oil pressure gauge, but in reality it is not. A conventional magnetic oil pressure gauge on a Ford consists of a gauge and a variable resistive sending unit. As oil pressure changes, the resistance of the sending unit changes, which in turn causes the gauge needle to move. This system worked well for years, but starting in 1986 a change was made on some models that altered the function of the oil pressure gauge as we knew it. The variable resistance sending unit was replaced with an open/closed switch and an in-line 20 ohm resistor was installed between the gauge and the switch. Anytime you have more than 4.5 psi of oil pressure, the switch closes, completing the circuit from the gauge (with the 20 ohm resistor in-line) to ground. This then results in a gauge that reads just above the middle. As you can see, there is no variation to the gauge needle in this circuit. It will either read no oil pressure or half-scale (normal oil pressure). On this style of gauge circuit, if the oil pressure gauge reads in the middle, it is telling you only one thing - you have more than 4.5 psi of oil pressure. How much more is anyone's guess at that point. When oil pressure drops below 4.5 psi, the gauge will return to 0 and alert the driver of a problem. Didn't warning lights tell us that much? Ford released a TSB (#88-5-14) for 1987-1988 F Series and Broncos that called for converting the variable resistive style oil pressure gauge circuit to the switch style circuit - if the customer complained of low or erratic oil pressure readings. The kit contains an oil pressure switch and an in-line 20 ohm resistor that is to be connected between the end of the existing sending unit signal wire and the new switch. The variable resistance oil pressure unit removed from the vehicle is then discarded. I want to point out some diagnostic problems that can arise from this circuit. First, if you are working on this switch style oil pressure circuit (with the in-line 20 ohm resistor) and install a variable resistive sending unit in place of the oil pressure switch, the result will be a gauge that reads lower than normal due to having two resistors in series, the fixed 20 ohm in-line resistor and the variable resistive sending unit. On the other hand, installing the open/closed switch on a vehicle that incorporates the conventional oil pressure gauge circuit will result in a pegged gauge needle after starting the engine. Sometimes you can tell that you need a switch instead of the variable resistive sending unit by the presence of the in-line resistor a few inches from the switch connector. This is not a good rule of thumb though, since many vehicles have the fixed resistor on the back of the cluster, and some of the digital dashes have that circuit built into the cluster. To determine what style circuit you have, simply ground the sending unit wire with the key on. If the gauge reads exactly half-scale, you can be fairly sure you need a switch-style sender. If you ground the sender wire and the gauge pegs, a variable resistor-style sender is needed.
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