86 GMC Oil Pressure Gauge Issue

[fast72beeper]

Putting the finishing touches on a 86 GMC Im putting together for my sis in law. But, Ive ran into an issue with the oil pressure gauge.

Right now I have a mechanical gauge installed as I’ll be breaking in a new cam and want to make sure on the pressure.

The stock gauge pegs when you turn the key on. If I ground the sender wire the gauge goes to zero. Is that correct? If not, where to look, check, etc to fix it.

Thanks for any help

[chengny]

"If I ground the sender wire the gauge goes to zero. Is that correct?"


No, when the lead to the sender is removed and grounded - these gauges (both the coolant temp and oil pressure) are supposed to peg high.

It might a bad coil in the gauge or a shorted hot wire - both of which rarely happen .

As a first step, why don't you see if it's systemic or isolated to the oil pressure gauge.

Pull the lead to the coolant temp sender, ground it and see if it's gauge reacts the same way.

[fast72beeper]

Originally Posted by chengny

[ Pull the lead to the coolant temp sender, ground it and see if it's gauge reacts the same way.

Coolant temp sender works correctly.

[fast72beeper]

Originally Posted by fast72beeper

Coolant temp sender works correctly.

Since the coolant temp sender works correctly does that point to the oil pressure gauge itself being bad?

[chengny]

Yeah, I'd say it's the gauge. It's acting completely backwards.

When the key is turned off (under normal conditions) the oil pressure gauge kind of hangs where is was when power was cut off.

When the key is turned back to RUN, and power is re-established, the gauge pegs CCW to 0 psi.

When the engine starts and oil pressure comes up, the sender starts to vary the resistance to ground.

Higher pressure = lower resistance = stronger pull CW (higher indication on the dash)

Lower pressure = higher res = lower dash reading

Here is a nice diagram of this type of gauge - it is showing a coolant temperature circuit but oil pressure is identical.



And a brief description of operation:

Gauge failures are often caused by defective wiring or grounds. The first step in locating trouble should be a thorough inspection of all wiring, terminals and printed circuits. If wiring is secured by clamps, check to see whether the insulation has been severed, thereby grounding the wire. In the case of a fuel gauge installation, rust may cause failure by corrosion at the ground connection of the tank unit.

VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.
When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil. When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.