Oil and water gauge issue, weird engine...help!

[cbbeard]

So my son and I are rebuilding a 1984 GMC stepside that my father found for very cheap. The downside is that the engine is a 1979 Oldsmobile 350. I know quite a bit about the Chevy 350, but the Olds is much different.

So here's the question...the water temp and oil pressure gauges don't work. So I'm wondering a couple of things...1) will the gauges work with the olds sending units etc? 2) if so, how do I test to see if the problem is the sending unit or the gauges themselves? 3) if not, any ideas?

I'm most weak in the electrical/wiring aspect, so any help is appreciated!

[geezer#99]

Eliminate the guessing and install some after market gauges.

[cbbeard]

Right on, that thought had definitely crossed my mind. But we are trying to make it "look" as stock as possible. (I know, sounds dumb when there is an Oldsmobile engine in it lol).

[chengny]

There is a dedicated fuse for the fuses - check that first.

I am not sure of the layout for a 1984 fuse block - that may have been the changeover year - but the fuse for the gauges will be in one of the two locations shown below.

Earlier models:




Later models:




If the fuse is good you can confirm gauge operation this way - this goes for all gages (except the voltmeter).

Go the the area of the sender and find the associated sensing leg. These color codes are standard across the years and models:

Wire to the coolant temperature sender is dark green (most trucks of this vintage will have a rectangular casing at the terminal)

The wire to the oil pressure sender (not to be confused with the oil pressure switch) is tan.

The sender has a bell shaped housing, one lead and the terminal is protected by a little rubber boot.

The switch is kind of a flat headed mushroom shape and has a two wire connector that is held in place by a snap on plastic cover.




These gauges will deflect up the scale as resistance in the associated sender falls.

In the case of the temperature sender, the resistance decreases with an increase in coolant temperature. Here is a chart showing the relationship between the temperature of the measured medium and the resistance across the sender to ground:



The oil pressure sender is a mechanical device that drives a variable resistance (basically a bellows/crank arm driven linear rheostat). Electrically, it works just like the temperature sender. But instead of reacting to temperature changes, it's resistance to ground decreases with a rise in pressure.



The reason I mentioned all the above is because of the method used to to test the gauges to see if they actually move. If the sensing leg is removed from the sender and short circuited to a good clean ground point, the associated gauge should deflect to the top of the scale (peg high).

If it doesn't react, the gauge it self may be bad. But it is more likely that the problem lies with the instrument panel's PC board, harness plug or connecting wiring.

[cbbeard]

Amazing help...so very much appreciated!

[Just call me Sean]

I dont see why you couldn't use the stock '84 sending units on the Olds engine, then you know the gauges would read right.