Oil pressure sender unit confusion

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Stephan D

Stephan D

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Got my newly rebuilt 360 back in my Lil Red, WOHOOO!!

The 1979 Lil Red Express truck has an original oil pressure gauge(no warning-light), but for some reason the previous owner had installed a non-electronic Bosch aftermarket oil pressure gauge, connected with a thin plastic tube.

I told the workshop to hook up the original gauge, so they installed an oil pressure sender that fitted the three terminal connector that was in place in the harness, but the gauge immediately pegged at 100 psi where the old showed 60 psi, something is wrong here, but what?

Picture of the connector that was in the truck and the new sender unit.
IMG_6642.JPG
Oil Pressure Sender Three terminals.jpg
 
It sounds like something is wired wrong, but looking at that sender it looks more like one for a light than for a gauge. Most of the ones I know of for a gauge have a larger body than the one pictured to monitor actual pressure, not just a minimum pressure.
 
This is the one listed for use with a gauge.
PS59T_Front__ra_p.jpg


Look for another terminal in the harness with a single, female spade-like connector. Use that one.
If there is no connector, then two things come to mind:
Was the harness replaced? A harness with "warning light only" wiring may have been used.
If not, was the instrument cluster replaced, using one with the pressure gauge instead of a light? They came both ways...
Trace the "hot" wire (ign. on) from the gauge to the sender plug, that is the one you need to connect to the gauge sender.
Remember, the gauge sender is nothing but a pressure-variable ground for the gauge. Find the wiring diagram for your year truck, it will simplify the process.
 
Professor Fate said:
This is the one listed for use with a gauge.
View attachment 1715640350

Look for another terminal in the harness with a single, female spade-like connector. Use that one.
If there is no connector, then two things come to mind:
Was the harness replaced? A harness with "warning light only" wiring may have been used.
If not, was the instrument cluster replaced, using one with the pressure gauge instead of a light? They came both ways...
Trace the "hot" wire (ign. on) from the gauge to the sender plug, that is the one you need to connect to the gauge sender.
Remember, the gauge sender is nothing but a pressure-variable ground for the gauge. Find the wiring diagram for your year truck, it will simplify the process.
Click to expand...

Yes!

This was the type of sender that I had expected as well, still confused why the three terminal connector is there, I will look for a "hot" wire in the harness behind the engine.

Is this the the only type of sender for the oil pressure gauge, or are the several types to be aware of?
 
Stephan D said:
Yes!

This was the type of sender that I had expected as well, still confused why the three terminal connector is there, I will look for a "hot" wire in the harness behind the engine.

Is this the the only type of sender for the oil pressure gauge, or are the several types to be aware of?
Click to expand...
It's the only type I am aware of, and the only type I find listed for a '79 Little Red.
 
Last edited:
OK had the same thing in my 88 w150. That sending unit you installed is for a light and a cut off you have to find the sending unit, it has three terminals one is the power for the gauge, one goes to the gauge and the other is for a idiot light ground. Hope it helps.
 
On that year truck, it will have a single wire to the sender.
Please note there are two senders that look identical, one for a gauge, and one for a light.
I might have an OEM gauge sender laying on my workbench I took off my 78 powerwagon that works.
 
67Dart273 said:
The simple answer is the type you pictured with 3 terminals is a SWITCH and not a gauge sender. I'm not familiar with the options/ wiring of these girls, cannot offer an explanation. Maybe originally there was a tee and used both?

Without a factory wiring diagram, no idea

EDIT

Found this, no answer........they show a single sender wire

Offroad Exchange - 79 Dodge Factory Service Manual

Page 2, far left, shows wire O-18WH (WHite)

http://www.offroadexchange.com/pdf/79dodge_wiring_diagram.pdf
Click to expand...

Thank you!

The workshop obviously installed an oil pressure switch which is wrong when connected to the oil pressure gauge, see picture below.

When this didn´t work, they cut off the three terminal connector in order to solve the problem, using two of the wires to the switch, the result is a gauge pegging att 100 psi with the ignition on, pulling some quite current in the same time.

So far I know:

1. The pressure switch is all wrong for my gauge
2. The gauge is moving, probably working
3. With the gauge pegging, it´s connected through the harness to the switch

There is one white wire of the three that went to the connector.

Is there a way to measure with a volt/ohmmeter which wire that should connect to the correct oil pressure sensor?

Feels a bit scary to connect wires randomly...
Hackad Oljetryckskontakt.jpg
 
Brown is the factory color for the sender.
 
Most test lights have enough resistance to test an oil pressure gauge. Install the light inline between the wire(s) in question and a good ground. Turn the ignition switch to the ON position and see if the gauge moves a little. If it does, you've found the correct wire for the sender. If it doesn't, try one of the other wires in the same way. You can also test those other wires to see if it will turn on a warning light, but, I don't think they had that option in a '79 pickup. I know my '78 doesn't have a light option along with a gauge. It's usually one or the other.
 
That doesn't look like the stock connector for a factory sender. Has the wiring been modified?
 
cudamark said:
Most test lights have enough resistance to test an oil pressure gauge. Install the light inline between the wire(s) in question and a good ground. Turn the ignition switch to the ON position and see if the gauge moves a little. If it does, you've found the correct wire for the sender. If it doesn't, try one of the other wires in the same way. You can also test those other wires to see if it will turn on a warning light, but, I don't think they had that option in a '79 pickup. I know my '78 doesn't have a light option along with a gauge. It's usually one or the other.
Click to expand...

Great advise, I will test that, starting with the brown wire suggested above.

The truck does not have an oil pressure warning light, but the wire harness might include that connector anyway, sort of generic harness for several truck models?
(Just speculating...)
 
cudamark said:
That doesn't look like the stock connector for a factory sender. Has the wiring been modified?
Click to expand...

I really don´t know, but the harness looks very stock to me, I though that connector should go to some sort of sensor on the intake manifold, uncertain of the function, see below.
20201201_182106.jpg
 
^^I agree with the test light. Use one with an incandescant lamp and not LED, because a lamp draws more current and will activate the gauge "at least somewhat." Also a stock voltage limiter (the gauges regulator) will pulse on/ off quite fast. This of course will show on the lamp
 
Thank you all!

Just wanted to round up my own thread, got the correct sender and located the right wire.

YES! - We have oil pressure!
20201204_115307[1].jpg
 
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