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1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks Discuss the Early Eighties Bullnose Ford Truck

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Old 11-28-2010, 10:58 AM
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Anafiel Anafiel is offline
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Oil pressure sending unit question.

I'm confused.

Yesterday, my dash oil pressure gauge dropped to the lowest position of the normal range, and just kinda stayed there, regardless of RPM.

As I don't have a mechanical gauge just this second to test true pressure, I thought I would do a couple simple tests to try to eliminate whatever I could. I read doing a Google search, that I could test the sender and gauge by pulling the sender wire.

According to the articles I read:

Key on, engine off: Gauge should read "0"
Key on, engine off: Pull wire off of sender, gauge should peg to high.
Key on, engine off: Ground sender wire and gauge should come back to "0"

But mine does the opposite. My gauge shows "0" UNTIL I ground the sender wire with key on. Then it goes full over!

Am I missing something? I'm assuming my gauge might be just fine. I'm hoping it's a sender. I can't afford replacing a pump just now.

Any ideas?
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Old 11-28-2010, 11:40 AM
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Ford changed the way the gauges operate sometime in the mid-late 80s or early 90s I forget
exactly. The early versions peg all the way to the high side when that wire is grounded and
flat-bottom to 0 when disconnected, the later models are the exact opposite. Same for the
gas & temp senders, they all operate in the same fashion.

In any event, it sounds like your gauge & wiring are good *at the present moment* (but IVR
problems are generally sporadic which could potentially cause weirdness).

Yes, the best test is to get a known-good gauge to hook up and run your test with that,
anything else is just spinning your wheels IMHO.
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Old 11-28-2010, 06:44 PM
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Anafiel Anafiel is offline
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Got a new sending unit today. Also got a little SunPro mechanical oil pressure gauge to mount down low where I can see it good. The guys at O'Reilly's were great in helping me build a little "christmas tree" so that I could have both the sending unit and the new gauge working, at the same time. No way I'm gonna have a "dead" gauge in my cluster after all the work I've done to get everything working.

Ran out of light, so I can't install any of this till tomorrow.

Quick question though...

Engine has been running smooth as glass, water temp gauge runs cool (replaced water pump last month, got good coolant), no lifter noise, nothing to indicate any major issues anywhere. If the engine was losing pressure due to wear, wouldn't the drop be gradual, and wouldn't I see that in other performance indicators as well? And what about the oil pump...would it drop to a low pressure situation, and just hang there?

This is a sending unit issue, right???

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Old 11-29-2010, 01:52 PM
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Fixed. Changed the sender, and added a little SunPro pressure gauge on a tee at the sender extension. Here's what the mechanical gauge tells me:

Cold start: 55-60psi
Full operating temp, in Drive (450rpm) 17-18psi
Full operating temp, in Park (650rpm) 22psi
1000rpm 35psi
1500rpm 45psi
2000rpm 50-55psi

Dash gauge indicated normal range, very little movement though... I'm glad I got the SunPro gauge!!
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1985 F150 XL 2x4 351W
92 Eddie Bauer Bronco 4x4 351W EFI

The system didn't fail...the jury failed the system!
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Old 11-29-2010, 01:52 PM
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1985, 2009, 300, block, f150, ford, gauges, ivr, light, oil, pressure, problem, sending, sporadically, straight, unit

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