f100 Oil Pressure Gauge Woes!

ceetwarrior · #1 (**permalink**) 06-19-2006, 10:23 PM

Hey guys, got something strange here... My oil pressure gauge stopped working. So I tear it apart to find what's the matter.

First thing I change is the sending unit. Start up the truck, still nothing. So I check it with an idiot lite and as soon as I turn on the key it lights up steady for 2 seconds then it starts blinking erratically. I checked all connections, sending unit, quick connect under dash then at the cluster itself.

I take out the cluster and pull off the "white" and the "black wires off the back of the gauge. The black/green striped one has power and it is blinking there.

So that eliminates the sending unit, connections, and gauge as being bad. Now why is the darn power wire blinking???
Also, if I had to, is it safe/possible to remote wire a 12v power to replace the blinking wire?

phkhgh · #2 (**permalink**) 06-19-2006, 11:02 PM

ceetwarrior,

Is the test light on the hot wire blinking steadily off / on, or it it random?

You said it blinks erratically - do you have the truck running when testing it? Is the wire to the sending unit running 'amongst' the spark plug / coil wires? It could be interference from the spark plug wires, espec if the insulation on either is old / cracked.

If it blinks randomly, the wire from the sending to the guage may be shorting on some metal. I had a 68 Camaro (in 1971) that died every time I turned a hard corner (I turned a lot of hard corners). Mechanics thought it was a fuel prob, but I finally found an ignition wire shorting on the block.

Also, how do you know the new sending unit is good? If the wire from the sending unit is suspect (and you know the sending unit is good - they can be checked), you could test a new wire from the sending unit back to the guage.

I don't remember if that's one that runs through a fuse. If it is, either run the test wire through the fuse, or just don't leave it hooked up very long. Sure, if the original wire is bad, you can replace all / part of it. But if it went through a fuse, either run the new wire through the fuse box, or use an inline fuse of appropriate size.

Hope this helps!

ceetwarrior · #3 (**permalink**) 06-19-2006, 11:28 PM

phkhgh, it can't have anything to do with the wire to the sending unit as It is first blinking before it even enters the gauge at the harness. The power wire coming from the harness nest straight to the push on connector on the cluster.

Good point, but yes it blinks erratically with the truck running or not.

Ford_Six · #4 (**permalink**) 06-20-2006, 12:01 AM

The gauge gets it's power from the instrument voltage regulator, which is just a calibrated blinker relay, so the reading on the gauges is actually an average. Do you get the same blinking pattern from the wire at the fuel sender?
If it is actually the power going to the voltage regulator, check the fuse box for signs of rot on the steel contacts. I had several rotted connections on mine, the brake light fuse fell out along with it's tabs.

ceetwarrior · #5 (**permalink**) 06-20-2006, 12:04 AM

Ford_Six, yes I get the same reading at the sending unit as I do at the gauge wire. Where is the instrument voltage regulator located at?

I'll check the fuse box. I'm guessing the fuse marked "instrument"?

What do you know about the blinking?? What would cause that? All of my other gauges work fine, so how would I tell if the "instrument voltage regulator" is good or bad?

workingdog · #6 (**permalink**) 06-20-2006, 09:54 AM

The voltage regulator is the small rectagular thing mounted on the back of the instrument cluster.

Peter

phkhgh · #7 (**permalink**) 06-20-2006, 01:42 PM

ceetwarrior,

Quote:

phkhgh, it can't have anything to do with the wire to the sending unit as It is first blinking before it even enters the gauge at the harness.

The oil pressure sending unit has a variable resistor which varies the current flow through the guage. FYI - I have gotten a bad (new) sending unit before. There is a quick way (I believe) to check the unit by shorting it for a fraction of a second. Don't remember off hand exactly how to interpret that test. I think the OEM manual also has some resistance values to check the unit - but I may be confusing it w/ another vehicle / part.

From the shop manual: If the guage does not indicate, make sure sending unit is properly sealed to engine w/ electrically conductive sealer. (Also, make sure all connections are clean and uncorroded and wires are sound.

Disconnect wire at the sending unit
Connect a test light or positive volt meter lead between the disconnected wire at sending unit and the neg lead to a ground

Turn ign sw to ON or ACC. Flashing light or fluctuating voltage indicates GOOD instr. volt. reg., and the guage circuit isn't interrupted. If that's how you did the test, sounds like your flashing light is indicating the volt. reg is good, but doesn't rule out a bad guage.

It goes on that the guage may be tested by using resistors. You need two resistors: 10 ohm / 5w and 75 ohm / 1 w. Connect the 10 ohm between the disconnected lead at the sending unit and a ground. The guage should show a high reading. With the 75 ohm resistor, the guage should read low. It might be as easy to get a 2nd sending unit.

It suggests that if the guage doesn't perform as indicated, replace the voltage regulator & see if the guage reads correctly. If it still doesn't, then replace the guage. Before I did all that, I'd check all the wiring & connections for this guage's circuit. It also says: Warning! Danger - Will Robinson: Do not ground or spark either terminal of the instr. voltage regul. It will burn out the dash wiring harness or the instr. voltage reg., or BOTH.

Hope I made this clear enough to follow.

ceetwarrior · #8 (**permalink**) 06-20-2006, 09:33 PM

phkhgh, thanks for the info. So does confirm that the blinking means the instrument voltage regulator is good?

We ohmed out both of my sending units to find one opens up and one does not. So being that it didn't tell us anything about what exact resistance it has, they both could be bad. I'm still guessing it's the sending unit as it worked fine then it stopped working, then it would come and go for the last few weeks.

I'll get a new sending unit and try it next....

ceetwarrior · #9 (**permalink**) 06-21-2006, 09:51 PM

Installed a Napa (Premium) oil sending unit ($15) and problem fixed (I hope). Needless to say I'm taking the other one back as it is only 3 months old.

What was throwing me off was I had two bad sending units, go figure. That's why I got into this whole dash mess. Had I had one good sending unit I could have known right away. Oh well, thanks guys.

phkhgh · #10 (**permalink**) 06-22-2006, 09:35 PM

ceetwarrior,

Yep, as I said:

Quote:

how do you know the new sending unit is good?

I learned the hard way never to automatically assume (when one assumes, it makes an ass out of u & me) that a new part is ALWAYS good. Glad that you got 'er done!

ceetwarrior · #11 (**permalink**) 06-22-2006, 10:03 PM

True to that. Oh yeah, we disected my original Ford sending unit. Pretty darn neat. I suggest cutting one open... it was pretty rusty on the inside too. I'd say it was a good part to last 40 years.

Bdox · #12 (**permalink**) 06-23-2006, 12:59 AM

To test the gauge: reconnect the wires the to gauge. Turn on the key and ground the wire that goes to the sender. The gauge should go all the way to the high side. If it does, the gauge is OK.

The blinking signal, as Ford_six pointed out, is the normal output of the instrument voltage regulator.

If the gauge proves to be good, the sender is at fault. If the gauge test fails, then use a jumper to test it directly on the side of the gauge that goes to the sender, bypassing the wire. If the gauge tests good that way, then the wire is open. Replace the wire.

If after all that you still don't read oil pressure, screw a mechanical gauge into the sender hole and check the oil pressure that way to insure that you have it.

Dave Severson · #13 (**permalink**) 06-24-2006, 07:13 AM

Quote:

Originally Posted by ceetwarrior

Installed a Napa (Premium) oil sending unit ($15) and problem fixed (I hope). Needless to say I'm taking the other one back as it is only 3 months old.

What was throwing me off was I had two bad sending units, go figure. That's why I got into this whole dash mess. Had I had one good sending unit I could have known right away. Oh well, thanks guys.

Yup, the cheaper parts aren't necessarily the best deal. I never argue with someone who sells his product for less, he obviously knows what it it worth!!!!

ceetwarrior · #14 (**permalink**) 06-24-2006, 06:17 PM

Well problem not fixed! On the trip up to Denver for Fordfest, half way up the gauge stopped reading. I'll just have to go all weekend with out it hah and sort it out when I get back..

Ford_Six · #15 (**permalink**) 06-24-2006, 10:00 PM

I have found in the last few days that just because it's blinking, doesn't mean it's right. My old IVR was burned up, running a full 12v all the time, the temp and oil pressure were pegged, and my gas gauge read a minimum of half a tank. Now, with the new one, the temp gauge doesn't move hardly at all, and the tank won't read over a half a tank. There seems to be an adjustment screw on the back, I just don't know what the frequency is supposed to be.
I just replaced the oil sender with a mechanical gauge, since I knew it wasn't running great oil pressure to begin with.