Oil Pressure Gauge Fluctuates

[Walt Fricke]

OP gauges normally fluctuate, based on pressure fluctuations. But mine fluctuates when the engine is off!

I have a standard 10bar/150 psi VDO 911 (style) big fat can oil pressure sender. It is plumbed into where the engine oil thermostat would be. I modified a thermostat so it just serves to direct the oil to where the cooler in would be no matter what, as I have a filter, not a cooler, there. I used this on my 2.3L motor, which was not set up for pulling oil pressures off the right cam oil line. That way I kept the idiot light. Worked fine.

I bought a motor without senders. Installed my stuff, including my custom wire loom from the 2.3 (blew up some years ago, alas). Everything looked OK. But at the track I noticed the OP gauge was pegged. No problem, must be a broken sender wire, or the connector pulled off, etc.

Well, no dice. And grounding the sender terminal put the gauge at 0, where it should be. So not likely to be a bad connection.

Tested two spare senders. One obviously busted. Other had reasonable resistance readings. Installed it. But the one I had been using also tested out about as well. I put regulated air pressure from 0 to 120 psi through a simple fixture into the gauges, and measure resistance.

I'm looking at relative changes, not any real VDO spec (though I've got one somewhere). One gauge read ~8.3 ohms at 0, and 123 at 120 psi. The other read 9.6 @ 0, and 116-117 @ 120. 20 psi was in the 10 ohm range.

Well, the replacement gauge pegged (engine off, should be no OP). After grounding it to check wires again, it sort of settled down. But the needle will flutter between close to zero and about 20 psi (first gradation on gauge.

What the heck? (that's not what I was thinking, but this is a family site, isn't it?)

How can I have poltergeists in my oil pressure measuring system? I can't see Brownian motion or some other random vibration do this. These gauges are a nice system, what with grounding driving them to zero, and higher resistance causing them to rise and reflect higher pressures. And how could oil pressure build (and fluctuate) in an engine that is not running. Not even hot.

This is a DC system, so I'm thinking this shouldn't be some stray capacitance causing resistance to vary a bit and bleeding off.

Walt Fricke

[304065]

Walt,

Put a ground strap from the body of the sender, attached with a stainless hose clamp, around the engine tin down to the trans ground.

[Walt Fricke]

John

Because what you suggest is so easy, I'll do something like that. I'll run it a few inches up to where my Electromotive ignition units are, and use their very good ground.

I don't use a trans ground, as I have solid mounts. And I have a ground wire I run from a valve cover stud to a rear motor mount 8mm bolt, just in case.

The sender case seems to be "grounded" in the sense that it is connected to the inlet 10mm x1mm hollow bolt part despite some plastic. Guess it is time to saw one apart. I know they just have a rheostat inside, and failure is usually due to the winding wires abrading.

Thanks

Walt

[304065]

Walt,

It is a good idea to run a ground strap from the trans to the body anyway. Remember that to complete all circuits on the engine-- alternator, ignition, plugs, and the oil sensing, a good ground is required, and the ground carries almost as much current as the hot wires.

[Walt Fricke]

I did the hose clamp thing. No joy. Whatever was doing this, it wasn't a ground issue. I traced the sender wire through various barrier strips I have to keep the wiring manageable. No bad joints I could find.

The sender that tested OK yesterday and was installed no longer tests good. Instead it has high resistances, though they vary. 1K or so. I'm guessing that maybe this represents oil getting past the diaphragm inside which keeps it away from the rheostat part and increases resistance. Doesn't explain the fluctuations when I turn the power on without starting the engine.

I could sort of kind of imagine that oil between the slider and the windings could cause the resistance to change on a one second or so time scale due to the effect of the current passing through it. I measured about 6V at the sender.

Anyway, I installed the sender that wasn't working last weekend, as it checked out on the bench. At least for now, when I power the gauge, the needle rises to the 0 mark, and stays put. Tomorrow, when I load the car, I'll see how long this lasts.

The local FLAPS said they could get me a new sender for $125. Wow, things have gone up.

The tranny strap is just a bit too much of a PITA given how often things come out and go back in in this track only car. I have one on the dual purpose, licensed SC ("club sport" rubber mounts). Dealing with a stout wire from the engine to a rear motor mount is a lot easier, though I think the solid metal mounts, between the four of them, actually are all that is needed.

Walt

[304065]

$125 is highway robbery for a stock VDO sender. These are between $25 and $50 for stock VDO-- of course you probably don't need M10x1.0 but can use an NPT threaded sender on a race car, they are cheaper. Make sure you get the correct resistance range, however.

[Walt Fricke]

I had the same thoughts. Racer Wholesale sells an adapter so you can use NPT, though fewer parts are always a good thing.

I'm still puzzled by the gauge's behavior, as I have little faith in my speculations.

In my experience, these senders are marked as 10 bar or 5 bar, or 150 or 75 psi, or 10 KPa (maybe 5 KPa also). A 5 bar sender (SC and later?) on a 10 bar gauge (2.7 and earlier?)gives you some pretty high apparent OPs.

They also come with at least two screw-in diameters - 10mm or a fat one. I've got two sizes of that adapter block for the right cam housing takeoff, one for fat, one for 10mm. And I think the fat ones will screw directly into the stock front right side top of the engine case location where the idiot light sender is normally located. For 10mm you need an adapter there. Which is already there for the idiot sender.

Walt