VDO gauge help

RodneyWright · **Posted:** Wed Jun 07, 2017 10:18 am

I'm running VDO gauges in the mustang. Why? It was a carry over from the BMW and I was retrofitting as I could. Short story is my electrical oil pressure gauge reads high on the gauge, past the 100 psi mark when I'm running over 2500rpm. I've actually changed this out twice, same results. Why did I do this, to keep the gauges the same make. I'm running the correct sending unit, so it should all be matched.

Instead of installing yet another oil gauge, I wanted to ask if there's a way to add a "filter" in the sending line to the gauge to have it read slightly less. The range on the gauge is 10-180 Ohms. As for which way the gauge reads, that I don't know. Not sure if the 180 is the "0" or the "100" on the gauge. Any help is appreciated!!

Vincent Keene · **Posted:** Wed Jun 07, 2017 10:25 am

No help, but my American-made Autometer mechanical oil pressure gauge in my Mustang works just fine. No sending unit required. :mrgreen:

FYI, mine usually reads about 60 PSI at WOT.

RodneyWright · **Posted:** Wed Jun 07, 2017 10:41 am

I'm gun shy of the mechanical gauges. I successful oiled down the interior of an old car I had....

Vincent Keene · **Posted:** Wed Jun 07, 2017 10:50 am

RodneyWright wrote:

I'm gun shy of the mechanical gauges. I successful oiled down the interior of an old car I had....

I'm using a stainless-steel braided AN line on mine. It will take a LOT more than 60 PSI to make it let go so I'm not worried.

Aaron Buckley · **Posted:** Wed Jun 07, 2017 1:58 pm

I think the question is, where have you mounted the sending unit? I wonder if it is getting some increase pressure readings due to the location? - AB

RodneyWright · **Posted:** Wed Jun 07, 2017 4:50 pm

Aaron Buckley wrote:

I think the question is, where have you mounted the sending unit? I wonder if it is getting some increase pressure readings due to the location? - AB

The only place I could that made sense, where the old sending unit was located and that was right by the oil filter.

Vincent Keene · **Posted:** Wed Jun 07, 2017 6:51 pm

RodneyWright wrote:

Aaron Buckley wrote:

I think the question is, where have you mounted the sending unit? I wonder if it is getting some increase pressure readings due to the location? - AB

The only place I could that made sense, where the old sending unit was located and that was right by the oil filter.

Yep, the only place I know of on the 4.6.

George Bright · **Posted:** Thu Jun 08, 2017 5:03 pm

Rodney on the VDO gauge 10 ohms is full scale and 180= zero. The only thing I can think of is the ground potential at the gauge is above ground at the motor. You could try running another wire from the gauge ground to the engine. You could also shift the reading down by adding a resistor between the sending unit and the existing wire to the gauge. A 25 ohm wire wound pot like this http://www.mouser.com/ProductDetail/CTS-Electronic-Components/026TB32R250B1A1/?qs=sGAEpiMZZMtC25l1F4XBU9ebFVlJKl0pCEErc12UVdk%3d should work.

RodneyWright · **Posted:** Mon Jun 19, 2017 10:04 pm

George, I purchased some 25ohm resistors from ebay, so not a lot of bucks in them. They technically read, "resistor 25 ohm 5w 1% axial". I put one in line to the oil gauge and nothing changed. Is there a different type of resistor I need to purchase?

Richard Casto · **Posted:** Tue Jun 20, 2017 9:57 am

What is the goal? To show something on your gauge that "indicates" below 100psi, or to show the actual pressure? Those are two different things. I think you could hack around with various stuff (including resistors) and you may eventually get the gauge to show lower. What I would do is first verify the sensor is working correctly. I know you mentioned you replaced it twice, but either way, verify it works.

It should not be hard to find docs that explain how it should measure (I.e zero ohms at zero psi or 180 ohms at zero psi.). As to verifying the calibration, I am not sure other than having someone do that professionally, but maybe the easier way is to just use a known good mechanical setup that you trust and plumb them up to work temporarily in parallel. Compare the output of the mechanical at various rpm/pressure values against what the electronic ohm values are.

Assuming you prove the sensor is working move onto the gauge. Maybe a variable resistor can be used to set values between zero and 180 ohms so you can see if the gauge displays correctly. If gauge works, then debug wiring. All of this should be doable with a trusted mechanical gauge, a multimeter and a variable resistor in the right adjustment range.

I personally would do it right vs. just experimenting with resistor values that makes it "look" right on the gauge. Otherwise you might be running more or less pressure than you think. What's the point of having numbers on the gauges if you can't trust them?

Richard

Richard Casto · **Posted:** Tue Jun 20, 2017 10:03 am

You know, after posting above, I would debug at the gauge end first. That might quickly verify that the sending unit resistance values are within range or not pretty quickly. Do this by disconnecting the gauge from the wires from the sensor and measuring resistance via the wires while engine off plus at higher rpm. If measures within range, then sensor and wiring probably ok so then debug the gauge, if not then debug sensor with variable resistor. If both sensor and gauge ok, then wiring is issue. This assumes oil pressure where sensor is installed is good.

Richard

RodneyWright · **Posted:** Tue Jun 20, 2017 11:35 am

I fried my hand held meter not too long ago, so I need to buy a new one before I can test. To your question, I just want the gauge to read a bit lower and not go off scale. I know the pressure is good. Just a matter of having it read in the scale of the gauge. Any recommendations on a decent testing meter?

Richard Casto · **Posted:** Tue Jun 20, 2017 11:55 am

Regarding multimeter... the sky is the limit of features and pricing. I was looking for something in the $50 range about a year ago and went with the extech ex330. It will probably do about all you need an more except clamp on amp measurement which I have a separate meter for as well (extech as well). There are cheaper models in the extech line as well. Just look at the reviews on Amazon. If money is no object, buy a Fluke brand, but that's more than you need.

I suspect you will ultimately fine an issue that will not require adding in resistors to fix this.

Richard

JamesMilko · **Posted:** Thu Jun 22, 2017 7:21 pm

I have something like this in my track box.

https://www.amazon.com/Digital-Multimet ... multimeter

Don't need anything fancy for automotive/home use really.

Chad Culbertson · **Posted:** Fri Jun 23, 2017 9:52 am

I just have a fairly cheap radioshack one, it's done everything i've ever needed as far as automotive and home uses.

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VDO gauge help

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