need some help from you electical types

RodneyWright · **Posted:** Mon Nov 03, 2014 10:23 pm

So now that I have my fuse blowing issue resolved, I'm back to my original issue. I put the old VDO gauge I had in the BMW many moons ago into the mustang. The reading was to 80psi. I purchased a new sending unit for the mustang and was able to purchase one w/ the correct thread pattern. I was kicking on the interweb looking for another way to spend my money when I happened across oil pressure gauges for the s197. They all read 100psi. Now I know why the gauge always read off the 80psi scale.

So being cheap, I thought I'd buy just a new gauge. Well that didn't help either, still reads over the scale. I'm assuming that both of the gauges are identical except for the read out. The magic is at the sending unit. My issue now is that VDO only sells one sending unit and it has adapters for the thread pattern. I played that game when I was installing the sending unit for the water temp. Almost impossible to get the adapter and the sending unit to seat properly. On top of that, it's impossible to get the old adapter out. Considering where the sending unit is mounted, I don't want to go down that path.

VDO gauges use 10-180 ohms for both the 80 and 100 psi sending units. What I need is a way to reduce the ohm's to the gauge by a small percentage so at least I can read the gauge w/ out it going off of the scale. So my question is, is there a way to put an "inline restricter" to reduce the input to the gauge? I'm NOT an electrical person, so I can use an assist here please.

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Do you know the resistance range that your gage needs to read full scale? Also, does reading increse with increasing resistance? (If the circuit goes "open circuit" is the reading high or low)?
It's easier to add resistance in this case. I suspect that there's a way to lower it but it will a affect accuracy and non linearity.

RodneyWright · **Posted:** Mon Nov 03, 2014 10:50 pm

Charlie Guthrie wrote:

Do you know the resistance range that your gage needs to read full scale? Also, does reading increse with increasing resistance? (If the circuit goes "open circuit" is the reading high or low)?
It's easier to add resistance in this case. I suspect that there's a way to lower it but it will a affect accuracy and non linearity.

I'm not sure about that information. I've been looking on line at ways to reduce the reading and I agree that it will goof w/ the gauge reading. I hate the spend the bucks on another style gauge. Let me see what info I can find...

BretLuter · **Posted:** Mon Nov 03, 2014 11:02 pm

My understanding is that you have to have a matching VDO sending unit to match the gauge. All VDO gauges have a range of 10-180 ohms, but if you have an 80 psi gauge, you have to have an 80 psi sending unit, etc.

I'm not sure if that's your issue or not, but I know it's how they work.

This *might* help: http://www.egauges.com/Pressure-Senders-s/23104.htm

if not, there's lots of other good info on that site about gauges and how they work.

RodneyWright · **Posted:** Mon Nov 03, 2014 11:07 pm

yeah, all of their gauges use the 10-180 ohm range. I'm currently running a 80psi sender w/ a 100psi gauge. I'm not going the adapter route and buying a new sending unit, I'll replace the whole thing if I have to. I was just hoping there was some "way" to at least keep the needle in the read range....

Vincent Keene · **Posted:** Tue Nov 04, 2014 9:13 am

The answer is don't put HANS PARTS on your Mustang. My Autometer oil pressure gauge in mine works just fine.

RodneyWright · **Posted:** Tue Nov 04, 2014 9:50 am

Vincent Keene wrote:

The answer is don't put HANS PARTS on your Mustang. My Autometer oil pressure gauge in mine works just fine.

I was looking at an autometer gauge in case I can't get this to work. I know right now I have the dummy gauge wire and the VDO gauge wire tied together and attached to the sending unit. Is that what you did? I know the car won't run if it can't see oil pressure.

Vincent Keene · **Posted:** Tue Nov 04, 2014 10:54 am

RodneyWright wrote:

Vincent Keene wrote:

The answer is don't put HANS PARTS on your Mustang. My Autometer oil pressure gauge in mine works just fine.

I was looking at an autometer gauge in case I can't get this to work. I know right now I have the dummy gauge wire and the VDO gauge wire tied together and attached to the sending unit. Is that what you did? I know the car won't run if it can't see oil pressure.

I'm a fan of mechanical gauges when it comes to oil pressure so I ran a AN-3 braided line from the block to gauge and attached the dummy oil pressure switch wire to ground to eliminate the message center problem and fool the ECU. Works just fine.

DickRasmussen · **Posted:** Tue Nov 04, 2014 11:17 am

Why a gauge if the car won't run without oil pressure? :-)

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Dick asked

Quote:

Why a gauge if the car won't run without oil pressure?

That's so in the middle of a long sweeper they can look down and say,"OMG, I need an Accusump."
Charlie G

Michael Westerfield · **Posted:** Tue Nov 04, 2014 12:15 pm

RodneyWright wrote:

I know the car won't run if it can't see oil pressure.

That is incorrect.

Car doesn't care about oil pressure (from a gauge perspective). It just feeds a dummy light.

Vincent Keene · **Posted:** Tue Nov 04, 2014 12:19 pm

DickRasmussen wrote:

Why a gauge if the car won't run without oil pressure? :-)

The point at which that switch is activated is so low it should have the message center read "Buy new engine" instead of "Low Oil Pressure".

I need to look it up for the Mustang low oil pressure switch, but on my Z-24 it's 3 PSI. :stick:

RodneyWright · **Posted:** Tue Nov 04, 2014 12:40 pm

Michael Westerfield wrote:

RodneyWright wrote:

I know the car won't run if it can't see oil pressure.

That is incorrect.

Car doesn't care about oil pressure (from a gauge perspective). It just feeds a dummy light.

It does trip the message center about low oil pressure and I think it does put it into a limp mode...

Michael Westerfield · **Posted:** Tue Nov 04, 2014 12:49 pm

RodneyWright wrote:

Michael Westerfield wrote:

RodneyWright wrote:

I know the car won't run if it can't see oil pressure.

That is incorrect.

Car doesn't care about oil pressure (from a gauge perspective). It just feeds a dummy light.

It does trip the message center about low oil pressure and I think it does put it into a limp mode...

It didn't on my 08'. When I installed my autometer gauges I orginaly ran the car without the oem sender hooked up. Further more, it isn't a gauge, but a switch that is normally open. It only "closes" when the oil pressure is low, so without the sender plugged in the car doesn't even know.

You must have had the sender plugged in, but uninstalled creating a low pressure condition.

DickRasmussen · **Posted:** Tue Nov 04, 2014 1:46 pm

Charlie Guthrie wrote:

Dick asked

Quote:

Why a gauge if the car won't run without oil pressure?

That's so in the middle of a long sweeper they can look down and say,"OMG, I need an Accusump."
Charlie G

Won't the rod rattle tell you that? :lol:

It did with my Lotus when the oil was cold before I added a baffled pan in Street Prepared.

Does a 4.6L Ford engine have that issue?

FYI the 2015 Mustang Performance Pack includes anoil pressure gauge . . . I think the correct sensor for Ron's is less expensive overall though.

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