Let's make the stock oil pressure gauge real

[markolson]

For those of you who don't know it, the stock oil pressure gauge is really just an idiot light. The sender is a switch that closes and turns that gauge on when the oil pressure is above 6PSI and turns the gauge off when it is below 6PSI. This is a waste of valuable cockpit instrumentation space.

I am an electrical engineer and would like to characterize the stock Mustang oil pressure gauge to see if it would be possible to develop a circuit that would allow us to change the sender to a real pressure transducer and make the stock oil pressure gauge work like a real oil pressure gauge.

In order to do that characterization, I need a couple of donor oil pressure gauges from a 96-98 Mustang. One or both may be destroyed during the characterization, so I can't guarantee they will be returned in good shape.

It is possible that the gauge is too crappy to use, so no promises. But I'd like to do some exploration. I'll be glad to share what I learn with all.

If anyone has an old instrument cluster with a working oil pressure gauge they would be willing to donate to the cause, let me know.

Thanks.

[SUP3RSTANG]

Originally Posted by markolson

For those of you who don't know it, the stock oil pressure gauge is really just an idiot light. The sender is a switch that closes and turns that gauge on when the oil pressure is above 6PSI and turns the gauge off when it is below 6PSI. This is a waste of valuable cockpit instrumentation space.

I am an electrical engineer and would like to characterize the stock Mustang oil pressure gauge to see if it would be possible to develop a circuit that would allow us to change the sender to a real pressure transducer and make the stock oil pressure gauge work like a real oil pressure gauge.

In order to do that characterization, I need a couple of donor oil pressure gauges from a 96-98 Mustang. One or both may be destroyed during the characterization, so I can't guarantee they will be returned in good shape.

It is possible that the gauge is too crappy to use, so no promises. But I'd like to do some exploration. I'll be glad to share what I learn with all.

If anyone has an old instrument cluster with a working oil pressure gauge they would be willing to donate to the cause, let me know.

Thanks.

i have one from my 01..would tat work

[DTShkr]

Genious. PLEASE do a detailed write up on this if your results are successful. GOOD LUCK!

[so low 5.0]

does this apply to the fox body unit as well

[01 blue bullitt]

that would be awesome best of luck ill try to find a cluster

[atari]

I have one out of the junk yard dash, its a V6 unit but I dont think it matters. I have no issues with hacking it apart, I was never supposed to get a cluster with the dash but it cam with one anyway. I was all happy for a spare, then I noticed the speedo.............

[Erich]

Is the problem the guage or the sender? I could see how the sender just sends an all or nothing signal to a perfectly good gauge. The 5.0 ones are real-correct? They certainly have a much different style sender. That thing looks like it has to be doing something.
Erich

[novanutcase]

Subscribing.....

John

[markolson]

Here's what I know so far.

I am working off a 96 Mustang Helms manual and the 96 Mustang EVTM plus what I have found on the forums. I have an 04 Mustang Helm manual and wiring manual, but all I have found there is the oil pressure switch. There are no details for the 04 gauge itself in those manuals, at least that I could find.

Somewhere in the neighborhood of 1989, Ford went from a pressure transducer driving the gauge to a pressure switch driving the gauge. After that, I know they put a plastic pin in the Fox body gauges to hold them at the high-normal reading when the pressure switch is closed, rather than going all the way to the top of the gauge.

The older Fox senders were variable resistor style pressure transducers. Based on the circuit diagram, it looks like the transducer varied the current flowing through an ammeter to show you the actual oil pressure. The resistance would vary from 9.7 ohms at the High mark to 74 ohms at the Low mark. In the newer ones, such as diagrammed in my 96 EVTM, there is only a switch to ground and an internal 20 ohm resistor. Other than that, the gauge mechanism for the old gauges look the same as the new gauges.

It also seems that the old variable resistor pressure transducers were notoriously unreliable.

I suspect that all of the gauges from 1990 and up are basically the same and all have the 20 ohm resistor and the pin.

I think we have a number of options here.

1. If we want full travel of the gauge, the plastic pin will need to be broken off if ford still puts them in. The 20 ohm resistor may have made the pin unnecessary. I need to look inside one to know for sure. We can run with limited high end range if we don't want to take the cluster apart. I am more interested in too low oil pressure than too high, so I am not sure yet. If anyone is willing to live with limited upper range, this may not be needed.

2. It is possible, if we can find a pre 89 presssure transducer sender, that it will replace the switch, and just work, although the calibration may be off due to the 20 ohm internal resistor. I also need to look inside the cluster to see if that 20 ohm resistor can be shorted out to eliminate it from the circuit.

3. If I can characterize the way the gauge works, It may be possible to make a small circuit that will take the accurate voltage output from a modern strain-gauge pressure transducer such as the ones Autometer uses, and translate that into the upside down current signal that the gauge apparently needs to operate. If this is the case, we might be able to make the stock gauge pretty darned accurate.

If anyone has shop manuals or wiring manuals that show the oil pressure gauge circuits for other years, it would be helpful for me to see them.

I will PM Atari and sup3rstang my address so they can send me the units for me to examine. If anyone wants to send me a Fox body unit, I'll be glad to look at it too. I just need to know what year it is from.

I promise I'll do a full writeup with pictures. I think this will be a fun project.

Thanks for your help

[markolson]

I will be documenting this project here: http://www.classictiger.com/mustang/...ssureGauge.htm

Here are the schematics I have:

Old Fox Body:



96 Mustang:



03 Mustang:

[chopper1]

What happens to the "guage" if you pass a low voltage signal to it?

If the needle moves in reference to the different voltages couldn't the sender be adapted to control the voltage to the stock guage? A good power supply would go a long way here to keep from frying a good guage.

[atari]

Ok, I got the PM and I will be sending it out in the next couple of days. Also, I work for a manufacturer of strain gage pressure transducers. Also, I have a very good source for technical info from ford and I have spoken with him and hes is very interested in this also. Anything I can do to help just let me know. -atari

[MFE]

Good stuff. FWIW I think the change to the guage from actual reading to idiot light with a needle took place no earlier than 1993.

[markolson]

Originally Posted by chopper1

What happens to the "guage" if you pass a low voltage signal to it?

If the needle moves in reference to the different voltages couldn't the sender be adapted to control the voltage to the stock guage? A good power supply would go a long way here to keep from frying a good guage.

Actually, it looks like it is an ammeter, not a voltmeter, so varying the resistance in series with the ammeter varies the current you get through the meter. Higher current gives you more needle deflection.

What we need to make this thing accurate is to determine how much current is required to deflect the meter to each point on the face. Then we can design a circuit that will take the voltage reading that comes out of a modern pressure transducer and translate that voltage into a calibrated current level to drive the ammeter.

Then we'll need to try several gauges to see if they are consistent in the calibration regarding how much needle deflection there is for any given level of current. If they are consistent, we can calibrate the circuts when we make them and just plug them in. If not, each circuit will need to be calibrated to each gauge.

Then we'll need to understand the effects of temperature on the gauge to ensure we don't need to add temperature compensation to the circuit.

[markolson]

Originally Posted by atari

Ok, I got the PM and I will be sending it out in the next couple of days. Also, I work for a manufacturer of strain gage pressure transducers. Also, I have a very good source for technical info from ford and I have spoken with him and hes is very interested in this also. Anything I can do to help just let me know. -atari

Atari, thanks for your help. I look forward to getting the gauge on my bench.

I'll need a data sheet for the pressure transducer we'll want to use. This leads us to the next question. Autometer sells oil pressure gauges with ranges or 0-100 and 0-150PSI. I need to know what ranges are available in your strain gauges. We will then need to decide which range or ranges we want to design for.

For those of you with aftermarket oil pressure gauges, what pressures do you nromally see during operation and what are the max pressures you see?

[BULLITT1992]

Good work!

[nickmckinney]

Here is a thread with pictures from TCCOA where they did the same thing to the Tbird chassis. It might be near the same fix for the Mustang.

http://forums.tccoa.com/showthread.php?t=75977

[markolson]

Originally Posted by nickmckinney

Here is a thread with pictures from TCCOA where they did the same thing to the Tbird chassis. It might be near the same fix for the Mustang.

http://forums.tccoa.com/showthread.php?t=75977

Great pointer. It looks like it is exactly the same as what we are trying to do. The only question is how well those senders work and what is the calibration.

I'll pick a sender up this afternoon for testing.

Also, it looks like the Mercury clusters up to and including 1996 have the 20 ohm resistor on the circuit board, while the newer ones have it inside the gauge itself. We still have more research to do. If anyone can look at their clusters to see if the resistor is on the circuit board, and then report what years have it and what years don't, then we will have more information for our Mustangs.

[Cougarstang]

Pretty cool idea. Reminds me of my abandoned pet project of getting Mark VIII illuminated cruise buttons to work with our mustangs. I couldn't figure out how to power the lights with headlamp/parking lamp engaged without swapping out the steering column. Maybe a possible next project for an electrical engineer?

[markolson]

I bought an old Fox Body sensor (Borg Warner S334, $21) today, and put a compressor quick release fitting on it:



I hooked it up to my compressor and a digital ohm meter, and took data at different pressures. (70 PSI was as far as my old compressor would go)

Here is the data I got:

PSI   Ohms
70     17
65     18
60     20
55     23
50     21
45     24
40     27
35     27
30     26
25     26
20     31
15     40
10     32

Somewhere under 10 PSI (my compressor pressure gauge is not calibrated below 10 PSI), the resistance goes infinite, so there is also a pressure switch built in, probably set at about 6PSI.

Here is a chart of the data with a linear regression trend line showing that resistance does generally increase as pressure decreases:



The good news is, if you are not too fussy, the old Fox sender looks like it will work to show pressure increases and decreases. The bad news is it will not be very accurate.

I plan to continue this project to create a circuit for a modern sender that will be accurate.

But for anyone who wants to have a functional (if not very accurate) oil pressure gauge, it appears that all you need to do is solder a wire across the 20 ohm resistor in your oil pressure gauge circuit and use the old Fox sender and it will work.

[chromeyellow]

cool project, I think I've got one in my parts bin. I'll get back to you

[markolson]

Originally Posted by SUP3RSTANG

i have one from my 01..would tat work

From the SVT & DOHC forum, I learned that Ford went to a microprocessor controlled instrument cluster in 99, so it appears that 99 and up are completely different circuits and will not work. I'll be glad to look at an instrment cluster to see if there is a hack that might work, but it will probably be different from the older units.

[BlackSunshine02]

Subscribing... also

[atari]

my cluster is out of a 99 according to MPS.

[markolson]

Atari, If you still want to send the instrument cluster, I will figure out if we can still hack it.

Still looking for a 96 instrument cluster. None of our local junk yards have any Mustangs. Grrr.

[Floordford]

Another problem that should not be over looked is that the fox sending units have a standard style threading and the modular engines have a tapered thread. I found this out when i put an older unit in my 2000 gt and cracked the oil filter housing.

Im also curious to see how the stock unmodified cluster would react if a changing voltage was applied at the sending unit plug. Then you would definately know all you had to do is mess with the sending unit and the stock cluster could get left alone.

[bs7781]

i have read a couple threads and i am still a little unsure what is happeneing with my oil pressure guage. At idle it drops all the way below the low red mark then i tap the gas and it jumps back up to normal pressure then when it idles back down the pressure drop below the low red mark. Anyone else have the same issues?

[atari]

Originally Posted by bs7781

i have read a couple threads and i am still a little unsure what is happeneing with my oil pressure guage. At idle it drops all the way below the low red mark then i tap the gas and it jumps back up to normal pressure then when it idles back down the pressure drop below the low red mark. Anyone else have the same issues?

I would say that your sender is bad or you actually have an oiling issue, I would get the oil pressure checked, Could be ugly.

[Floordford]

yeah i had a bad oil leak i couldnt find and the same pressure issue. It turned out to be all from a bad sending unit. The unit leaked oil all over the place and id clean everything up and the wind would throw the oil around to where i thought i had a bad oil pan gasket.

[bs7781]

just got it fixed. it was a bad oil pressure sending unit. i bought a mechanical gauge and hooked it up and it reads 10psi at idle when the car is at operating temp.

[markolson]

Originally Posted by Fastangboi

Another problem that should not be over looked is that the fox sending units have a standard style threading and the modular engines have a tapered thread. I found this out when i put an older unit in my 2000 gt and cracked the oil filter housing.

So the newer oil pressure switches do not use a standard pipe thread fitting? That is very interesting.

Originally Posted by Fastangboi

Im also curious to see how the stock unmodified cluster would react if a changing voltage was applied at the sending unit plug. Then you would definately know all you had to do is mess with the sending unit and the stock cluster could get left alone.

I was planning on connecting my circuit to the sender plug. It will be easier than trying to run the wiring under the dash.

By my estimation, if you leave the stock cluster alone (with the 20 Ohm resistor still there) and use the Fox sender, the oil pressure will vary from about 1/4 of the way up from Low at 15 PSI to a little over half way up at 70 PSI higher above that. If you remove the 20 Ohm resistor, then you should see the gauge read at about half scale at 15 PSI, going up to about 1/8 of the way below High at 70 PSI.

These estimates are based on the resistance readings I got from the sender I bought and the meter diagram above. I will validate this when I get an instrument cluster to play with.

[Christopher Ihara]

If I recall correctly the 1999+ cars the instrument cluster (HEC) itself is programmed to simply act as an on-off indicator. What I can't remember is if the actual oil pressure is located in the GEM or the EEC-V. If I remember from talking to a Team Mustang guy the reason they turned off the true "pressure reading" is because 99% of the customers aren't smart enough to figure out that oil pressure is going to drop after going to idle or running hard and getting the oil hot. It also presented a lot of warranty issues with inaccurate sensors needing to be replaced, thus they simply turned it into an idiot light.

The way to make it work properly is to replace the pressure sensor with a variable rate unit and then reprogram either the HEC or the EEC. Which he said could be done.

[markolson]

Originally Posted by Christopher Ihara

If I recall correctly the 1999+ cars the instrument cluster (HEC) itself is programmed to simply act as an on-off indicator. What I can't remember is if the actual oil pressure is located in the GEM or the EEC-V. If I remember from talking to a Team Mustang guy the reason they turned off the true "pressure reading" is because 99% of the customers aren't smart enough to figure out that oil pressure is going to drop after going to idle or running hard and getting the oil hot. It also presented a lot of warranty issues with inaccurate sensors needing to be replaced, thus they simply turned it into an idiot light.

The way to make it work properly is to replace the pressure sensor with a variable rate unit and then reprogram either the HEC or the EEC. Which he said could be done.

From what I see in my 03 Cobra schematics book, the oil pressure switch wire goes straight to the HEC (Instrument Cluster) and it doesn't appear that the EEC-V has anything to do with it, unless the cluster processor does some sort of other communcation with the EEC-V regarding the oil pressure data.

If anyone sends me a 99+ instrument cluster, I will look into it. I am still looking for a 96 cluster, BTW.

[IRSmart]

somewhere i have a cluster out of a 2003 automatic Mach 1 that i took the needles off of to get to the gauge faces. currently it is just a cluster with the needles not attached, and my original GT face in it. if something like that will work for you, send me $20 and you can keep it.

[markolson]

Originally Posted by IRSmart

somewhere i have a cluster out of a 2003 automatic Mach 1 that i took the needles off of to get to the gauge faces. currently it is just a cluster with the needles not attached, and my original GT face in it. if something like that will work for you, send me $20 and you can keep it.

I was hoping for a cluster that had never had the needle removed from the oil pressure gauge. If I put the needle back in the wrong place, I won't be able to calibrate my circuit for stock gauges. (assuming I can make it work at all for the newer gauges.)

Thanks for the offer though.