Leeroy_25

I want to run an aftermarket oil pressure gauge in my car but do not want to run two sender units. I also would prefer that the factory gauge was left not working in the dash. So I want to check that if I put in the new sender unit and then wire both gauges to it will the new gauge function correctly or will the old gauge draw current away from it and make it read incorrectly. I don't mind if the OEM guage reads incorrectly but I prefer it just does not sit idle.

Thoughts please.

Thank you

misterstyx69

iTrader: (142)

Kind of hard to answer when you do not list what series of car you have.
But then again you could always get a oil pedestal adapter and put a sender there,retain the old sender with it's gauge intact.

Akagis_white_comet

iTrader: (4)

I'm a bit confused here as to what your goal is. You wish to use the stock gauge sender on a new gauge, but you say that you want the stock gauge KO'ed and then say you want it to work and don't care if it's off, then you say that you want to run a second sender...

The stock senders, no matter which series you have, all vary their resistance based on pressure. This value is then translated into a voltage reading by a Voltage Divider housed inside the gauge cluster. I'm not sure if the cluster does this using 12v or has a stepdown converter to give the gauge 5v for the divider,

I don't believe it would "pull power away" from the stock gauge or vise versa enough to cause a visible discrepancy, even if power was coming from the Meter fuse.

A 3-wire gauge sender has its Voltage Divider inside the sender, making the gauge itself just act as a rescaled voltmeter. The only difference between this and the stock setup is where the voltage divider is located. Mazda just put it inside the cluster so it wouldn't be subject to heat induced failure as easily. Probably easier to package it this way too.

Using the FC as an example, its gauge and sender read Infinite ohms at 0psi and 80 ohms at 110psi. Just by mathematics, it's impossible to find what half of Infinity is, so let's substitute a known large value. On my gauge tester device, I used a 100k potentiometer for this. At 100kOhms of impedance, the gauge was flatline to the best of my observation. The exact ohms may be lower than this, but for this example, let's say it is 100k for clarity.

So to make a Voltage Divider, a 50k resistor would be suitable. This emans that hen the oil pressure sender has 50k ohms of resistance, the new gauge (which is a re-badged voltmeter) will read half of the supply voltage. If using a 5v supply, the gauge will be seeing 2.5v from the sender. Ideally, you want it to read at the middle of the sender's pressure range when in the middle of its resistance range. Since the stock sender reads 0-110, 2.5v should point to 55psi...

The stock S4/S5 sender reads 141.2 ohms at 60psi. So based on this, the Pressure vs Impedance readings are not linear, but logarithmic (exponential curve). If I were to take a guess, the stock gauge's voltage divider resistor would be 150 ohms and 1/8 watt. That would put the new gauge at 2.5v at just under 60psi. The exact impedance for the resistor you need may be SLIGHTLY different, but you can zero in on it by using a potentiometer, multimeter and an air compressor. Give the sender 55psi and read its impedance.

Leeroy_25

Seems like there is confusion I what I am after. So to start my car is 1994 FD3.
I want to take out the old single wire sender and fit a new after market sender in place of it. I noticed the new sender is 2 wire which may be an issue but I am guessing there is a way around that?
My main goal is to fit the new gauge, which goes with the sender, to a pod so I have a nice accurate gauge. At the same time I would like to power the Oem gauge off the same sender if possible just so it is not sat idle in the dash.
I am assuming I can take a single feed for the OEM gauge off one side of the new sender? I just want to be sure doing so will not effect the accuracy of the new gauge.
I don't really want to run two sender units as they are quite chunky and take up space I might need.

Akagis_white_comet

iTrader: (4)

Sorry, but using an aftermarket sender won't make the stock gauge read properly. The stock gauge is scaled to read the values (see the FSM) from the stock sender only.

However, using the Voltage Divider idea I hammered out above, you can use the stock sender and have it send information to both the stock gauge AND an aftermarket one. Google "Voltage Divider" to learn more about how the circuit works.

If your intended sensor is a two-wire one, I would bet that it is still a Ground-referenced one, functioning just like a three-wire TPS does

misterstyx69

iTrader: (142)

Sort of curious to know just how big an Oil pressure sender would be to "take up space that you might need"?
I have 2 senders in my car and I use an adapter.NO problem.
Your trying to make a vanilla cake mix also taste like Chocolate.
Funny that they make chocolate Icing so you can still have vanilla cake.
(damn it!..now I want Cake!)

Leeroy_25

Maybe I am being a bit picky, but if I can do the job with one sender why fit two? I have been led to believe the stock senders are inaccurate anyway so I may as well do away with it? I am not fussed if the OEM gauge read correctly from the other sender as long as it reads something instead of being ideal. if it is going to sit near on zero then I might need to think again. With my 20B conversion space is at a premium and I will have to fit a map sensor and FPR in around that area. I am sure I could fit 2 senders there if need be but like I said if i can avoid it it would be nice to do so.

Akagis_white_comet

iTrader: (4)

+1 for Styx. The only "big" senders I know of that would be found on a rotary are the stock FC/FD ones. The contemporary GM style 10 bar ones are quite compact for what they deliver, though some aftermarket ones have the diaphragm much closer to the BSP fitting than the stock sender. Might make for some clearance issues depending on how 'busy' the area is.

But back to the matter at hand now. An aftermarket sender/sensor, such as the kind I mentioned above will NOT operate the stock gauge correctly because they are SCALED DIFFERENTLY. Google "Voltage Divider Potentiometer" and the relationship between Voltage, Current and Impedance will make more sense. I would not risk damaging the stock gauge or cluster on it. You have 4 options.

1. Stock Sender with Stock Gauge only
2. Stock Sender with Stock Gauge and Voltage Divider to run Aftermarket Gauge
3. Stock Sender & Gauge plus Aftermarket Sender & Gauge
4. Aftermarket Sender in stock location with Aftermarket Gauge

If space is at a premium and you're running -10AN oil cooler lines, you can add in a pass-through gauge adapter to place the new sender elsewhere. Here is an example of one:
JEGS Performance Products 100530 JEGS Fuel Pressure Gauge In-Line Adapter Fittings - Free Shipping on All Orders @ JEGS

If all else fails, your map sensor doesn't care where it is mounted or plumbed at on the UIM. The 20B's UIM has four barbs on the firewall side and four more on the passenger side. It also has a handy threaded mounting boss near the firewall barbs which would be reused as a mounting location for the map sensor.

Leeroy_25

Thank you guys..
My aftermarket sender is the same size as the OEM one in my case.
I will look up the option of a voltage divider more closely. I just thought stock senders were renowned for being poor and unreliable?!

Cheers
Lee