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Old 02-22-2008, 01:08 PM   #1
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Default Oil pressure LED help and 12VDC signal for engine running?

Hey there all I have afew questions and was hoping that some of the experts here can give me a hand with a few issues:

For starters I've installed a BEGi-S kit onto my Cali / Auto miata and am looking to do a few things to feel more comfortable with handing my car over to the wife for daily driving.

The oil pressure gauge is great but really could use an idiot light. Does anyone here know of a way to wire in an LED that will illuminate when there is no oil pressure? Since the gauge is so slow to react, it would be good to have an instant response if the oil pressure goes out. I've heard of a person doing this mod but cannot find a decent write-up on how to do this. I know enough on how to pull the cluster out and install an LED but the wiring is where I'm stuck... can you all help with this, please?

Also, I have installed an autostart system with turbo timer and whatnot. System works great and all but seems that it needs a +12VDC feed that tells the car the engine is actually running. Is there a good place to tap into that will provide this signal? The oil pressure only gives me like +3 to +5 VDC and is not sufficient for the alarm to see. What do you all suggest for this? Could I use the +12VDC feed from the fuel pump as a good indication that the car is running?

Finally, I bought a decent EGT gauge and don't really know the best location to install the thermocouple. Should this go into the exhaust manifold or into the downpipe? Also, what is a good range for healthy readings in your opinion?

Anyhow, this is my first Miata and there are still a lot of areas that I'm green with... so please don't flame me for being new here.

Thanks and cheers all!

Prospero (AKA Matt)
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Old 02-22-2008, 02:12 PM   #2
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Are you thinking the LED turns on when the oil pressure is gone? That's the problem with idiot lights- they're too late. I think what you really want is a low oil pressure warning light... might as well add one for coolant temp too (IMO that would be a more likely problem). You can get the light and sender from most race oriented shops (summit) - just search for "oil pressure warning".

I'd need a wiring diagram to see what actually is ONLY ON when the car is running. I know my fuel pump is powered before the car ever cranks, so that's probably not what you want.

You want your EGT probe in the manifold "collector" as close to the turbine housing as possible. Optimum TIT (turbine intake temperature) is often stated at 1650f under boost. There is quite a tolerance and it's very tune dependent.
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Old 02-22-2008, 04:33 PM   #3
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Agreed that it's too late when the oil pressure is gone but you read between the lines with my request. I'll look at summit and jeggs and see what they have in house for this purpose.

Good point on the fuel pump...I'll need to keep looking for sources that will do the trick.

As for the EGT, I was under the impression that it would be in the maifold but wanted a confirmation to be sure. I hate to drill into places that would not be happy with it.

Thanks and if anyone else has an idea, please shout my way!

Cheers,
[email protected]

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Originally Posted by m2cupcar View Post
Are you thinking the LED turns on when the oil pressure is gone? That's the problem with idiot lights- they're too late. I think what you really want is a low oil pressure warning light... might as well add one for coolant temp too (IMO that would be a more likely problem). You can get the light and sender from most race oriented shops (summit) - just search for "oil pressure warning".

I'd need a wiring diagram to see what actually is ONLY ON when the car is running. I know my fuel pump is powered before the car ever cranks, so that's probably not what you want.

You want your EGT probe in the manifold "collector" as close to the turbine housing as possible. Optimum TIT (turbine intake temperature) is often stated at 1650f under boost. There is quite a tolerance and it's very tune dependent.
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Old 02-22-2008, 05:05 PM   #4
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You could do a few things if your willing to do your research and build some simple circuits. You could buy a stock oil pressure switch at a diff oil pressure. I checked and there's a 10PSI one on ebay, and a search for 15psi switch came up with part number NC 8659 for a 15psi switch. So looks like you could just swap out the switch for a different pressure if you so desire.

All the switch does is send out a ground below X psi, whatever it's rated at. So a 7 PSI switch sends ground below 7psi, and it's open above that. It would be easy to hook up an LED, just figure out what current the LED pulls and get a resistor that will limit it given a 12-14V voltage. + to ignition on, negative to sending unit.

Or you could buy a sending unit that sends out a variable signal, and build a circuit comparator circuit. The sending units are somthing like 10-180 ohms for 0-100 PSI. The circuit would look at the ohms from the sending unit, and compare that with a 0-200ohm rheostat you put in the circuit. When ohms drops below what you specify with the ****, it sends out a ground to light your LED.

Or you could just covert you dummy guage to a real guage. Or add a real guage seperately.
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