Bus master and vdo pressure switch

[jcaplins]

For those using the Bus Master for your power and fuel pump switching, I have questions:
(if you're using a Grt EIS4000, even better)

What does your fuel pressure switch circuit look like? what is the input voltage, and fixed resister?

Here is what it should looks like for the GRT EIS to read fuel pressure, but I'm not sure 4.8V is OK for use with the Bus Master:

http://www.grtavionics.com/documents...ux%20Input.pdf

I believe I need to read about 2.5V (somewhere between 2 and 3V) for the Bus Manager to work properly at the switching point.

Setup according to the GRT document above, I measure:
(10 ohms, no power)
0psi = 0.136V (fuel pumps off)
35psi = 1.2V (calculated 100 ohms)
41psi = 1.36V (calculated 120 ohms)

Calculating with 12V instead of 4.8:
0psi = (10 ohms) = .385V
35psi = (100 ohms) = 2.99V
40psi = (120 ohms) = 3.85V

This seems like what I need, but that would mean I can not use the same pressure sensor with the GRT and the Bus Manager.

Anyone else have this issue?

Thanks. Happy Holidays.

[rzbill]

I don't understand why any device other than the GRT EIS 4000 is involved in the fuel pressure sensor connection. 4.8v comes from the EIS, sensor input is EIS, ground is same ground as EIS. Done.

Provide 12v power to the EIS module though whatever master switching system you choose.

***EDIT**** I see a Bus MANAGER from Spruce that asks for fuel pressure input in order to automatically turn on a backup electric fuel pump. I assume you are wanting to manage fuel to an electronic fuel injection system. It's overkill for a Bendix.
I don't see any specification for power input requirements for the pressure signal to the bus manager. You are welcome to change the grt specified resistor as you see fit but just remember that the EIS inputs are max 5 volt and the slope/offset values for the input will change to values you will need to figure out on your own.

An alternative (assuming you have an GRT EFIS for engine display and limit setting of all engine parameters) is to use the EFIS for all limit setting except for the fuel pressure you are interested in. Use the EIS 4000 limit setting for fuel pressure and then use the warning light output as signal to the bus manager. It is a sinking output so you will need to reverse the voltage swings to make a bus manager input.

[jcaplins]

Originally Posted by rzbill

I don't understand why any device other than the GRT EIS 4000 is involved in the fuel pressure sensor connection. 4.8v comes from the EIS, sensor input is EIS, ground is same ground as EIS. Done.

Provide 12v power to the EIS module though whatever master switching system you choose.

***EDIT**** I see a Bus MANAGER from Spruce that asks for fuel pressure input in order to automatically turn on a backup electric fuel pump. I assume you are wanting to manage fuel to an electronic fuel injection system. It's overkill for a Bendix.
I don't see any specification for power input requirements for the pressure signal to the bus manager. You are welcome to change the grt specified resistor as you see fit but just remember that the EIS inputs are max 5 volt and the slope/offset values for the input will change to values you will need to figure out on your own.

An alternative (assuming you have an GRT EFIS for engine display and limit setting of all engine parameters) is to use the EFIS for all limit setting except for the fuel pressure you are interested in. Use the EIS 4000 limit setting for fuel pressure and then use the warning light output as signal to the bus manager. It is a sinking output so you will need to reverse the voltage swings to make a bus manager input.

Thanks for responding. couple answers and clarifications:
- Yes to the electronic fuel injection system and using the Bus Manager to control the primary and backup fuel pumps.
- I don't have a GRT EFIS.
- I do have a GRT EIS4000 that seems to be working perfectly.
- Trying to tap off the same fuel pressure sender to control the Bus Manager.