Low Pressure Switch?

[Tony Hanna]

I'm wanting to run a relay between my zener diode and the map signal wire.
I'd like to use a pressure switch in the alky injection line to activate the relay so if I run out of alky or the pump dies, the relay will open and break the connection to the zener diode. Hopefully if this works, a loss of pressure in the alky system will cause the stock cutout to come into effect and save the engine. The problem I'm having now is trying to decide what to use for the pressure switch. I have a low pressure cutout switch out of an old set of air conditioner lines. I'd like to use that, but I don't know how well it would react to the alky (methanol).
Any thoughts?
Thanks,

[zin]

Don't know if you've already got a switch, but, depending on the pressure you want to use, an NOS 15750 fuel safety switch would likely do the trick. It's pre-set at 4 PSI but is adjustable up and down a few PSI. It's a normally open switch so it'll only make contact if there is pressure. You can find them at alot of speed shops and should only be about $35.00 including the adapter fitting.

Mike

[Una]

I'd imagine the AC switch would be just fine. The environment inside an AC system can get highly acidic when moisture gets in there.

[Tony Hanna]

Cool, I'll go with the AC switch then. I'd like the cutout pressure to be a little higher. I'm guessing that by the time the pressure in the alky line dropped to 4psi, I'd be in trouble. I was thinking something in the nighborhood of 25-30. I think the AC switch is somewhere around that.

[Una]

Yeah.. Chevy ones have an adjustment screw to raise and lower the cutoff point.. ;)

[zin]

If the A/C deal doesn't work for whatever reason, there are other switches available, 14, 35, 50PSI normally closed and open; all about the same price. I just used that particular pt# as it is very common and didn't know what pressure you needed. I would expect that if you ran out of liquid the pressure would drop to practically nothing in a heart beat. Bottom line though, is you need to run what you feel comfortable with.

Mike

[Tony Hanna]

Thanks for the ideas guys! I think I'm gonna go with the Dodge AC switch to start with because it's what I have. Hopefully the only time this setup will come into play is when I test it with a multimeter, but if it works, it'll be nice to have as a safeguard.

As for the liquid pressure dropping, I'm kind of concerned that the switch would read boost pressure back through the line if the alky failed. If I'm running a 4 psi switch and it's seeing 20 psi of air pressure, it would stay closed right? I'm sure a checkvalve between the switch and the nozzle would eliminate that, but I'm trying to build this mostly out of junk I have laying around.
Thanks Again!

[zin]

If I'm running a 4 psi switch and it's seeing 20 psi of air pressure, it would stay closed right? I'm sure a checkvalve between the switch and the nozzle would eliminate that, but I'm trying to build this mostly out of junk I have laying around.
Thanks Again![/QUOTE]

Yup, didn't think of that, don't really have a way to boost compensate the switch, so you would have to use a switch set to just higher than max boost for it to work like it should. Good call!

Mike

[Tony Hanna]

Cool! Now all I need to do is get off my butt and start building it. I haven't got nearly as far as I'd like to have because of the move to the new house and being out of work. On top of that, the only flat ground where I have a place to work is the garage which is piled full of our stuff until we get the floors finished. Hopefully things will come together soon and I'll get a chance to get started.