Activating a solenoid with a PAC?


New Member
Dec 17, 2005
Well, I'm new here, but not exactly a newbie (I did extensive searching before starting this thread).

How are you guys activating a solenoid with the PAC (alkycontrol kit)? Options that seem viable to me, in order of preference, are:

1: Mosfet circuit that runs off of the 'pump on' LED to turn on a 12v solenoid via relay. (Problem here is, I don't have a schematic. Ideally, somone could post a schematic and parts list, but that might be asking a bit much..... :rolleyes: )

2. Run a 12v solenoid in parallel with the pump. (Problem here is, will it open at low pump voltages?*)

3. Standalone relay circuit with HOBBS switch to open at low boost pressures (4-5lbs). **

* - I have an M90 roots type blower, and it is a damn heat gun. I can get some good flash KR at the start of a WOT run, and really need reliable instant activation. I also have some TCC lock/unlock KR when stepping on it from a roll, very high engine load instantaneously, coupled with instant boost, the combination of going slightly lean and that high load is a recipe for KR. I have a check valve, but would prefer to use a solenoid.

** - This is viable, but I am worried about siphoning since my nozzle is behind the throttlebody, and its not a turbo car so it is under constant vacuum (nozzle is before the compressor). So if I open the solenoid before the pump is on, it could siphon a little and then I would have air in the lines, and that brings me back to the flash KR problem. This effect would probably be neglible, but a concern nonetheless. I will probably go this route if I cannot find a schematic

I can tune, and have addressed the flash KR problem as well as I can, at the expense of some gas mileage :rolleyes: . Not the ideal situation. Lower tps% for PE activation, more acceleration enrichment, initially richer PE vs time table.....Basically any way to add fuel is marginally effective, and is hurting my gas mileage.

The grand prix community hasn't really embraced methanol injection in the past, I believe due to (up until fairly recently) the lack of mainstream tuning equipment, and lack of favorable results without tuning. One of our vendors recently put out some very well priced tuning/scanning hardware and software, so that might change, especially since water/air intercoolers are basically the only option with an M90 in our configuration, and a good one is about 1000 bucks...... But enough rambling...

So cliff notes, if someone runs a solenoid off of a PAC controller, and has a working schematic, I would really appreciate it. If not, I think I am doing a simple standalone circuit with a low pressure HOBBS switch and a relay, that approach is sounding better by the minute. So in that case, this is just a hello post from a new member :) Any advice or opinions either way is appreciated, thanks in advance.
I sell a module for 15.00 that does this.

If you want to build your own, the best I can do is give you a descipt.

You'll need a Mosfet like an IRFZ44, two resistors a 56K and a 1k, make a small heatsink(small piece of aluminum) and then the wiring as follows.

You'll need to unhook the LED from the Gray wire on the PAC.

Then gray wire from PAC to one leg of 1k resistor. Other leg of 1k to Gate on Mosfet. From the Gate leg to Drain leg connect a 56k resistor. Drain leg will go to ground.

The solenoid you'll wire a diode across it, 1n4007 is fine. One leg of the solenoid will go to 12 volts. The other to the Source on the Mosfet.

When you apply a small voltage to the Gate, the source and drain connect and activates the solenoid.

Now for the Gray wire going to the LED.. two ways. One is wiring the LED across the solenoid. That way when the solenoid has activated..your LED is on. You'll need a 1k resistor in series with the LED so it doesnt cook the LED.

The other.. get a 1k resistor and attach to the Gate on the mosfet as well. Leaving the black wire on the LED grounded.

If any of the above is confusing or in question... buy the module :wink:
Well, it all makes sense, but without seeing a schematic I am useless... No mosfet experience.

$15 isn't bad at all, so I think I will go that route. How do you prefer to do the transaction? You want me to PM or email you and go from there? I will check back late tonight or tomorrow. Thanks
Contact me off board for business please.

Phone is listed below and on website..

Didn't want to waste your time during the holidays, so I built the circuit, final question. The diode on the solenoid points which way? Thanks :confused: :redface:
I'm assuming you mean the diode parallel with the coil? If so it goes from the more negative leg towards the more positive leg (band facing more + leg). If your using the FET to switch the ground side (again assuming N-mode devices here), the diode goes from the drain/coil to V+ with "band" facing V+.

If your using a BJT, it will go from the collector/coil connection to V+ (band facing V+). Note: with a FET, this diode is even more critical! There is also an "improved" avalanche diode method to do suppression which is a little better, but probably not really needed.

Thanks for the reply. I assumed that the purpose of the diode was to avoid applying a backwards voltage to the mosfet when the solenoid is deactivated, and in that case it would not allow voltage to flow back through the ground...... So it should allow voltage to flow towards the +12v side of the solenoid.

I will figure it out, I built two of the circuits in case I somehow fry one (a voltage on the wrong side of the mosfet should fry it pretty easily). You know the saying, why build one when you can build two for twice the price
If you need any help, lemme know... i gotta get some mileage outta this edumakashun. :p