I *think* you hosed U1. The 2N2222 is likely fine, unless you did something like hook your coil to the A/C command input...

That's exactly what he did. The coil of the purge valve solenoid was jumpered into the A/C command input with no protection against flyback voltage.

Yeah, he used 1K and not 1G, which would have had been protected.

So I should start by locating the faulty transistor? There's only 4 on the board so it shouldn't be too difficult.

Yes, check all the 2N2222's on the MSPNP daughter board.

Readup on "flyback voltage" or "protection diode" (Wikipedia works fine). In the MS1 Extra Hardware Manual, there are a lot of examples of flyback voltage protection diodes. See, for example, the circuit for "Outputs 1, 2, 3 and 4." The 1N4002 in that circuit is there to dissipate flyback voltage. To be safe, you need something like this going across the coil of your purge valve solenoid. Note that 1N's are pretty much interchangeable, with the larger numbers able to resist larger reverse currents. I typically use 1N4004's, as they are readily available in packages of two at Radio Shack.

So, I went back and looked at the thread that started this whole thing:

https://www.miataturbo.net/showthrea...ht=purge+valve

I think there is some information missing from this. In fact, there seems to have been at least one other person on that thread who experienced Dutton's problem (although his failure mode was AC clutch not engaging instead of stuck engaged). I'm getting ready to do this myself to give a little more kick when I engage A/C. I really don't feel like replacing my compressor or burning up something in my MSPNP.

First of all, a little bit of wiring definition because the ECU pinouts have changed over the years:

Wire ** Function ** (1.6 ECU Pin/1.8 ECU Pin)
Y/R ** Ground for Purge Valve ** (2X/4T)
LG/B ** AC Command Input ** (1Q/1K)
L/B ** Ground for AC Relay ** (1J/1G)
W/R ** Power for ECU and Purge Valve ** (1B/4B)

Joe's design jumpers Y/R into LG/B without providing for flyback voltage protection. Brain has suggested that Y/R should be jumpered into L/B, which makes more sense to me. What I am after is activating the purge valve when the A/C clutch engages, and L/B is closer to the AC clutch than LG/B. In addition, L/B has some flyback protection because it is designed to be connected to a coil whereas LG/B is only designed to be connected to switches.

However, I don't think the existing flyback protection on L/B is necessarily sufficient, because the purge valve solenoid can be powered in run and start, whereas the AC clutch can only be powered in run. You might potentially get flyback voltage damage during start in this case.

So, I would propose the following implementation and would appreciate feedback.

First, use a diode jumper to ground the Purge Valve Solenoid when the AC Clutch is engaged (note: diode used to prevent a ground on Y/R from interfering with L/B function):

Y/R ----->|----- L/B

Second, a diode jumper to dissipate flyback voltage on Y/R and protect L/B:

Y/R ----->|----- W/R

I think this will be safe. Thoughts?

Ok, so I may be getting another MS in a trade with a guy building a track car. If that falls through I'm contacting DIY about repair, and if that falls through I have a plan.

So the LG/B wire and the Blu/Blck wires that go to the ECU are just grounds correct? Other than connecting the two all the ECU is doing is turning on the seconday fan.

So what if I just wired those two directly together? When the dash switch is turned on it would ground the A/C relay directly which would turn the system on. If the pressure or temperature switch get out of range it would turn the compressor off. Since the Blu/Black wire only grounds the relay I can't imagine there's a ton of amps running through it so it should just ground through the switches, thus bypassing the ECU completely.

Now, what am I missing? There's no way it's this simple.

No I meant ignition coil 25k+ volts. My money is on U1 needs to be replaced, and likely that's it. LM324N. Come by and I will give you one.

I would also check the car's a/c switch to make sure it still goes to ground when depressed.

It pretty much is that simple.

Here's the gist, the LG/B wire is an INPUT. The stock ECU uses this input for many things. You already mentioned the fan. It also kicks up the idle speed internally and it grounds the L/B OUTPUT to the AC relay.

In our case, the MSPNP connects the LG/B INPUT to the L/B OUTPUT and takes care of the fan. There is no idle speed compensation. We're trying to assist that with the purge valve, but you got burned because you connected the purge valve solenoid to an INPUT instead of an OUTPUT.

If you do what you suggest, you still have to handle the fan, but otherwise it should work (you can also wire the purge valve into that connection without worry). It's a bit more wire surgery than I want to do on my car, however. I'd like to leave the wiring and connectors as intact as possible and add my jumpers and circuits inside the MS. That's just me though.

I think you would be surprised at how much flyback voltage a simple solenoid coil can generate. Not much power, but a heck of a lot of voltage.

My fans are wired in parallel so that's not an issue for me. This isn't by any means my #1 choice. First I'm going to check U1 and the dash switch like Ben suggested. Also I'm hoping this trade goes through and it will fix a lot of problems for a few people (my unit would go to a track car).

I'm aware. But I also have access to the schematic. ;) U1 would take the hit.

Thanks for the offer on the chip Ben. I live so far south I'll probably just have to order one though (the only day I have off is Sunday and you guys are closed).

Is there anyway to test the chip before I pull and replace to confirm?

Ahhhhh . . . . Access to schematics, eh? Sneaky.

BTW, as long as the schematics are in front of you . . . care to weigh in on my suggested use of diodes to make this safe? In particular, is the isolation diode between Y/R and L/B needed?

I figure you owe me since you were so coy.

Whoo hoo Ben it looks like you're spot on. So I found another user who had this exact same problem (only they have a 95) and it was the same op-amp you were saying. Hopefully (fingers crosses) all I have to do is locate this chip on my daughterboard and replace.

Of course my compressor is probably still fried but at least that's something I know I can fix. Besides, my a/c had a slow leak anyway so it was probably time to replace all the o-rings.

I plan to order the chip tomorrow (maybe I can send DIY a few bucks so shipping will be fast) unless someone knows a local place to buy them. It looks like radioshack sells an LM324 but is that the same as a LM324N?

probably not. the DIY one I assume is an 8 pin, not 14pin.

Glad you found your problem. So, you definitely had a flyback voltage issue. You're probably OK with the RS part, but if I were you, I would get the exact op-amp from Ben. Unless you did the design or have access to design data, it can be hard to tell why an exact variant of a chip has been used.

Also, since you're talking to Joe about this, can you run the diode jumpers I posted above by him? I want to do this, but I don't want to repeat your experience. I dabble in electronics, but Joe is a EE (I think), and I'd feel a lot better if a trained person would weigh in. Thanks for creating a thread about this. You've certainly helped me.

To protect from flyback when the solenoid closes? Probably couldn't hurt to put a 1N4001 between Y/R and +12V.

Drew, send an email to websales@diyautotune.com with your shipping info, and I'll send it to you.