MSPNP, intermittent loss of RPM signal.
 

Yesterday the car wouldn't start. Just cranks and cranks. Booted up laptop to see what was going on and then it just started. Drove around for quite a bit for 2 days with no problem. Parked it last night, and then this morning go to start it and same thing happened. Reconnected the laptop and I'm finding no rpm signal. Unplugged cam sensor and coil and reconnected, just to make sure the connector was fully seated and there was no change. Before I start pulling my hair out is there something that commonly happens to the MSPNP that causes this? Something I should check first? It's been working fine since August.

Nothing common that I know of. I'd look to the CAS itself. They are known to fail, although it's not SUPER common

No RPM signal in MegaTune is definitely indicative of a problem with the CAS signal. You can ignore the coils and the rest of the ignition system- they're not causing this.

I'd spend a little time triple-checking the CAS connections, both at the CAS itself, at the ECU connector, and at the junction connector between the two, X-17, whose position I unfortunately do not know on a '94. (Perhaps someone with a complete '94-'95 FSM can chime in?)

One issue which is not entirely unheard of with MegaSquirts in general is that they are somewhat less tolerant of flaky CASs than the stock ECU appears to be. Normally however this manifests itself as an occasional misfire, rather than just a "no RPM" condition. edit: Sav beat me to it.

Power to the CAS comes from the same wire that powers the MS (red/white, from the INJ fuse, via the Main Relay) so if the MS has power, the CAS should have power.

Short of a failure of the MS itself (which seems unlikely) the CAS circuit is about the only thing that could be causing this. Well, that, or a broken timing belt. :rolleyes:

Well I guess with 155k miles a dead CAS is believable. Anyone got a pinout so i can test it? It's an optical unit right?

The white / red wire is +12, the black / green wire is GND, the white wire is CKP (4 pulses per revolution) and the yellow / blue wire is CMP (two pulses per rev.)

To test it on the car cranking, you need an oscilloscope. In fact, to really thoroughly test it you need a scope to see if it's doing anything ugly. You can do a quick-n-dirty test with a voltmeter by turning it over slowly.

Both outputs (CMP and CKP) are open-collector, meaning they require an external pullup to operate, and provide a closure to ground when active. You can connect a 1k resistor between +12 and the output under test to simulate a pullup.

Actually, an idea test would be to forget the pullup resistor and test it in-car letting the MSPNP provide a pullup. This will require tapping into the CKP and CMP pulses, which can be done by inserting a very fine wire, such as a single strand from a 20ga stranded cable into the connector, bending it over, and plugging it in. CKP is at position 2E and CMP at 2G on your ECU connector.

Thanks. I'll check it out later when I get home...if the problem still exists. Seems like it works the same way as the CAS sensor on my 626, which I had to wire up for Megasquirt, pullup resistor and all.

I just checked and a CAS is $400. Used ones are between $150-$175. I'd like to make damn sure I'm right before I buy one.

You can probably buy one from one of us much cheaper.... I just sold one for $50

My CAS used to do this because the spring loaded tabs inside the socket that keep a positive connection with the pins on the CAS lost their spring. I solved it by bending the pins on the CAS ever so slightly towards the front of the engine to maintain a firm contact.

Hope it works for you.

P.S I also have a working optical CAS from a 1.6 that will work on a 1.8 with MS. PM me if you want.

Yep. CAS is dead. First thing I did is plug the stock computer back in to see if it would start. Nope. Then I grabbed my DVOM and probed the connector with the key on. I'm seeing 12v, the pullups on the signal circuits, and the ground. So MS and the harness seem to be in order. Then I ohmed out the sensor and I'm getting an open circuit inside the sensor. I tried taking it apart to see if there was something I could possibly fix inside of it. Turns out it's a hall effect sensor instead of an optical sensor like I thought. I just gotta get a new one.

Oh and ironies of all ironies, the first autox of the season is this Sunday, and my car is now boosted as of 2 weeks ago. I really really want to make this event so I'm going to try and find one of these things this weekend. That's my mission.

Expect a PM on the Mob from Bryan.

Thats actually normal. Hall effect sensors on the miata are in an open collector configuration, when they generate a signal they ground their output. Also I dont think that you would read a resistance on the 12V and GND pins. I'll check my working one tonight for you.

Took a trip out to planetmiata today to pick up a used CAS. That did the trick. Car's alive again. :) Thanks for the help guys.