97 Miata, crank no start, not what I was thinking it was

I have a 97 Miata that died on me driving one day. No fuel, no spark. I tested the CAS, and it is pulsing 5v appropriately. CEL doesn't come on with key on, engine off, but it didn't when I bought it, and it's been a reliable car for 8,000 or so miles. At one point, back when it was running fine, I did my weekly code check, and it was throwing a code that Torque claimed was for ECU main power, but it didn't come back after reset.

Another issue that may be related is my non-functional tachometer. That hasn't worked since I owned the car. PO stated it was due to it having a replacement ECU out of an automatic. First of all, would that affect the tach? Second, do you think that a bad ecu could be at fault?

This is normally stuff I can figure out if I had a shop manual, but I haven't found a decent one yet. Which one should I spend my money on? Do I need multiple?

Best manual is the factory one. Get one of the self-bound half-size ones and it has a wiring diagram. Or get the full size. 1996 and 1997 are basically identical, and eBay has the big 1996 mechanical manual for $75 and the wiring diagram for $50. Grab 'em if you want the good stuff.

If the CEL doesn't come on with the ignition, I don't think you've got power to the ECU. Can you still read codes? Check your fuses.

Fuses are good. I'll check for power at the ecu when I can.

If it's easier, plug in with your scanner. There are multiple power wires to the ECU.

Ooo, main relay. That's a common failure. Check for power on the red/white wires at the injectors with the ignition on. https://www.flyinmiata.com/main-relay.html

If the car has an auto ECU, the CEL will be on all the time, so the PO likely removed the bulb.

Just checked for power at the ECU, and I've got 12v there.

Now, for some reason, the PO left the old ECU in place when he put the auto one in. I did try both ECUs, just to be sure.

12v on all the appropriate pins?

I'll have to check that it's all. I saw it at a couple and called it good.

There is a loose ground there. What is it supposed to ground on? It seems that I'm working through something that was possibly badly repaired in the past.

If it's a ring terminal, you can basically ground it to anything. I forget exactly where the ground points are in a 1997, there are a bunch of them around the car.

Any power at the red/white wire?

I do have power at the white/red wire, both at the ECU and at the fuel injectors. I also have power at the pins listed on this page, and ground at the grounds listed there as well.

What is the proper way to test the CAS on the car? Is there a way to test it without pulling it? I should be able to hook up my VOM to between ground and the 5V lines, and see it pulse, right? I was getting it to pulse before, but I'm not now.

Since I wasn't getting the CAS to pulse when cranking, I ordered a used one from eBay. When I put the old one beside the new one, I noticed it looked different. Turns out my engine has the optical 1.6 sensor instead of the hall effect 1.8 sensor. Should interchange? I've read differing "facts" on the internet.

Now, with the sensor off the head, but connected to the harness, I'm getting the appropriate pulsing to 5v when I turn the sensor, from both sensors. I can also hear it pulse the injectors.

I haven't reassembled it yet, but I'm thinking I should put in the correct CAS since I have it on hand.

Is there any good flowchart for this that isn't in the manual? That's not something I can afford right now, when I really need both of them. All of the ones on the internet are pretty lacking, so I'm relying on how-tos and forum posts to troubleshoot anything.

The CAS are interchangeable electrically, but you may run out of range of adjustment for timing.

I'll put the correct one on, then. Got to get a new seal tomorrow, and I can continue to troubleshoot this thing.

I've also ordered a back probe set and piercing probes so I can troubleshoot this stuff easier. Cutting into the harness like I've had to do is never a good idea.

I just checked my savings, and there's enough in there that I can grab those manuals without worrying too much. That way I won't be in the dark about this car.

Most of the cars I've owned have had the OEM manuals leaked to the internet. What I have found for the Miata didn't seem to be of the quality I'm used to from Honda, Nissan, and even Isuzu and Jeep. Hopefully this is what I'm looking for. If it is, I'll probably scan it so I have an electronic version I can search.

Sweet, I won them for $112 shipped for both. Asking price was $152 shipped for both. I feel I did well.

Alright, it's still not running. FSM said to replace the coilpack, so I did, no change. Time to replace the ECU? That's the only thing I can think of at this point.

No codes? When you hook up your code reader, does it "talk" to the ECU or do you get nothing?

No codes, Torque seems to be communicating with the ECU as normal.

Will torque show if the motor is cranking? I'm not familiar with that ap. I am also not a Mazda guy. I'm a Toyota guy. I think I may have fixed a Mazda once. Did you check ALL the 12V supplies going to the ECU? It would appear the ECU itself is getting 12V or you would not be talking to it. But, it will switch a bunch of stuff like the fuel pump, etc. Also, look at the schematic and see where that stuff picks up power. Fuel pump, ignition, etc. Relay mentioned above? Ignition switch? Two wires scotch taped together under the seat?

All of the 12v supplies are good, that was the first thing I checked when I got the FSM.

There is some electrical hackery going on with this car. I haven't found any skinned wires or splices, but I have had to undo a few additions that were wired straight to the battery, including the radio.

Edit: Torque shows 0 RPM when cranking. Not sure if this is normal or not, I've almost always had it show something for RPM on other cars.

If I'm understanding your troubleshooting process so far you're getting a CAS signal when you turn the sensor but not when you install it and crank the engine. If that's correct have you determined that the cam is, in fact turning (broken timing belt?) and that you have power to everything you should when the engine is cranking and not just with the key in the run position?

Derick Freese wrote: Edit: Torque shows 0 RPM when cranking. Not sure if this is normal or not, I've almost always had it show something for RPM on other cars.

Not normal at all. I'd chase that down- for some reason, the ECU thinks the engine isn't spinning when it is. Why is a good question.

Figured it out. Timing belt broke, or at least sheared off a bunch of teeth. Shards everywhere underneath the timing belt cover when I took off the valve cover.

Now, I've cranked this motor over a lot since this happened. 1.8 Miata motors are interference motors, right? Hopefully I can source a 1.8 somewhere local for not much money.

It's not interference. Throw on another timing belt and see if it runs.

The timing belt fixed it. Finding the time to work on it took forever, and I kept running into parts that I had to wait on. All of the timing covers were broken, the crank pulley was literally about to fall off, so I needed a new woodruff key and crank pulley boss. The thermostat housing gasket wasn't the easiest to find. Of course I did the water pump while I was in there. The PCV valve was beat into the valve cover with no grommet. Just a total mess.

The Flyin' Miata timing belt tool kit is a must to do the timing belt, though. I'm glad such a great tool kit is available for these cars.

Thanks everyone for helping me out. This was a stupidly stressful repair for me.