P0507 & P0850 High Idle issues on MSM

[roninsoldier83]

So, I've had a bit of an issue; a couple of them actually. To give a bit of background, my 2004 Mazdaspeed Miata that was running fine until a few days ago. The car actually hadn't been driven for about a week due to lots of cold rain in my area. As such, it sat outside in my driveway for almost a week while it was raining. It was working perfectly fine prior to that.

A few nights ago I started the car and noticed immediately that the Bose stereo system wasn't acting right. It had lost the time setting and all of the sudden, the button mapping was all mixed up! Like for instance, pressing the power/volume button no longer turned the head-unit off- it now changed CD tracks! None of the buttons functioned as they were supposed to anymore- they now all had a completely different function. Although, this isn't a huge issue; it was just something that I thought was odd to have occurred at the same time as my "main" issue.

When I started driving my MSM I noticed that after it was warmed up, the idle wasn't right, as it was much higher than normal. Symptoms:

-With the car in neutral and the clutch released, the idle now jumps up to about 1500rpm.
-If I press the clutch in, the idle drops to its normal ~900rpm mark without a hiccup.
-The car otherwise seems to display no driveability issues and is bone stock.

After driving the car for a couple cycles, it illuminated my check engine light (CEL) and I used my OBDII scanner to find the following codes:

- P0850 - Neutral switch circuit malfunction
- P0507 - Idle control system RPM higher than expected

Now, based on a bit of a quick search, I assumed one of the neutral switches is likely faulty. There's actually 2 neutral switches on the car- one on the clutch pedal (LA01-66-490A) and another one on the transmission (Y602-17-640). I'm crossing my fingers that its not a wiring or ECU issue.

I also have a hard time understanding how the idle issue & stereo issue could have occurred at the exact same time. Electrical surge perhaps? Sheer coincidence? Anything I am missing and should be looking into?

Either way, after doing a bit of research, it appeared that the most common cause was the neutral switch on the clutch pedal. So, I ordered one and installed it this evening. Unfortunately, it did not solve my woes.

On another note, I disconnected the battery in order to remove the clutch neutral switch and cleaned the switch & wiring. Disconnecting the battery seems to have returned my Bose stereo head-unit back to normal.... but unfortunately, the car's idle is still the same, even with the new clutch neutral switch.

So, at this point, I'll be ordering the transmission based neutral switch. There's only a couple of small caveats:

-Apparently its recommended to use a lift in order to get to the switch, which is apparently on the upper driver's side portion of the transmission. Anyone possibly have access to a lift I could use? Or at least a set of super high jack stands?

-Also, the switch itself is 27mm and has a wiring harness attached to it. I happen to have a 27mm 1/2" drive socket, but it won't fit over a wiring harness. A few guys have recommended using a cutting wheel or something to that effect, to slice a slit in the socket like so:


^^^Anyone have a cutting wheel or any recommendations on modifying my socket?

Any help or feedback would be truly appreciated! Thanks in advance!

-Brandon

[RX-7 Chris]

it almost sounds like maybe something got wet.

[I`m Batman]

it almost sounds like maybe something got wet.

Yes, put it in a bucket of rice overnight. It should dry right up.

[roninsoldier83]

it almost sounds like maybe something got wet.

Yeah, that was my first thought due to it sitting out in the rain.... but, I have no idea what might have gotten wet. The interior of the car appeared to be as dry as can be. Hopefully I don't replace the second switch just to discover there's another issue I'm missing.

[millertime]

Check to see if the drains for the cloth top are clogged, water can build up in there, that's how rust starts but it might run near some wires and could cause damage.

[Learjet45]

For lift access, you could take the car to DIY Auto Repair. It's up north off of 36 more towards Denver than Boulder IIRC. I took my Subaru there right before my drive back to NY to change my diff fluids, atf, and oil. It made life so much easier. It's $30/hr to rent the lift and perform the work yourself.

[roninsoldier83]

Check to see if the drains for the cloth top are clogged, water can build up in there, that's how rust starts but it might run near some wires and could cause damage.

Duly noted! I just checked them and they seemed to be fairly clogged, but I'm not sure if it was enough to stop water from running down. Either way, I cleaned them out as best I could and they now seem to pass water just fine... I really should have checked them before I cleaned them out. Oh well, thanks for the tip.

For lift access, you could take the car to DIY Auto Repair. It's up north off of 36 more towards Denver than Boulder IIRC. I took my Subaru there right before my drive back to NY to change my diff fluids, atf, and oil. It made life so much easier. It's $30/hr to rent the lift and perform the work yourself.

Awesome! Thanks for the tip man! Just looked them up and saw that they have 10% discounts for police/EMTs/Firefighters- sweet! That should make this process much easier.

[roninsoldier83]

So, I think I'm going to replace the tranny neutral sensor this upcoming Sunday around ~11:00am at DIYAutoRepair if anyone feels like stopping by.

In the meantime, rather than modding a 27mm socket to fit over the sensor wire, I actually just spent a whopping $6 on a 27mm thermal sensor socket that should hopefully do the trick:



^^^I actually dislike using 12 point sockets- I prefer 6 points for lack of likelihood of stripping; but I think it should be just fine.

[roninsoldier83]

So, I put the car on a lift today and I swapped out the neutral sensor on the transmission. I was easily able to see the problem with the old sensor:


^^^As you can see, one of the sensor wires broke off/was cut. When I replaced it, the car was back to idling like normal and the CEL is gone. The car is back to normal.

On another note, swapping the sensor really wasn't that bad. The time I actually spent swapping it out was likely around 1 hour. It didn't require dropping the exhaust or the transmission. Although it did require a little bit of creativity with the wire clamp being on the top of the sensor where there was very little space for my larger-than-normal hands. Overall though, it wasn't too bad.

Unfortunately, I also found another issue while I was working.... the sensor wire to the back-up lights has also been cut. Not a big deal, it doesn't throw a CEL or affect reliability/driveability, I just hadn't noticed it until now. To my dismay, none of the local parts stores had one in stock; but fortunately it is MUCH easier to replace, as it is in an easier to get to spot on the passenger side of the tranny. I think the back-up light sensor is on the driver side of the tranny on the 5-speed Miatas (based on searches), but on my 6-speed its pretty accessible, nice and low on the passenger side. I'll be replacing it shortly.

I think I was also able to deduce how the sensor wires were damaged. It appears that the transmission was previously removed from the car; likely to replace the rear main seal, or something to that effect (clutch maybe, who knows). Either way, it looks like the sensor wires were crushed between the tranny and the transmission tunnel/body when it was put back in. I'm pretty confident that this won't be a repeat issue, which is nice.

Either way, nice to have the car running properly again.