Miata Blower Motor Issue

Car: 1993 Miata Auto (base, No A/C, No power steering, no power locks, no power windows)

The HVAC(heater A/C) blower on my mom's Miata is behaving strangely. When driving the HVAC blower will randomly shut off (bumps in the road seem to be one of the triggers). Her car is an automatic and you can restart the blower by bumping the car into N or P. The blower will then stay running until it shuts off again. Sometimes it does not stay running in D even after restarting the blower with N. You can often shift and get the following behavior from P (blower on), R (blower off), N (blower on), D (blower off). It seems bizarrely enough related to the Shifter starter interlock (only allowing car to be started in P or N)

Initially this sounded to me like the typical corroded/gummed up ignition switch problem. So last week I pulled the switch apart cleaned it thoroughly buffed the contacts and rings lightly with some emery cloth till they shined and then greased with dielectric grease and re-installed. This seems to have helped. Initially the blower would not work at all unless in P or N. Now when it shuts off while driving it can be restarted like mentioned above.

I cleaned up the chassis to engine ground as well which didnt seem to help.

Has anyone experienced this before? Could this be a failing blower motor relay or resistor pack? A totally failed ignition switch(it looked fine other than be corroded and gummy, no melting or exceptionally worn parts) I have not had a chance to do anymore diagnostics. Thoughts on what to check next?

Looking at the wiring diagram, I suspect your 10amp ST SIGN fuse is blown.

In reply to foxtrapper:

Is that in the interior or under the hood? Why would the blower work at all if this was blown?

If the blower works at max speed - ie defrost, but not lower speeds it's almost surely the resistor pack. It's accessible w/o dropping the blower all the way out if you're limber enough to get your head up in the passenger footwell and still see what you're doing.

The resistor pack, if it's bad will have an obvious failed solder joint. The case says "Do not repair by soldering" molded right in the plastic. So I promptly cleaned the joint, soldered it, and reinstalled. It was July in MD - I needed the AC to work...

ahaidet wrote: In reply to foxtrapper: Is that in the interior or under the hood? Why would the blower work at all if this was blown?

Don't know. It's apparently on a main box with breakers. In looking at the diagram, if I'm understanding it correctly, if that fuse is blown the blower relay would ground back through the interlock switch, which only closes in P and N.

Resistor pack and or burnt/loose connections are the usual problem.

Ok thanks all. It was too cold to mess with it last night. I will be in the garage working on other things tonight with heat on so I will take a look at the fuse first then other items.

The blower when it works, works in all speeds not just the high position.

I checked the fuse mentioned above and it is good. I started digging deeper into the system starting at the blower motor circuit breaker. It is getting 12V no matter where the gear shift lever is located even when the fan is not working. The circuit breaker has continuity through and has not tripped. I then traced the wires to the blower motor relay under the hood. It is also getting 12V through the blue wires in the diagram below. I then traced the blue-white wire coming out of the relay to the motor and it has continuity from the relay to the blower motor regardless of gear lever location. I then checked the blk red wire in the diagram that goes to the "Starter". It goes to ground when the gear lever is in P/N but has no continuety when the gear lever is in R/D. What is this "Starter" it is referring to? Is this the ignition switch? I feel like this should get ground all the time perhaps? Thoughts?

I scanned the blower motor troubleshooting page from my 91 factory shop manual. Here are the two pages if they will help.

Some of it got cut off. The part that is missing on the first page says to check the circuit breaker in the fuse box.

On the second page it got cut off a bit at the top. It should say at the top:

Step 3
1. Turn the ignition switch ON.
2. Turn the blower switch and A/C switch OFF.

I did not do anything on it last night... Thanks for the repair manual section. I have done some of that on my own but there are few things that are certianly worth testing. If I get all the way through that and it still is doing its baffling behaviour of running when the car is in Park or Neutral and off in Reverse or Drive I am considering something drastic. I am pretty certain that the "Starter" box labeled above is related to the ignition switch/ car being on.. It is so when you turn the car off that the blower shuts off too. I may try just running that red/black wire to a ground. The other more drastic thought I had was defeat the starter interlock on the gear leverl. Other than the odd chance my mom would try and start it in gear (which might not even be all the eventful, not sure the starter spins fast enough to stall the converter) is there any other down sides/dangers?