Reverse Switch Blowing Fuse

[Roadrunner]

Halfway through last weekends track day, the speedo and tacho gauges died. (Wife was a trooper - went out in her session, no tachometer and still managed to put down similar lap times to previous sessions )

Have done some investigating and after 6 blow "Meter" fuses eliminating tail light wiring etc, I have determined it is the reverse switch on the 6 speed SE gearbox. (unplugged it, no more blown fuses when put in reverse - would only blow the fuse when put in reverse).

Where can I buy one of these switches from? Is direct through Mazda the only answer and if so does anyone have a part number?

Also, being that the switch is towards the very bottom of the gearbox, does fluid need to be drained first before swapping over switches? Or can the car be raised high enough on the drivers side to not have any fluid piss out.

And lastly, what is the electrical plug/switch thing on the very top of the gearbox in line with the reverse switch?

[OMY005]

I can't see how it could be the switch causing the problem. Most reverse switches are just single contacts (just making or breaking the circuit). Sounds like a short somewhere else in the circuit.

Hope this helps, Andrew.

[manga_blue]

Likewise. I'd be looking for problems in the tail lights and their wiring first.

[project.r.racing]

yup, could be the white reverse light also. as they only come on when the reverse switch it activated. pull out one globe and trial, should be able to narrow down the issue to a certain area if it is the wiring or reverse lights.

[Roadrunner]

I did remove both reverse lights.
The disconnected both loom plugs that connect to the whole tail light assembly.
Still brew the fuse.

When I google MX5 reverse switch there seems to be lots of people have it just randomly go.

Being that I replaced the rear bumper last week I had assumed I’d pinched a cable but I can’t find anything between the loom and the light bulbs that would suggest an issue. Reverse globes aren’t blown either.

I’ll get the multi meter out and check the cables for continuity and earthing before ordering a new switch. But all roads lead to the reverse switch itself.

[Roadrunner]

Had the brain thought (fart?) to help narrow this down.
I bridged the loom side the reverse switch plug together (to simulate the reverse switch being active)
Turned the ignition on and the reverse lights came on and no blown fuse!!

So that means there's not a short in the cabling to the reverse lights.
Can only assume there is a short inside the reverse switch itself. I'll contact Mazda on Monday and see how horrendously priced these switches are

Seems to be a common issue with the early NA's, not so much for NB's.

[OMY005]

Can only assume there is a short inside the reverse switch itself.

That should be easy to check with a multimeter to see if either of the contacts are going to earth when the switch is operated?

Andrew.

[Roadrunner]

Can only assume there is a short inside the reverse switch itself.

That should be easy to check with a multimeter to see if either of the contacts are going to earth when the switch is operated?

Andrew.

Strangely enough no. Doing this test led me to assume the switch is fine. But when only it was eliminated from the circuit, everything was fine. Add it back in, pop.
In my hours spent googling lots of others seem to have similar results where for all money the switch isn't shorting out but something internal must be somehow as they all had zero issues after replacing the switch. Weird.

[OMY005]

Strangely enough no. Doing this test led me to assume the switch is fine. But when only it was eliminated from the circuit, everything was fine. Add it back in, pop.
In my hours spent googling lots of others seem to have similar results where for all money the switch isn't shorting out but something internal must be somehow as they all had zero issues after replacing the switch. Weird.

Strange indeed, something must have come adrift inside the switch, and its shorting to the threaded body. Let us know what you find.

Andrew.

[Roadrunner]

So to close this thread off if anyone searches for this problem in the future, it was the reverse switch.
Plugged a new one in and reverse lights come on and no blown fuses.
Easy to replace but you do have to drain the transmission fluid.

[takai]

Haven’t had the issue on an MX5, but have had other cars where the reverse switch would heat up and melt the little plastic liner on the plunger. This would allow a short to gearbox ground within the switch.

On one rally car we resolved it by changing to an external switch off the gear stick.

[Red_Bullet]

So to close this thread off if anyone searches for this problem in the future, it was the reverse switch.
Plugged a new one in and reverse lights come on and no blown fuses.
Easy to replace but you do have to drain the transmission fluid.

Thanks for the info.

[Okibi]

Glad it's solved, thanks for sharing.