BoxheadTim
BoxheadTim MegaDork
2/21/23 10:01 p.m.

My Maxton is running a 1st gen RX7 gearbox as best as I can tell, and the speedometer never worked. I know the PO replaced the (plastic) speedo gear at the gearbox end and mentioned that lasted a couple of miles of working speedo before it lunched itself again.

The car uses a little "gearbox" to translate the RX7 speedo input to a mechanical output that the VDO speedometer in the car actually understands, and I suspect that the problem is actually that gearbox, not the gearbox end of things. If my suspicion is correct and the gearbox needs taking apart and lubricating, that might solve the problem other than - I need the plastic gear that goes into the RX7 gearbox and drives the speedo cable. I don't think that part is available anymore - heck, most of the RX7 specialists don't even seem to list it anymore, and even the ones for the FC appear to be discontinued.

The alternative - because I really want a working speedo and odometer - is a GPS-based speedo. VDO makes one that looks very similar to the original speedometer, but it's clearly more modern and I'd really like to get the one in the car working again.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
2/21/23 10:19 p.m.

You need to know what, exactly, you have for a transmission, because Mazda changed gear pitch partway through the first generation RX-7 run.

I suggest the old Solomiata drivetrain page, as it has a lot of info on these:  https://fixjunk.com/solomiata/solomiata/Drivetrain.html

You are in luck if you have a later pitch gear, as Miata bits will work.  And B2200 truck, for that matter.

 

BoxheadTim
BoxheadTim MegaDork
2/21/23 10:28 p.m.

Thanks. I guess I have to pull the gear out for that unless some of the sparse docs with the car contain that information.

Let's hope it's the later pitch gear as I just found out that the electronic VDO speedometers I considered as a replacement are NLA...

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
2/21/23 10:36 p.m.

If you don't have a later pitch gear, it is fairly easy to change the drive gear with the tailhousing off.  The hardest part is getting the tailhousing off if you have an '81-up trans, some of them use a roll pin to hold the shifter block to the main shift rod instead of a bolt.  If it uses a bolt, it's fairly easy, just tedious and messy.

 

IIRC the mainshaft is the same all the way up through 2005 or whenever the last of the 5 speed NCs was, so there is a lot of depth in swappability.

jgrewe
jgrewe Dork
2/22/23 12:51 a.m.

Show me a picture of what you need when you figure it out. I have about 10 Mazda transmissions in various states of taken apart. From RX2,4,7,T2 and Miata

93gsxturbo
93gsxturbo UltraDork
2/22/23 10:20 a.m.
GameboyRMH
GameboyRMH MegaDork
2/22/23 10:44 a.m.

Another option, if you can get the specs of the original piece you could get a replacement gear 3d-printed.

nocones
nocones PowerDork
2/22/23 11:03 a.m.

I actually may have what you need.  The MG originally used a entire 12A RX-7 cluster but I never bothered to hook up the speedometer.  Everything "Should" still be present on the gauge cluster which I no longer utilize in the car.   Assuming what you are talking about is the actual Gauge end, I will see if the drive converter is still there tonight.   

Keith Tanner
Keith Tanner MegaDork
2/22/23 11:29 a.m.
93gsxturbo said:

Buy once, cry once.

https://speedhut.com/accessories/sensors/speed-sensors/speedbox-cable-included-gps-vss-to-mechanical-drive-speed-converter/

 

 

We've used similar devices in some of our LS conversions - NA Miatas have a cable-driven speedo and we had an electronic signal from the T56. We called them a squirrel box. They work, but they require a custom cable and we found those to be problematic. They also lag behind actual speed. It's not a good option overall IMO. I never installed one in my own car.

There are aftermarket self-contained GPS speedos - I have one for my Vanagon. I'd skip the squirrel and go straight to one of those. It's quite possible that Adam at Revlimiter could make a set of gauge faces that look like what you want them to look like so it doesn't look like a brand new gauge in an old cluster.

Or just determine how old/new that drive gear is and get one out of a Miata or a B-series truck or one of the other Mazdas of that era. That's the cheapest answer. If you can get me a VIN off that trans I might be able to pull up a part number. Color of the gear, too, mazda color-coded them for ratios.

amg_rx7 (Forum Supporter)
amg_rx7 (Forum Supporter) SuperDork
2/22/23 11:44 a.m.

Contact Riggs Rotary 

https://www.riggsvintagemazda.com/

sevenracer
sevenracer HalfDork
2/22/23 1:34 p.m.

I probably have some spares of both the main shaft plastic gear and the gear that meshes with it that plugs into the trans if you can figure out what you need.

BoxheadTim
BoxheadTim MegaDork
2/22/23 9:33 p.m.

In reply to nocones :

The gear I think I need is the one at the transmission end that goes into the transmission.

BoxheadTim
BoxheadTim MegaDork
2/22/23 9:35 p.m.

In reply to sevenracer :

Thanks, I'll keep that in mind. First order of the day after getting the car running again is to determine what does and what doesn't turn freely. Based on that I'll see if I can manage to get the original speedo working again if the speedo cable and the translation gearbox are both in good order.

BoxheadTim
BoxheadTim MegaDork
2/22/23 9:36 p.m.

In reply to Keith Tanner :

I found out last night that Speedhut builds GPS speedometers and with a bit of the customizations they offer, I can get a GPS speedo that looks pretty close to an early 1990s kit car VDO speedo. Not the same, but hopefully close enough.

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