Accelerator Pedal Failure

[Rich]

Hi all,

After reading a few threads and having a few conversations about failing throttle pedals I thought I ought check my own - glad I did!
For a bit of background, here's the information on the Main OC Forum:-
http://www.mx5oc.co.uk/forum/forums/p/342/526.aspx#526

Mine was equally fractured, and you cold wobble the pedal quite easily by hand - I suspect it would have broken off by the end of the year (or another track day). See below:-



Looking at the design, it's poor. Therefore rather than purchase a replacement, which would probably only do the same again, I decided to fix my own, strengthen it and remove the cause of the fracture (modifications actually done by Dad - thanks ).



To avoid further issues we also put a bolt throught the base to brace it against the bulkhead and remove the stress that caused the original fracture.



and here it is back in the car:-



I looked on a later mk1 and could see the pedal box design was changed to provide additional strength. I'm not sure what year this was done so either way I'd recommend checking yours for signs of failure...

Rich.

[AndyT]

The cause of the failure is a sticking throttle cable (cable sticking to the sheath); the routing is slightly different on the 1.8, so it is not as prone to faliure. Remember to fit a new cable, after your repairs. Additionally, this only affects rhd cars. Miatas use pretty much the same pedal box, but the cable is routed completely differently.

[Rich]

The cause of the failure is a sticking throttle cable (cable sticking to the sheath); the routing is slightly different on the 1.8, so it is not as prone to faliure. Remember to fit a new cable, after your repairs. Additionally, this only affects rhd cars. Miatas use pretty much the same pedal box, but the cable is routed completely differently.

This certainly wasn't the cause on mine. The cable and throttle body have always been well lubricated and there's no sticking. I can see this would cause an issue, although on mine I put the blame on my heavy right foot as it's when the pedal is floored that it was putting strain on the housing causing the fracture. I was able to recreate this before removing it from the car, hence the bolt to brace it against the bulkhead.
The later car (1.8 ) that I checked had a different design of pedal box and was better braced past the point where the fracture had started on mine. Specifically, if you look at the 2nd photo where the rubber bump stop is - this bracket runs up past the point of fracture.

Rich.

[des]

A quick and easy way to check out your accelerator pedal bracket is to use a digital camera and flash. Fire off a few shots then use your PC to look and zoom. Mine looks good and to see it only took a few minutes without a lot of fuss and removal of seats etc

[AndyT]

The cable is supposed to be self lubricating, like a bike brake cable. A slick inner layer, perhaps teflon, guide the cable. It is only really possible to add additional lubrication at either end of the cable, as you cannot remove the cable from the sheath without destroying it. I cannot see how it is possible to fully relubricate a 4 foot throttle cable in situ; to remove the cable generally means breaking it, unless you are very agile. On my '91, it always had a "firm" throttle. A hand on the gas pedal could induce movement in the bracket quite easily, as you fought against the resistance in the cable; this is before any kind of movement at the throttle body. Replacement of the cable totally eliminated this bracket movement. The cable is very cheap, and is easy to install (hard to take out, easy to fit).

In retrospect, the most bum bit of advice about MX5s is this adage about constantly adjusting the throttle cable. The throttle cable should not need that much adjustment; some people were and are doing it on an annual basis, when there is no need. There's about 10mm of "adjustment" on the throttle cable. I would suggest that any thick metal cable that needs 10mm of adjustment is due for a replacement. Those that constantly adjust their cable, because "it stretches", may in fact be exacerbating the pedal box damage.

I suspect over time, the cable either corrodes, and snags the sheath, and/or the sheath wears, causng the cable to snag some more. The "slackness" at the TB people interpret as cable stretch is actually due to the pedal bracket bending.