Stuck in Reverse

By Lance Schall
Peak To Peak Miata Club
Boulder, Colorado

Some 1990 Miatas (VIN 130498 or less) have been known to become stuck in reverse gear while shifting out of 5th or out of reverse. (Editor's note: We also had one report of this happening in a '92.) The failure mode leaves the shift lever drifting, out of the shift gates, in no man’s land. You find that with the shift lever in the neutral position, the reverse lights are on; and indeed, driving backwards is possible. The shift lever can not be moved into the position for 3rd, 4th, 5th, or reverse. 1st and 2nd gear may be selected, but the gearbox is "locked". Upon teardown, no broken or out-of-tolerance parts are found. So, what happens?

The gearbox has a propensity for reverse. You may sneak the gear shift out of reverse slowly or move it out of 5th sharply (giving the 5th/reverse rod the necessary velocity) and in one quick, continuous movement, position the gear shift to center neutral position. If motion is exactly correct, the internal control lever end is out of the way and momentum carries the 5-R rod into reverse even as the gear shift is heading toward neutral. The failure condition is created when the extension housing control lever is not properly located within the rod end [5] tangs and the 5-R rod [4] is in the reverse position.

I would not remove the gearbox to upgrade internal parts unless you are stuck in reverse or alternately, if the gearbox is already out of the car to facilitate clutch replacement. Recommended minimum steps are to review the field repair procedure below and keep a stubby screwdriver in your trunk.

Mazda’s solution was the implementation of Special Service Program - 11. SSP-11 has three steps: Firstly, the two copper washers under the detent spring retaining bolt [1] are removed. When the bolt is replaced, it compresses this detent spring [2] about 2.4 mm more. Interestingly, the shop manual is not clear on the existence or count of these washers. The deletion of these washers is a factory attempt to increase the detent spring force by increasing the compression of the detent spring. The 5-R rod [4] has 6 detent notches, three on each side. The center pair of notches are the largest ones. They correspond to neutral position. The increased force on the detent ball [3] causes the 5-R rod to snap more positively into position. SSP-11 does not affect the matching spring and detent on the other side of the 5-R rod.

Secondly, the reverse light switch is replaced. The reverse switch plunger rides in a notch on the rod end [5]. A stronger spring in the switch also helps resist the motion of the rod into reverse. My dealer has identified three different reverse light switches. I have compiled the following data for each part number. It appears that Mazda increased the stroke and made the spring stiffer with each revision.

Lastly, to complete SSP-11, the gearbox is marked on the bell housing with a dab of black paint to indicate that the repair has been done. The position shown in SSP-11 is high on the right side of the engine near the starter. You’d almost need to remove the engine for this step. This item is not always completed, so the lack of a mark does not necessarily indicate that SSP-11 has not been done. The sure way to tell is to look under plug [1] and count the washers. Several jackstands, a well lit garage with a firm level floor, and a flashlight should be sufficient tools to check for the washers. Two washers means no rework, zero means SSP-11 is done.

Fortunately there is a field fix. Keep in mind, we experienced our failure with a gearbox that had already had SSP-11 performed! This is why I detail the field repair below. The mission is to get the gearbox out of reverse and back into neutral without using the shift lever. It turns out this can be done using the mounting hole for the backup light switch to gain access to the rod end [5]:

Mazda wanted to make it harder for a gearbox to slip into reverse. A desire to minimize the cost of the SSP dictated that the gearbox would stay in the car. Their fix is the removal of two washers under the detent plug and the replacement of the reverse light switch with a stiffer spring version. In my opinion, this fix is inadequate.

The micro film at the dealer's parts counter showed that the 5th/reverse rod (P/N M506-17-431) had been revised (P/N M506-17-431A). There is no VIN break after which all cars have the new rod, it is a compatible supersession. Ordering a 5-R rod will automatically get you the new one. New and old rods can be fitted to a gearbox interchangeably.

Subsequent measurement revealed that on the revised rod, the neutral detent relief is deeper on both sides of the rod. This would appear to be a design change to eliminate our problem. Both neutral detent notches are ground 40% deeper in the revision A rod. This will certainly cause the rod to snap more securely into the neutral position! To replace the 5-R rod, the gearbox must be removed from the car and the housings separated. Thankfully the gears, bearings, shims, synchronizers, etc. can be left alone. As a matter of fact, the entire contents of the gearbox hangs off of the bearing housing [6]. The gearbox can be operated as an instructional tool when disassembled to this point.

To eliminate any possibility of failure, the removal of the gearbox and substitution of the M506-17-431A reverse rod is necessary. The reverse light switch should be the newest, P/N M506-17-640A. Verify both washers at the detent spring retaining bolt have been removed. And if the gearbox is disassembled for replacement of the 5-R rod, why not replace the detent springs [2]? Renew both springs if they are less than 17 mm in length. According to the shop manual, the detent springs should be 17 mm long. No tolerance is listed. My old ones were 16.4 mm, the new ones 17.5 mm.

A variety of c-rings, split pins, copper washers, and special gasket goo are required to complete the repair. If I remember correctly, $100 will buy all the parts. If you have the dealer do the whole job, the gearbox removal and replacement will add a few hundred to your cost. If the mechanic already has the gearbox out for a clutch replacement, the additional labor to disassemble it will not be excessive.

Lance Schall
Peak To Peak Miata Club
Boulder, Colorado

Note from Jonathan Rubin

It's much easier to bring the rod back into position if the clutch is depressed.

Also, as of this writing (12/2007)the replacement switch spring, is VERY stiff. Don't be surprised if you have to use more force than is comfortable to shift into and out of reverse.