Backup light switch for transmission?

[Matthew Ziegler]

fourth

yes if your switch is open, when the button or plunger is depressed,

[maineSS]

I finally got around to modifying my '49 crashbox for backup lights as part of a complete rewire with a modern harness. It's not as simple as the newer synchro box, as the rails are deeper in the bore -almost 1", and the Torqueflite/904 switch won't make contact with them simply by screwing it into the case.

You"ll have to remove the top of the tranny for this mod, and the sheet metal "tunnel" will have to come out- not a lot of fun. The shift, PTO, and Transfer case levers have to move around as you work the cover up and off, and I managed to somehow lock up BOTH the Shift lever and the PTO in the process. The shift lever can be freed up by placing the top casting in a heavy vise, and working the rails around. The PTO can be freed by loosening up all the attaching bolts, and moving the PTO out (have a pan under to catch the gear oil).

Pic #1 shows the tranny with the "tunnel" off. The rail plug closest to the driver has to come out, I drilled a small hole next to the edge and pried it out. The plug is lying on top of the center rail housing in the pic. The tranny cover is held by (6) 3/8" bolts, move the lever to Neutral before taking it off. Pic #2 shows what the rails look like in Neutral, Pic #3 shows gear position in Neutral (to be continued)

[Jerry Henry]

Thanks, very much, for the great details and pictures. Every time you put your truck in reverse you will be proud of all the effort. Some other things are priorities to me right now, but maybe down the road. Thanks again. JH

[im02crazy]

Magnetic proximity switch and relay

It might be easier to fit a magnet to the shift rods or shifter and the reed switch on the chassis. WWGrainger has a couple industrial control magnetic prox sets that would probably work well. Easy to set up the alignment and no moving parts. some of these have up to about a 2" working distance.

You can still also wire a SPDT (center position off) dash switch in parallel and also use a timer. Left side turns backup lights on through timer, right side turns backup lights on through mag switch and relay to switch the high power lamps using the low power mag switch.

[maineSS]

Continuing on, Pic #4 shows the rail position in Reverse, and Pic #5 shows the gear position. You need to have the gear selector in Neutral for easiest installation and removal of the cover, and you need to insure both forks engage their respective "slots" on the gear clusters. The clusters move very easily on the center shaft, and it's real simple to push the rear gear cluster back as you're dropping the top on- which leaves you in gear all the time. I found this out after putting the entire floor back together, so I ended up doing it all over again. Check by tilting the top up slightly, and looking thru the crack before bolting things up.

In my tranny, the rail was .930" in from the face of the top casting in Neutral and all forward gears, and 1.880" in at Reverse. There was about 7/32" between the rail and the end of the switch in all but the Reverse position, and the switch had to be depressed another 1/32" to work.

I originally planned to drill and tap the rail for a bolt, figuring a 1/4-20 hex head would do the trick. The rail was hardened to the point where a file wouldn't touch it, which required a new approach.

I took the top to my machinist, and he set it up in the mill. He faced the irregular casting surface flat (Pic #6) to provide level mating surface for the switch gasket, and bored the rail hole down 3/4" with an 11/16" drill to prep for the 3/4-16 tap. The original hole is too small for direct tapping, and the seal plug seat is NOT accurately aligned with the rail hole, so don't skip this step, or you'll likely end up with a jammed/broken tap. (to be continued..)

[maineSS]

Pic #7 shows a 3/4-16 full tap (top), and a 3/4-16 plug tap (bottom). If you're tapping by hand, you'll need both, as the full tap provides the starting thread. In blind or short holes that are threaded to the end, the nose of the full tap hits before you're done threading, so you switch to the plug tap to finish up. A mill can use just the plug tap if everything is solidly held. You can cut the end off a full tap with a cutting wheel to make your plug tap, just be sure you're done all your thread starting work. Pic #8 shows the fiber washer for the switch being tapped with the full tap. This is a Dorman oil drain plug washer PN #017 (17 mm). Tapping it allows it to screw over the threads without tearing, after the switch is screwed tight, the threads compress, capturing it on the switch (pic #9). To be continued...

[maineSS]

After the Dodge Boys sank my bolt plan with their superhardened rail, I thought a sliding plug would take up enough room to keep the switch depressed in all but reverse gear. Unfortunately, the diameter of the plug would be greater than it's length, which would allow it to tip over in the bore. There's no danger of it falling into the tranny, since the rail blocks the passage in all gear positions, but if the plug tipped, it could prevent the tranny from going into gear. The machinist and I put our heads together, and came up with the idea of extending the plug's length by attaching it to a short spring. The plug didn't have to stay in contact with the rail over it's entire travel, it just had to be long enough to depress the switch, and not tip forward. The switch can accomodate up to 1/4" travel before bottoming out, so the plug can be longer than 7/32"- ours was .280" long, .505" dia x .170" long inside the spring, with an end ridge .618" dia x .110" long at the end to cap the spring (pic #10). The end of the spring is bent 90 deg to fit a hole in the plug.

The machinist made up a plug from Delrin rod. A trip to several hardware stores showed that a #45 spring (pic 11) would suffice. Interestingly enough, #45 springs fom different stores were slightly different in dia, buy several and fit, as some will hang up inside. You need an O.D. of .618 to avoid this. You can crush and stretch the spring to fit with pliers, but it takes awhile. (To be continued...)

[maineSS]

Pic #12 is a dimensioned sketch of the plug to supplement my description, pic #13 is sketch showing how the shoulder/thread area of the switch has to be cut back on the lathe a bit to allow a seat for the 3/32" fiber gasket. Pic #14 is a dimensioned sketch of the modified spring. The spring supplier is Hillman Fasteners, who supply several hardware store chains. It's close to the right dia, but will need some fitting work. Your finished dia needs to be .618", and the coil spacing has to allow the spring to compress without coil binding before the plug actuates the switch.

Keep in mind that your tranny rail travel and top dimensions may be a little different, measure the rail travel between gear positions before doing any machine work.

[maineSS]

Pic #15 shows the complete switch/spring/plug assembly, ready for installation. Next comes the electrical connection.

The two outside pins are the backup switch, the center is the Neutral Safety connection. Unfortunately, the pin dia is too large for an 18-22 gauge butt connector, and too small for a 14-16 butt connector to fit, so there's no cheap way to connect up. Pigtails are available from NAPA (~$23) and CarQuest (~$20). I don't have the PN's now, I'll post them next time. A less expensive supplier is scoutparts.com (888-288-0550), PN SP10270 at $8.25. Shipping will be extra, depending where you live, CQ may be competitive.

The pigtail cap has a half-moon insert at the bottom that prevents the cap from covering the base of the switch (pic #16). I found another cap at a tranny shop that lacked this, apparently it's not made anymore. If you're good with a Dremel, the half-moon can be removed (pic #17)- but one slip will really mess things up. (to be continued...)

[maineSS]

Pic #18 shows how the pigtail fits over the switch after the half-moon is removed.

After butt-connecting the outer (purple & black) pigtail leads to my backup light wiring, I used a 5/8" cable holder bought at Home Depot to secure the switch wiring to a tranny cover bolt (pic #19). The driveshaft U-joint will make very short work of any loose wires near it. I had to enlarge the 1/4" hole to 3/8" with a chainsaw stone in my Dremel (pic #20). Not much material left around the outside of the hole, but it'll last til I find something better.

The PN's for the pigtail are: Mopar #4085383, CarQuest 57-4724, NAPA EC187.

[maineSS]

There's plenty of room in the PW tunnel (pic #21) for the switch and wiring- about 1" over the top of the tranny. Matt mentioned earlier in this thread that the M-37 is different- one of the things M-37 owners may want to check before using this switch.

For those who are inclined to the doorswitch mentioned earlier in this thread, remember that it's an "interrupted ground" rather than "interrupted power" like the Torqueflite switch. This means you'll have to either use it with a relay, or modify your tail lights to ground at the tranny switch instead of the chassis. Using it with a "normally closed" relay would be more convenient- the doorswitch would allow current to flow thru the relay to the lights when reverse gear is engaged. Costwise, a quality relay would bring you up to the Torqueflight switch, but if you didn't want to bore & thread the tranny cover, it would allow the use of an O-ringed plug as an adapter.

Those who are running the Dennan overdrive can use this mod to cut out the solenoid when shifting to Reverse. I intend to wire a backup alarm into my circuit, so if my safety fails due to switch malfunction, I'll know something's wrong before that expensive crunching noise...

[JSabah]

Hoping to revive this thread a bit to help with the brain **** Im experiencing. I have an NP420 from a 59FFPW and as Im installing the Denman Overdrive system, Id like to put a lockout switch on the reverse shift rod. I've removed the freeze plug on the far left that the reverse rod is in. when the vehicle is in any forward gear, the rod is back and when shifting into reverse, it moves forward. The NAPA switch seems to be a NC switch which I believe needs to be normally OPEN and depressed while in forward gears but un-depressed/released while in reverse (and Normally Open) so that the circuit is NOT complete when in reverse....what am I missing here.

EDIT: I just figured out that the center pin is like a ground interrupt (and isolated from the switch body. Therefore I think I need to put this switch in the ground side of the OD solenoid. Does that make sense ? Here is my OD switch circuit showing that Im interrupting the typical ground/solenoid and putting the Napa switch in line. Does this look correct?

[Doc Dave]

I am in the process or re-registering for this forum.
Over on Joe's site, in the archives section, you can do a search and I am pretty sure you will find your answer about the reverse shift switch.
The Denman OD has worked flawlessly for me for 10 years now.