Electrical musings

The Tremec T56 gearbox comes with a reverse lockout solenoid designed to prevent you accidentally selecting reverse during forward motion. On a modern vehicle this is controlled by the ECU which effectively locks the car out of reverse once the vehicle speed exceeds a certain value say, 3mph. There has to be a failsafe in case of a malfunction so in fact the lockout can be overcome with a bit of brute force against the spring loaded plunger. The ECU I’m using doesn’t have provision for this functionality so I’m left with either using a bit of force each time I want reverse or rig something else up. I’ve decided to go with the “rig something else up” option although I understand many choose to use brute force or even take the mechanism off altogether. The problem with removing it comes with the reverse selection being across to the right and forward which could potentially lead to problems from an enthusiastic shift from fourth to fifth if you miss the gate and crash into reverse. The other option is to mount a switch somewhere that needs to be pressed before you select reverse but this option doesn't really appeal. The spring is quite stiff and so as an experiment i’ve temporarily wired the solenoid and powered it direct and it does make shifting into reverse so much easier so I think it’s something worth pursuing.

Another aspect I’ve come across in relation to the engine management system is the use of a Neutral Safety Switch (NSS), again a modern “gadget” to ensure that you’re either in neutral with an auto box or have the clutch depressed in a manual before you can start the engine. Again, it can be overcome, the ecu provides an output which must be grounded to allow the engine to start, you could just permanently ground the wire but I’ve decided to make use of it and fit a switch to the clutch pedal requiring the clutch to be depressed (grounding the wire) before the engine will start thus avoiding the accidental lurch forward when you’ve forgotton you left it in gear.

This is where it gets interesting, I’ve going to try and power the reverse lockout solenoid on the gearbox from the brake light switch on the brake pedal and ground it through the safety switch on the clutch pedal. This will mean it’s only possible to select reverse (without brute force) whilst the brakes are on (providing power) and the clutch is depressed (providing an earth) which is generally a good position to be in before you put the car in reverse! I guess there’s still a possibility of a mis-shift whilst changing down and on the brakes but in this scenario I’ll be shifting away from the reverse position i.e. right to left. The other benefit is that by grounding the solenoid through the switch on the clutch, it won’t keep being activated each time you brake during normal driving. There’s a little bit of figuring out to do with the wiring, I don’t know yet the output voltage from the ECU for the NSS and don’t want to risk the solenoid trying to earth itself back through the ECU so I’ll have to fit some diodes in place to limit the direction of the current. Once I’ve got it all figured out I’ll post some more details