Reverse (backup) lights without any hardware inputs

[Philip Lochner]

Neither my 4L60e, nor the gear selector in my Jensen offer any physical switched outputs with which to activate the reverse lights on my car. My GPIO based Transmission Control also does not have any programmable outputs with which to toggle an output when in Reverse to activate the reverse lights.

The pressure switch logic in the 4L60e is such that pressure switch circuits N and R (This is NOT Neutral an Reverse!!) are BOTH active (grounded) ONLY when the transmission is in reverse (and engine running).

I therefore built a circuit that would activate the reverse light relay only when circuit N AND R are BOTH grounded. I used transistors and built the circuit into a dead relay housing.

Internally to the GPIO, there are however pull up resistors to 5V. These caused both the N and R circuits of my "controller" to be permanently active. This problem was overcome by installing diodes into J4 of EGT4 (line towards the nearest edge of the board) and the other from the top of C17 directly to pin #6 of the AMPSEAL connector and then breaking the PCB track somewhere between C17 and pin6. It would have been just so elegant if EGT3 was used instead of GPI2 for the "R" circuit cause then a diode in J3 would have done the trick. These diodes are NOT shown in the circuit as they are internal to GPIO.

The reverse lights now come on in Reverse only

[WestfieldMX5]

Nice, I like this kind of small hacks

[jsmcortina]

Microswitch on gear selector?

James

[DaveEFI]

Microswitch on gear selector?

James

Too simple.

[Philip Lochner]

Microswitch on gear selector?

James

Easier said than done. A programmable output would have been so elegant.....

[DaveEFI]

Microswitch on gear selector?

James

Easier said than done. A programmable output would have been so elegant.....

Not familiar with your box - but how does it inhibit the starter working in gear? Many have a combined inhibit switch/reverse light one.

[Philip Lochner]

Microswitch on gear selector?

Not familiar with your box - but how does it inhibit the starter working in gear? Many have a combined inhibit switch/reverse light one.

At the moment there is no starter inhibit (not good I know...) due to the absence of any suitable switches both on my 4L60e selector shaft and selector lever. Another good use for programmable outputs....

[DaveEFI]

Doesn't that GM box have a limp home mode? Ie pure mechanical selection of drive (probably just one gear) neutral and reverse? If so, does it have enough position sensors to do what you want in electronics? How do the car makers that fit this box achieve it?

[Philip Lochner]

Doesn't that GM box have a limp home mode? Ie pure mechanical selection of drive (probably just one gear) neutral and reverse? If so, does it have enough position sensors to do what you want in electronics? How do the car makers that fit this box achieve it?

Dave, Yes, the 4L60 does have a mechanical limp home mode which gives you 2 and 3 and Reverse based on the gear selector shaft. But this has NOTHING to do with my problem.

The box has 3 "ground active" built-in switches on pins N, R, and P of the transmission connector. These are the pins TransControl (GPIO) uses to know in which gear the transmission is. All that is needed is a bit of code for a few programmable pins from TransControl similar to what Megashift offers.

[Marek]

If it somehow mutiplexes the outputs on these pins to indicate gear then you can display that easily using transistor logic. If it accepts multiplexed inputs on these pins then you could remotely shift the same way.

Enter "transistor logic" into a search engine to see how simple AND, OR, NOR, NAND etc gates and inverters are built using just a few transistors.

I've used similar circuits on the calibration wires of my LC1s to flash up when they are failing (and remain blank/off when working correctly) and then multiplex them with a CEL and low coolant sensor all in one warning light

kind regards
Marek

[DaveEFI]

Doesn't that GM box have a limp home mode? Ie pure mechanical selection of drive (probably just one gear) neutral and reverse? If so, does it have enough position sensors to do what you want in electronics? How do the car makers that fit this box achieve it?

Dave, Yes, the 4L60 does have a mechanical limp home mode which gives you 2 and 3 and Reverse based on the gear selector shaft. But this has NOTHING to do with my problem.

The box has 3 "ground active" built-in switches on pins N, R, and P of the transmission connector. These are the pins TransControl (GPIO) uses to know in which gear the transmission is. All that is needed is a bit of code for a few programmable pins from TransControl similar to what Megashift offers.

If there is a built in switch for R which is used for other purposes, it's not going to be difficult to share that with a relay to switch the reverse lights. You may need a couple of diodes on its output to stop the two functions interfering with one another, and obviously a driver transistor for the relay. A darlington type might be best as that needs the smallest current to drive it.

[Philip Lochner]

Doesn't that GM box have a limp home mode? Ie pure mechanical selection of drive (probably just one gear) neutral and reverse? If so, does it have enough position sensors to do what you want in electronics? How do the car makers that fit this box achieve it?

Dave, Yes, the 4L60 does have a mechanical limp home mode which gives you 2 and 3 and Reverse based on the gear selector shaft. But this has NOTHING to do with my problem.

The box has 3 "ground active" built-in switches on pins N, R, and P of the transmission connector. These are the pins TransControl (GPIO) uses to know in which gear the transmission is. All that is needed is a bit of code for a few programmable pins from TransControl similar to what Megashift offers.

If there is a built in switch for R which is used for other purposes, it's not going to be difficult to share that with a relay to switch the reverse lights. You may need a couple of diodes on its output to stop the two functions interfering with one another, and obviously a driver transistor for the relay. A darlington type might be best as that needs the smallest current to drive it.

Dave, N, R and P in this instance does NOT refer to Neutral, Reverse and Park. They refer to transmission fluid pressure switches called N R and P. If both N and R are grounded the transmission is in Reverse. b This is why I needed a logical "AND" function for "N " AND "R" as per post #1.