Something else to consider
 

I recently had an engine/transmission (915) out service and specifically told my mechanic about the reverse light switch issue and he was very familiar with the metal pin not mating properly.

So I get the car back and had my wife check the reverse lights to see if they were coming on and no lights! WTF! :mad:

I bring it back to the shop and he pulls the switch and swaps it out with a new one, still no reverse lights. He checks out the pin and winds up spilling my recently replaced gear oil....great! :mad:

Anyway to make a long story short, he checks the engine harness pin connectors and he plays with the prong and receptacles, plugs it back in and enagages the reverse gear...and walla reverse lights works!

Of course I don't know which pin # controls the reverse lights, but I guess it is another possible solution. :D

+1 on the drill bit solution. My reverse pin also shrank 7mm in the wash while the transmission was rebuilt

This thread helped me out big time years ago

OMG you are right - on checking my original pin more carefully, I found this:

http://forums.pelicanparts.com/uploa...1421730664.jpg

:)

I'm still having back-up light issues. My turn signal fuse was blowing every time I drove the car, and I isolated it to the back-up light circuit.
If I disconnect the harness from the transmission reverse switch, no fuse blows.
If I apply 12V to the directly to back-up light circuit (at the transmission switch harness), the lights come on.
If I test the switch in the tranmission, it operates correctly (ie ignition on, engage reverse gear and my 12V test light inserted in the switch output illuminates) and no fuse blows.
I checked all the bulbs and connections, all look good.
All the individual components seem to function correctly, but as soon as I plug it all back together and engage reverse, the fuse blows. I even tried a 25A and that blew.

You don’t mention which of your p-cars this is, but switch operation appears similar for both.

Sounds like pedal push shorts 12V to ground… If you want to futz with it, cheap multimeter with beep on short setting helpful (Harbor Freight or Fry’s). If not, just replace the switch!… our host sells both, ≈$15 for the 996, ≈$42 for the ’72 911. Maybe get a cork as described above to keep trans oil out of your face … might be same size for 996, dunno!

Futz route: With ignition off, use multimeter (set to beep on short) and:
- check to see if either switch contact is grounded to chassis with pedal up, or grounds when down;
- check switch operation across the pins;
- plug in harness (IIRC you can pull back boot to get to wires), repeat check to ground on both wires with pedal up, then down. Reverse lamps will be a few ohms, might trigger beep…

If it does, or you don’t get any beep at all, you can repeat above using lowest multimeter resistance scale … you’d need to be seeing ≈0.5Ω or less to blow a 25A fuse.

Good luck!

Thanks Barry, the fault is with my 72.
I have tried your troubleshooting steps (except the part about pedals, I am confused by that as no pedals are involved in the back-up circuit).

I get the following results (using a Fluke 88 multimeter on Ohm setting with beep on short) with ignition off and wiring harness disconnected from back-up switch:

Transmission not in reverse:
Open circuit in the switch, neither contact goes to ground.
Green and white wire goes to ground.
White wire goes to ground.

When transmission is in reverse:
Both switch contacts go to ground.
Green and white wire goes to ground.
White wire goes to ground.

If I unplug the rear light harness connectors, the white wire no longer goes to ground, the green/white still grounds. (I never get higher than 0.0 ohms in any of the tests, other than when I get open circuit).

When I plug the backup switch harness back into the switch and reconnect the rear light connectors, all wires and switch contacts go to ground all the time, regardless of whether reverse is selected or not.

As far as I can tell, there are at least 2 faults - one in the backup switch, one in the green and white cable, but I am at a loss how to proceed from here. The backup circuit worked well before the engine and transmission were rebuilt.
I even bought a FF310 faultfinder, but that thing just tells me the entire circuit is shorted everywhere in the car.

SpawnyWhippet,

Looking at the '72 schematic, and after building complete wiring harnesses for your model year, I am thinking you may be testing the wrong wires.

The Green/Yellow wire from fuse 8 or 5 in the Frunk supplies 12 volts to the reverse light circuit.

THEORY

12vdc flows from the front fuse panel through the body harness to pin 5 of the rear fuse panel side 14 pin connector, from there to over pin 5 of the rear 14 pin connector, then to the switch through the engine harness.

When the switch closes, the power flows through the switch contacts back through the engine harness along the Grey wire to pin 6 of the 14 pin connector at the rear, then under the fuse panel to the side 14 pin connector pin 6. From there it splits out to both rear 6 pin connectors feeding the tail lamps.

At the Female tail lamp six pin connector the wire color changes to Grey/Red and goes to the lamp, illuminates the lamp and goes to ground after passing through the filament.

TESTING

To isolate and troubleshoot you should disconnect the 14 pin rear connector and test the engine harness' female pins 5 and 6 to ground and to each other. they should normally show open and then short with the transmission in reverse.

If that is working correctly, re-install the engine harness 14 pin connector and now you can test the wiring under the fuse panel by removing the side 14 pin connector and test male pins 5 & 6 to each other and ground the same way as you just did.

The now removed fuse panel side female 14 pin connector can be used to test wire continuity and integrity to ground by first removing the 2 tail light six pin plugs and metering only pin 6 of the 14 pin connector to ground and then probing both of the 6 pin male pin connectors to find the grey wire's terminal and test for continuity to pin 6 of the 14 pin connector. (Make a note of which pin is the grey wire on the 6 pin male connectors)

The Green/Yellow wire can be tested by checking that pin 5 has continuity to the Front fuse 8 and/or 5. (and is isolated from ground)

If all that checks out OK, then find the corresponding 6 pin female socket feeding the tail lights to the grey wire you just located in the male 6 pin connector, and test continuity to ground and the center post of the reverse light socket.

Make sure bulbs are out when testing.

Let us know what you find. :)

Sheesh!!! :eek: Confusion appropriate, cerebral flatulence on my part, was researching hooking up the 993 cruise motor to the 911 controls and clutch switch so my mind was clearly stuck in neutral when I wrote the "pedal push" nonsense.

Will get back to you once I've had a chance to go through your reply with the wiring diagrams and see if anything looks promising, just returned from biz trip and have paperwork to clear first...

Have to wonder if the harness got pinched & shorted somewhere, as you say it worked before...

Cheers

Just want to thank those who started and contributed to this thread. VT would not pass my '78SC for inspection because of no back up lights. Checked the bulbs and circuit -- all good. Then, read this thread and took out the back up switch -- also good. But, I had no pin! I have had the car for about 5 years and never had reverse lights. Who would have removed it and why is a mystery. Anyway, cut a drill bit to 52 mm and tapered the end that engages the switch a bit and after some experimentation and reduction of length to 51.7 mm, I am good to go. Thanks again!

This is an easy fix --- especially with a lift.

Hello,
Just removed the reverse light switch. No Pin. If on the transmission's reverse gear shift rod the pin was not installed (or doesn't line up with switch hole axis): Shown on picture from How-To: Porsche 915 Transmission Repair Tutorial Part 8 - Porsche Wiki
and I try to insert a test drill bit as suggested for the repair, do I run the risk of losing the drill bit into the front of the transmission housing? If so, how can I tell if the transmission rod has the pin attached (and lines up), do I simply look into the hole? What should I see? Thank you,
Phil


http://forums.pelicanparts.com/uploa...1606313065.PNG

update, placed a correct gauge electrical wire in the hole to hit the rod shaft pin. It contacted something and between R and N, about 1cm of travel.

BarryJB wrote:
"Just FYI if it's the same switch, it takes a minimum activation stroke of ≈1mm and can handle a maximum stroke of ≈4.3mm before it bottoms out. The reverse pin in my 915 shows ≈10mm stroke on engaging reverse."

My switch activates at the complete end of the stroke, nowhere near 1mm from relaxed position. I can't see how that would be a long term solution, it does go to zero Ohms but the alignment would be hard to achieve. Does anyone else know when a new switch makes contact? Is it as I understand BarryJB wrote, 1mm in and the switch closes?

Thanks,

Phil

Do I dare bring this thread up again?
There are bullet shaped contacts that go into the reverse switch. On my car these are messed up and don't contact very well. I'm thinking to replace them. Any hints on how to remove old and solder/crimp the new contacts in? Here is a picture of the contact I'm thinking of.
Thanks.

http://forums.pelicanparts.com/uploa...1617634650.jpg

The contacts should each pull straight out. If you can't re-use the old terminals, I think you can get new ones (911-612-907-00 or -01). The wires can easily be soldered into the cups on the contacts; just use a soldering iron to heat the terminal/wire, etc. Be sure to use good silver solder.

Thanks. I need to look into the silver solder deal. I usually use rosin core solder for electronics. I'm leery of doing this. Maybe I should make a two wire additional connector that uses crimp-on contacts. This would allow me to solder on the bullet contacts on the bench and not under the car. ??

My current plan is to solder or silver solder new pins to pig-tail wires on the bench and cut the existing wires and connect to the car harness using Posi-Tite Watertite Connectors. This avoids soldering under the car.

I believe the harness to the switch’s long enough that you can probably extract the connector to the side of the car so you don’t have to solder under. It’s not attached to anything else from the engine tin onwards.

Thanks, I'll take a look. I don't crawl around on the concrete as well as I used to.

I got the new "pins" that go on the ends of the wires.
Installed those with soldering iron, solder joints look good. I tested the lights by shorting the pins and the lights come on.

The switch didn't work well on the bench (high ohms) so I ordered one from our host. Coming in a couple of days.

I made a diagram so that folks could have some basic dimensions.
The throw looks to be only 2mm (reverse engaged vs. not engaged).
I found I had 2 shims (1mm each).
On the bench I expect the new switch to short with not much movement - maybe only ~1mm.

In my case with 2 washers there is ~ 0.74mm of compression of the switch when not in reverse. The question is do I take off one shim to get more pre-compression?

Does all this make sense with what others have seen?

http://forums.pelicanparts.com/uploa...1621291296.jpg

Today I got the new switch from our host. Installed with the washer it comes with - didn't work. Lights stayed on no matter the shift lever position. I replaced with the two washers I made years ago and got it to work. IMO - the range of the switch and rod are too finicky. I hope it stays working for awhile.