Reverse-sense (backup) switch grounds out

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ChickenPinz
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Reverse-sense (backup) switch grounds out

Post by ChickenPinz » Sat Jun 27, 2020 10:49 pm

There I was, blissfully happy about my back-up light solution that sends power through the transmission switch to the rear of the vehicle. I even have a low-brightness for reverse gear, and a toggle-switch override for full blinding brightness.

Well, today when adding a back-up camera (Garmin) the front panel breaker tripped.

Turns out the switch on the transmission is not isolated, and may short the two contacts to ground. Perfectly fine if you'd grounded one side of the switch in your installation, but bad if you route power through the switch.

Things had been happy for months, but apparently that's how long it took to wear a sufficiently conductive path to start shorting things out.

I solved this with a relay and all is well now.

Just offering a heads-up to others that if you wired power through the transmission switch it might just be a matter of time until it starts shorting to ground.
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rmel
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Re: Reverse-sense (backup) switch grounds out

Post by rmel » Sun Jun 28, 2020 9:56 am

Bad switch. Like the BRAKE switch the two terminals are isolated from ground.

The BRAKE switch supplies power and is not a grounding switch, the reverse
switch is the same but mechanically actuated. The plunger may be too long
causing your switch to misbehave? There are two switches both Bosch, one
does require the plunger to be replaced with I believe a shorter plunger. The
other uses the existing plunger -- but is a more expensive switch, older SKU.
Both have isolated terminals by design.

The housing on these ZF Tranie's were pretty commonly used (with different
gearing) for cars and trucks where the electricals where different, thus these
are not grounding switches.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712M

Driver: Ron // KO0Q

ChickenPinz
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Re: Reverse-sense (backup) switch grounds out

Post by ChickenPinz » Sun Jun 28, 2020 10:31 am

The utilized switch is the one sold by SAV, and does require a pin removal & swap.
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ChickenPinz
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Re: Reverse-sense (backup) switch grounds out

Post by ChickenPinz » Tue Oct 13, 2020 7:59 pm

So...the new switch has started failing like the first one -- the contacts short to the plunger which shorts out to ground. Anyone else had this failure, or does everyone else tie one side of the switch to ground?

The load on the switch is about 0.1A, so very light.
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rmel
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Re: Reverse-sense (backup) switch grounds out

Post by rmel » Fri Oct 16, 2020 1:00 pm

You may not get a lot of feedback on this as reverse switches are not all that common.
And I would suspect most are a switched ground application leveraging the 24V
available in the tail harness so people just may not have noticed an issue.

Both my trucks have them and are issue free. One Truck is connected as +12V switched
power. The other I'd have to confirm but as I recall it's 24V switch ground. But -- here's
a couple of thoughts.

First make sure the switch is Bosch and not a substitute, though it's also possible it's
Bosch but the wrong P/N for the plunger. Second I'd be suspicious that the plunger
is too long for this particular switch -- pushing beyond the travel limit. One test
of that would be to unscrew the switch to find the make/break point. If that turns
out to be a considerable number of turns out before break, this may be the problem.
It's not impossible the plunger is correct but the SW is the wrong P/N or visa versa.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712M

Driver: Ron // KO0Q

ChickenPinz
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Re: Reverse-sense (backup) switch grounds out

Post by ChickenPinz » Fri Oct 16, 2020 1:51 pm

The switch and plunger is the combination sold by Willy @ SAV, and is unbranded. The version sold by EI looks different in their photographs.

I'm cutting it open on a lathe today, and will report back what is found internally as the failed unit fails (shorts to the case) on every actuation as soon as it's depressed just a few mm.

FWIW, switched ground would be backward wiring relative to the other tail lamps used on the Pinz which are all switched power, and the bulb cans are grounded at the fixture. Switched ground also would require twice the # of wires into the rear of the vehicle. Thus I see switched power as the preferred configuration, but either would be functional. Also, I prefer to not route power to places unless it's powering a load. In the switched ground configuration to a back-up light, you'd need to run "hot" lead to the rear. Of course if one has already done that for other reasons, it's less of an issue.

In order to work around the failed switch, I simply added an inversion relay so connection to ground would be benign.

More details later after the switch is cut apart.

PS: As to the commonality, I'd most Pinz owners who operate their vehicle on US roadways would add the reverse switch as backup lights are required by federal DOT laws: https://www.govinfo.gov/content/pkg/CFR ... 71-108.xml and https://leginfo.legislature.ca.gov/face ... nNum=24606. Alternatively, people could just have added a manual switch as I didn't see a required automatic linkage requirement in the regs.
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Re: Reverse-sense (backup) switch grounds out

Post by rmel » Fri Oct 16, 2020 4:11 pm

Just for clarification purposes -- for those considering adding a back up light.
This was an option on the Pinz, if present the harness would have an Orange
wire from breaker #9, routed to the Tranie switch then to the tail junction box
then to the reverse lamps.

If you don't have this option Orange would have a different use. There are many
schematic versions reflecting the various configurations not to mention the
Austrian config. You may not find the reverse lamp option on your schematic.

On most truck versions, 24V power is routed to the rear, either in the junction
box or to an AUX charging port, if you have that you can tap that (with an in-line
fuse) to power the reverse lights. Then route a switched ground line to the Tranie SW.

Most Pinzies I have seen have red reflectors in the rear, as did mine. That did
not stop getting titled and plated. I made the conversion to avoid backing up
over a cliff or obstacles while on the trail in the dark :shock: This is a good
upgrade to do. I would also recommend using an OEM switch which EI seems to
be stocking.
Puller: 71' 710K 2.7L EFI aka Mozo
Follower: Sankey MK 3, 3/4 Tonne
Rescue Pinz: 73' 712M

Driver: Ron // KO0Q

ChickenPinz
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Posts: 134
Joined: Tue Feb 11, 2020 7:16 pm

Re: Reverse-sense (backup) switch grounds out

Post by ChickenPinz » Sat Oct 17, 2020 10:48 pm

We cut the switch open on a lathe, and identified the failure, which can can be seen in the attached image as the hole in the otherwise solid rubber disc.

Digging deeper to understand why there is a hole in the disc, the conclusion is that the kit of parts from SAV results in the plunger being depressed too deeply, and compressing the rubber sealing disc between the two convex plungers which eventually erodes a hole in it.

The switch supports up to 5mm of overtravel, and can be compressed to a minimum of 15.5mm from the mating face. The supplied Cu sealing washer is 1.3mm thick. This dictates the concave pin in the transmission must be no less than 14.2mm at maximum outward travel. On my truck, I measured about 14.5mm, which is precariously close to the limit.

As a test, I measured how far the switch screws in past initial contact closure, and it goes in 2.5 turns before seating. At 1.5mm thread pitch, that's 3.75mm for a switch that has about 5mm allowed.

As a fix, I will install the new switch with an extra Cu washer, reducing the overtravel experienced by 1.3mm.

For anyone installing the SAV switch, I would recommend looking at how many turns the switch needs past contact closure (transmission in reverse) before seating. If it's close to 2.5T (3.75mm), I'd add a second copper washer which will reduce the turns by 1.3/1.5 --> just over 5/6 of a rotation.
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