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68 RS backup lights blowing fuse in reverse

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Techsan

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Discussion starter · #1 ·
Hello all,
Picked up a 1968 last week and working through a few things to get it registered. Added a horn and seatbelts all of which was completed on the advise and conversation from this site, THANK YOU!

The last item I had on my list was to get the backup lights operational. I have a 327, with a Muncie 4 speed. From what I could gather on many of the other discussions here, most of the wiring seems to be in place. I just added the Hurst switch to the shift linkage and also had to add the boot and wiring from the firewall on the engine side down to the new Hurst switch. Checking voltage and continuity along the way, everything seems good. I can operate the switch by hand with it wired in and the backup lights light up. However, if I use the shifter to put into reverse, it blows the fuse.

Not sure if this is something I am doing wrong, but it seems to be a new issue that wasn't present until I added switch. The blinkers are also connected through this fuse and have historically worked fine.

Any ideas on what is causing this fuse to burn out when the shifter connects with switch?
 
#2 ·
Techsan said:
Hello all,
Picked up a 1968 last week and working through a few things to get it registered. Added a horn and seatbelts all of which was completed on the advise and conversation from this site, THANK YOU!

The last item I had on my list was to get the backup lights operational. I have a 327, with a Muncie 4 speed. From what I could gather on many of the other discussions here, most of the wiring seems to be in place. I just added the Hurst switch to the shift linkage and also had to add the boot and wiring from the firewall on the engine side down to the new Hurst switch. Checking voltage and continuity along the way, everything seems good. I can operate the switch by hand with it wired in and the backup lights light up. However, if I use the shifter to put into reverse, it blows the fuse.

Not sure if this is something I am doing wrong, but it seems to be a new issue that wasn't present until I added switch. The blinkers are also connected through this fuse and have historically worked fine.

Any ideas on what is causing this fuse to burn out when the shifter connects with switch?
Click to expand...
I suspect a defective switch…grounding through the housing when mounted.
 
#4 ·
(y) If you connect your ohm meter to one of the leads on the switch and check resistance to the housing, you should see close to infinite resistance. The switch only completes and breaks the 12vdc circuit when the backup lights are used.
 
Discussion starter · #5 ·
R66 said:
(y) If you connect your ohm meter to one of the leads on the switch and check resistance to the housing, you should see close to infinite resistance. The switch only completes and breaks the 12vdc circuit when the backup lights are used.
Click to expand...
R66, here's what I found on the resistance readings:
Terminal1 to Terminal2: 0.005
Terminal1 to Switch Housing: 1
Terminal2 to Switch Housing: 1
Terminal1 to Ground: 0.004
Terminal2 to Ground: 0.000
Wire to wire: 0.004
 
#6 ·
If this is the switch you have, it is a momentary switch, meaning the backup lights come on when the plunger of the switch is depressed if it is a NO (normally open) switch. You should have close to 0 ohms between terminals 1 and 2 with the switch depressed. Infinite ohms between 1 and 2 with the switch not depressed.
If it is a NC (normally closed) switch, you should have close to infinite ohms between terminals 1 and 2 with the switch depressed. close to 0 ohms between 1 and 2 with the switch not depressed.
In either case, you should have infinite ohms when measuring between either terminal to ground (switch case) in either position.
It appears you have a bad switch. They are very common, I have a couple in my junk stash, and can be bought cheaper from your local parts store or on-line from an electrical supply house.
I would send that one back.

Hurst 2480003 Hurst Back Up Light Switch for Hurst Manual Shifters

This switch is designed to be used in conjunction with the OEM back-up lights and either a Hurst Competition/Plus 4-speed, Super Shifter 3 and MasterShift 3-speed shifter.
www.holley.com www.holley.com
 
#7 ·
I am not sure about your wire to wire measurement. If I recall, the wire is fed from the fuse block 12vdc and goes to the backup lights to light them up. The two wires at the switch are actually the same wire 12vdc from the fuse block. When you turn the ignition on, you should measure 12vdc at one of the two wires and when you connect them together, the backup lights should illuminate. Think of your switch as just completing the circuit by connecting them electrically thus copper contacts.
 
Discussion starter · #9 ·
Hey all, thank you for all the input!

So the switch we have been troubleshooting is the one R66 provided a link for. It is the NC switch from Hurst. So, the wire to wire measurement above, which measures almost no ohms, was me just testing the circuit for resistance just prior to being connected to switch.

If the switch is disconnected from the circuit, I am showing an Ohm measurement of 1, infinite resistance. The terminal to terminal resistance measurement provided above is with the switch wired into the circuit. All of these measurements were taken with the key off.

When I turn the key to the on position, the voltage reading is 11.4 volts at the switch terminals with a resistance reading of 1. Pressing the switch button and closing the circuit with my hand the resistance reading is between 0.2 and 0.3.
 
#12 ·
When testing any device for resistance, such as your switch, it should be isolated from the circuit - wires removed. No power to the switch or wire you are testing. The VOM battery provides the power to do resistance tests.
Thus, your readings appear to be OK as follows:
Measuring between the two terminals (lugs) on the NC switch (plunger out) with the wires disconnected should give you ~0 ohms or a bit more as that is when the switch contacts close to complete the circuit and feed the power to the lights. Measuring between the two terminals with the plunger depressed, you should read infinite resistance as the internal contacts in the switch are open (not in contact) and prevent power from passing to the lights.
Then to verify the switch is not shorted, measuring between each terminal to the metal case (ground) of the switch or to the transmission if mounted, your reading should be infinite for both terminal to ground reading.

If your readings are as above, proceed to the wiring. You have found that one wire is hot (12vdc) when disconnected from the switch or the switch is open (plunger depressed) and the other should not be hot.

If the fuse does not blow with the switch open (plunger in), then it is likely the circuit (wire) from the fuse block thru the ignition switch is good and there is no short in the power circuit to the switch and the short is in the wiring to the backup lights.
If it blows after the switch closed (plunger out), you have a short in the wire to the backup lamps.
Either circuit can be tested with the VOM for resistance once power is isolated (shut off). One end or both ends of the wire must be disconnected to test it to ground. The bulbs in the backup light sockets will provide a path to ground and thus you should remove them also to do the test.
If you connect one lead to the wire and the other lead to a good ground, you should read infinite resistance or at least very high resistance in the highest range for ohms of the meter. If you get a low reading in the lowest range of the VOM, you have a short to ground and need to move down the wire (circuit) to find the bad spot in the wire or a device in the circuit.
I hope I didn't get confused trying to explain how to do this, as I am rather old.
Whatever you do, DO NOT install a larger fuse in hopes of resolving the problem, it will find a short by burning the wiring (and possibly the car) up.
Good Luck. I am happy to see you own and use a VOM. It is as important as your socket set and wrenches IMO.
 
#13 ·
i tried that switch and was jinky when trying to set up for adjustment. Went with this one and worked perfectly, but like Corey went with TKX and its sitting in a box with the Comp + shifter.
Image
 
#15 ·
StevieBB54, the switch you show on the Muncie is original or a good reproduction of the GM switch that comes on the car from the factory. They can be difficult to adjust, but they do work well. I have an original on my 68RS.
 
#16 ·
Its a repo. I checked it out with my ohm meter when I installed it. Adjustment was a simple tweak of the aluminum arm. Most complicated part of the installation was trying to install those microscopic hair pins. Lol sux
 
#17 ·
Its a repo. I checked it out with my ohm meter when I installed it. Adjustment was a simple tweak of the aluminum arm. Most complicated part of the installation was trying to install those microscopic hair pins. Lol sux
 
Discussion starter · #18 · (Edited)
R66 said:
When testing any device for resistance, such as your switch, it should be isolated from the circuit - wires removed. No power to the switch or wire you are testing. The VOM battery provides the power to do resistance tests.
Thus, your readings appear to be OK as follows:
Measuring between the two terminals (lugs) on the NC switch (plunger out) with the wires disconnected should give you ~0 ohms or a bit more as that is when the switch contacts close to complete the circuit and feed the power to the lights. Measuring between the two terminals with the plunger depressed, you should read infinite resistance as the internal contacts in the switch are open (not in contact) and prevent power from passing to the lights.
Then to verify the switch is not shorted, measuring between each terminal to the metal case (ground) of the switch or to the transmission if mounted, your reading should be infinite for both terminal to ground reading.

If your readings are as above, proceed to the wiring. You have found that one wire is hot (12vdc) when disconnected from the switch or the switch is open (plunger depressed) and the other should not be hot.

If the fuse does not blow with the switch open (plunger in), then it is likely the circuit (wire) from the fuse block thru the ignition switch is good and there is no short in the power circuit to the switch and the short is in the wiring to the backup lights.
If it blows after the switch closed (plunger out), you have a short in the wire to the backup lamps.
Either circuit can be tested with the VOM for resistance once power is isolated (shut off). One end or both ends of the wire must be disconnected to test it to ground. The bulbs in the backup light sockets will provide a path to ground and thus you should remove them also to do the test.
If you connect one lead to the wire and the other lead to a good ground, you should read infinite resistance or at least very high resistance in the highest range for ohms of the meter. If you get a low reading in the lowest range of the VOM, you have a short to ground and need to move down the wire (circuit) to find the bad spot in the wire or a device in the circuit.
I hope I didn't get confused trying to explain how to do this, as I am rather old.
Whatever you do, DO NOT install a larger fuse in hopes of resolving the problem, it will find a short by burning the wiring (and possibly the car) up.
Good Luck. I am happy to see you own and use a VOM. It is as important as your socket set and wrenches IMO.
Click to expand...
R66 said:
When testing any device for resistance, such as your switch, it should be isolated from the circuit - wires removed. No power to the switch or wire you are testing. The VOM battery provides the power to do resistance tests.
Thus, your readings appear to be OK as follows:
Measuring between the two terminals (lugs) on the NC switch (plunger out) with the wires disconnected should give you ~0 ohms or a bit more as that is when the switch contacts close to complete the circuit and feed the power to the lights. Measuring between the two terminals with the plunger depressed, you should read infinite resistance as the internal contacts in the switch are open (not in contact) and prevent power from passing to the lights.
Then to verify the switch is not shorted, measuring between each terminal to the metal case (ground) of the switch or to the transmission if mounted, your reading should be infinite for both terminal to ground reading.

If your readings are as above, proceed to the wiring. You have found that one wire is hot (12vdc) when disconnected from the switch or the switch is open (plunger depressed) and the other should not be hot.

If the fuse does not blow with the switch open (plunger in), then it is likely the circuit (wire) from the fuse block thru the ignition switch is good and there is no short in the power circuit to the switch and the short is in the wiring to the backup lights.
If it blows after the switch closed (plunger out), you have a short in the wire to the backup lamps.
Either circuit can be tested with the VOM for resistance once power is isolated (shut off). One end or both ends of the wire must be disconnected to test it to ground. The bulbs in the backup light sockets will provide a path to ground and thus you should remove them also to do the test.
If you connect one lead to the wire and the other lead to a good ground, you should read infinite resistance or at least very high resistance in the highest range for ohms of the meter. If you get a low reading in the lowest range of the VOM, you have a short to ground and need to move down the wire (circuit) to find the bad spot in the wire or a device in the circuit.
I hope I didn't get confused trying to explain how to do this, as I am rather old.
Whatever you do, DO NOT install a larger fuse in hopes of resolving the problem, it will find a short by burning the wiring (and possibly the car) up.
Good Luck. I am happy to see you own and use a VOM. It is as important as your socket set and wrenches IMO.
Click to expand...
Thank you for taking the time to provide all this detail. It’s extremely helpful and I’m trying to soak it all in.

I have power to one wire coming into the switch (light green) and one that doesn’t (dark green).
As you detailed, with the switch wired into the circuit, there is no short. If I close the circuit by operation the switch with my hand, the lights illuminate as desired, this I have been able to repeat without burning the fuse. It’s only when the reverse lever contacts the switch that the fuse blows.
Sounds like I need to continue this investigation with the lights removed. I have not considered that angle yet as the light is grounded via the housing.
 
#19 ·
It sounds like you have a bad or wrong switch.
Put one lead of the VOM on either connection lug of the switch and touch the other lead to the plunger (NC position-plunger out) and check the resistance. Should be infinite.
Stroke and hold the plunger of the switch to the NO position and hold it while you check resistance to the plunger, you should still have infinite resistance
Move the VOM lead to the other connection lug and repeat the test in both the NO and NC positions of the switch.
If you get anything but infinite readings, you have found your problem and need a new switch.
If the switch is still installed, put a couple of layers of electrical tape on the plunger knob and energize the switch to see if the ground is gone. You could BUBBA it by slipping on a rubber cap for vacuum ports, but it wouldn't last long and you want it right I am sure.
Contact Hurst and tell the the results of your test and get a new one free. If they won't send you one, let everyone here know their switches are defective. You should not have a circuit to ground from either connection lug thru the plunger.
I tested two similar switches and neither one uses the plunger as a ground.
You may have never found your problem without the proper tool, your VOM meter. (y)
Ron
 
Discussion starter · #20 ·
R66 said:
It sounds like you have a bad or wrong switch.
Put one lead of the VOM on either connection lug of the switch and touch the other lead to the plunger (NC position-plunger out) and check the resistance. Should be infinite.
Stroke and hold the plunger of the switch to the NO position and hold it while you check resistance to the plunger, you should still have infinite resistance
Move the VOM lead to the other connection lug and repeat the test in both the NO and NC positions of the switch.
If you get anything but infinite readings, you have found your problem and need a new switch.
If the switch is still installed, put a couple of layers of electrical tape on the plunger knob and energize the switch to see if the ground is gone. You could BUBBA it by slipping on a rubber cap for vacuum ports, but it wouldn't last long and you want it right I am sure.
Contact Hurst and tell the the results of your test and get a new one free. If they won't send you one, let everyone here know their switches are defective. You should not have a circuit to ground from either connection lug thru the plunger.
I tested two similar switches and neither one uses the plunger as a ground.
You may have never found your problem without the proper tool, your VOM meter. (y)
Ron
Click to expand...
Hahaha! Well, I’m currently working through the diagnostic, from start to finish again. This time I’ve added the steps you suggested by removing the lamps and testing that part of the system (switch to lamps) to get an infinite reading, regardless of VOM ohm setting.
And I’ve bubba’d it as well tonight. I’ve concluded that Bubba knows a thing or two about getting a job done. It works perfectly with just one tiny piece of duct tape (bubba style).
 

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Techsan
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