[FJtest]

OK, let's look at this step-by-step.

I assume that you have actually measured battery voltage, and confirmed that it is somewhere between 12.6V and 12.9V with the engine NOT running.

First, let's look UPSTREAM of the sockets, at the B+ supply to the backup light circuit. You stated that you had previously tapped into this wire to run a backup camera, had disconnected the camera, made a voltage measurement at the tap point, and saw only 8V.

The backup light electrical system is very simple. B+ for the backup lights is supplied directly from the battery through the ignition switch, the IG1 fuse, and switched via the BACKUP LIGHT SWITCH on MT vehicles, or via the PARK/NEUTRAL switch on AT vehicles. The switched power then goes directly to the backup bulb sockets.

IMPORTANT! Please clarify exactly which wire you tapped for your backup camera. The YELLOW wire with RED stripe (which you identified as the location of your tap) is the constantly-on B+ supply for the backup light circuit. If you actually tapped into this wire for your backup camera, it would have been on all the time.

The RED wire with YELLOW stripe is downstream of the backup light switch, and voltage is only present when the backup switch is closed (transmission in reverse).

(In Toyota schematics, the first color is the basic wire color, the second color is the ID stripe.)

So, if your tap point is truly on the YELLOW/RED wire, and you measured only 8V to a solid chassis ground, you either have high resistance in the wiring somewhere between your tap point and the battery, or the battery voltage is down around 8V.

If you actually did tap into the SWITCHED side of the backup light circuit (RED wire with YELLOW stripe) wire, and you are measuring 8V, then the suspect is high resistance in the backup switch contacts, or high resistance where the wiring harness connector plugs into the backup switch.

Go back and verify which wire you actually tapped into, and repeat the voltage measurement, making sure the ignition is on (but obviously with the engine not running), the shift lever is in reverse, and that you are picking up a solid ground point for your meter.

While monitoring the voltage at your tap point, move the shift lever rapidly between the P and R, or N and R, positions multiple times and see if the voltage stays at exactly 8V, or varies slightly with each shift. If it varies, the problem is most likely contaminated contacts within the reverse switch.

(Also, what type/make/model of meter are you using to make these voltage measurements?)

 

[FJtest]

OK, I think the photo of the wire and the damage caused by your "crimp-on, insulation displacement" tap may tell the whole story.

Unless they are very carefully installed, the crimp-on taps end up cutting multiple strands of conductor in the wire. Depending on how many strands were cut, this decreases the effective wire gauge, increases the resistance of the wire, and increases the voltage drop, so the voltage seen at the load is less than the supply voltage at the battery.

The fact that the voltage DROPS to 8 volts with the bulbs installed, and INCREASES to 11 volts with them removed, clearly indicates that you have a high resistance somewhere in the backup light circuit. The voltage drops as soon as you begin to draw any current through the circuit.

ACTIONS:
1. Get the battery replaced, verify the charging circuit is operating correctly, and that the battery voltage indicates that the battery is fully charged (around 13V).

2. Re-measure the voltage at your tap point WITH THE ORIGINAL INCANDESCENT LAMPS INSTALLED, ignition ON but engine not running, and shift lever in REVERSE.

3. Repeat measurement #2, but at the bulb socket with the bulb installed, back-probing from the wire-entry side of the socket.

Let us know what your voltage measurements are.

If you are going to try to perform any real electrical troubleshooting, you're going to have to get a modern, mid-grade digital mulitimeter that can accurately resolve tenths of a volt and tenths of an ohm.

Your grandfather's 40 year old "Micronta" (Radio Shack) analog meter probably isn't going to cut it. Comparing it against another uncalibrated meter of unknown accuracy doesn't achieve anything.

And never, NEVER us a crimp-on, insulation-displacement wire tap for ANYTHING.

 

[FJtest]

Let's wait and see what your voltage readings are with the new battery before attempting any repair of the damaged wire.

Are you experienced with electrical soldering, have a good quality, appropriate wattage soldering iron, rosin-core solder, and heat-shrink tubing?

The best solution would be to cut the wire, perform a lap-splice, and insulate the splice with heat-shrink tubing, but if too many strands are NOT cut, you may be able to just tape the damaged area to insulate it.

One other thought ... do you have ANY other electrical loads that are being driven by the backup light circuit? Additional backup lights, etc.?

 

[FJtest]

Ok sounds good. Not sure I will have a new meter by Tuesday, but I'll order one for sure. I have another one that I found in my family's stuff CB-instruments GMT-18A meter. Not sure that is any better.

I have a done a small amount of soldering over my lifetime. From RC cars as a kid, to stereo/speaker work, and now tinkering with aux lighting in the FJ. I have some 60/40 Rosin Core Solder wire, plenty of shrink tube.

I don't have any other loads currently hooked up to the backup light. I removed my backup camera from the reverse light at the present time, but hoping to somehow tie it back into the reverse light at some point. My AUX rear flood lights are manually controlled through an aftermarket switch that I installed.

As I previously mentioned it sounds like the rear backup sensor system is powered through the same cable because it won't work when I have lightbulbs in the socket.

Thanks.

The backup sonar system is powered through a separate circuit, but has a "sense" line connected to the backup light circuit to provide a signal to the backup sensor ECU to tell it when the transmission is in reverse and turn on the sonar system.

The "beep" you heard was from the backup sonar system as it was getting spurious low-voltage trigger signals from the backup light circuit. If the voltage in the backup light system is too low, it won't properly trigger the backup sonar system ECU to turn on.

 

[FJtest]

If there is sufficient wire free length, go ahead and perform a lap splice at the damaged area, overlapping the conductors by 3/8-1/2 inch and carefully soldering. Make sure to install the heat-shrink tubing before soldering, and slide it far enough away from the splice so it doesn't start to shrink during soldering.

I'm a little skeptical that your only problem is the damaged wire strands where you had your tap; there may be another high-resistance point in this circuit.

Perform the splice, and see if that completely resolves the voltage drop problem.

If not, the next steps are to check for any corrosion at the IG1 fuse, and check the backup switch for internal resistance.

 

[FJtest]

I suspect that the BACKUP switch itself (or the connector at the backup switch) may be the real culprit.

With the incandescent bulbs installed, ignition on, and shifter in reverse, measure the voltage at the following points:
1. Downstream side of the IG1 fuse;
2. B+ supply side of the backup switch;
3. Switched side of the backup switch.

If the voltage is low at the IG1 switch, you have excessive resistance in the wiring somewhere between the fuse and the battery;

If the voltage is low at the B+ side of the switch, you have excessive resistance in the wiring between the B+ side of the switch and the IG1 fuse;

If the voltage is OK at the B+ side, but low at the switched side, the resistance is within the switch itself.

One other thought ... are you ABSOLUTELY sure you don't have some other electrical accessory connected to, and drawing current from, the backup light circuit? Like a "converter box" or other jury-rigged wiring for trailer lighting?

 

[FJtest]

Ok good notes. One clarification...when you say "backup switch" you are referring to the rear backup sensor activation button and not my aftermarket backup lights that I installed? My aux backup lights are running to their own relay under the hood and powered in one of the available KEY ON open slots in the fuse panel. I can get the exact here shortly.

I'll triple check everything again, but I don't think I have anything else wired to that. My lighting and electrical mods at this time include the following PLEASE NOTE - ALL DISCONNECTED DURING THESE TESTS):

1- Interior LED trim and accent lights.
2- Some LED PUCK lights underneath side mirrors that are tapped into the ceiling lights.
3- Fog/driving lights.
4- Rear Aux LED backup lights.
5- Aftermarket Pioneer stereo with Backup camera. Camera was originally tapped into that YELLOW/RED line. I have removed it completely.
6- Interior LED Map light

I did install the factory OEM trailer hitch harness that didn't come standard when i purchased my FJ new back in '06. I bought the part from Toyota directly. I could disconnect that perhaps?

The "backup lamp switch" is located on the transmission, and senses when the transmission shift linkage is placed in the REVERSE position. It's item #42 in the illustration below. It has nothing to do with the backup sonar system except to provide a signal that the transmission is in reverse.

Because of its location, it is exposed to mud, road salt, dust, and possibly oil leakage from the engine.

None of the other electrical accessories you mentioned or OEM trailer lighting harness should affect the backup lights.

 

[FJtest]

I think you are 100% right on the switch being the potential culprit. I'm just going to go ahead and order a new one since its only around $40. I'll solder the tapped wire as well and see what happens. Thank yo.

But ... rather than just blindly replacing parts, why not measure the voltage drop across the switch contacts, or the resistance when the switch is closed?

This is where your mid-grade digital multimeter will instantly pay for itself ... if the voltage drop across the switch is more than 100 millivolts when the switch is closed (in-circuit test), or the resistance is more than 0.1 ohm (switch disconnected from harness), then the switch is bad.

If these measurements are good, then the switch is NOT your problem.

 

[FJtest]

No clue where you're located, and I usually find that the majority of Harbor Freight tools should be considered "one-use, disposible", but I did purchase several HF Cen-Tech multimeters years ago for my kids to learn with, and they were actually of decent quality.

However, I don't see those models currently offered, but they do have an "Ames DM600 Digital Multimeter" that is relatively inexpensive ($39) and might meet your needs. Bear in mind that a good-quality multimeter like a Fluke will cost you $200 - $500, but that's way overkill for your application.

AMES
DM600 Compact Digital Multimeter

 

[FJtest]

Hi, so I went ahead and swapped out the backup switch and BOOM! its working! I really appreciate all your time and efforts to help me pinpoint this issue. I have definitely learned a few things from this experience and feel liked a saved myself a bit of money at the same time. I also thought it would be helpful to attach this document that I had stored about the entire backup system and how it is wired and laid out. It help me understand all of this a bit better.

Glad to hear that you verified that the root cause was high resistance across the switch internal contacts.

I sent you a Private Message requesting the opportunity to perform a failure analysis on your switch; please don't throw it away!

 

[FJtest]

Hi, so I went ahead and swapped out the backup switch and BOOM! its working! I really appreciate all your time and efforts to help me pinpoint this issue. I have definitely learned a few things from this experience and feel liked a saved myself a bit of money at the same time. I also thought it would be helpful to attach this document that I had stored about the entire backup system and how it is wired and laid out. It help me understand all of this a bit better.

dantonb14, are you still out there??

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