Sooo.. 8.3vdc at the reverse connection for the trailer.

[71PA_Highboy]

Working on the trailer wiring and initially had no reverse light power at the connector.

Found a bad connection in the jumper and repaired it and had 12 vdc at the connector in the engine compartment with the factory reverse lights disconnected, but 8.3 vdc with the factory reverse iights connected. Same at the trailer wiring connector. Has a new 14ga wire for the trailer connector running from the engine compartment to the 7 pin connector, so old wiring isn't the issue there.

The factory incandescent lights look bright and would not have guessed a lower voltage based on visual queues.

But 8.3 vdc won't run the LED reverse lights on the trailer.

As I see it, fix options are as follows:

1. Try replacing the B/U switch in the NP435 to see if the switch contacts are the issue.
Pro: simple switch replacement. All factory so nothing requires a change or Mod.
Con: In the lid so probably have to pull the carpet, cover, cut the sound deadening to swap. If it doesn't fix the problem gonna have to do one of the below as well.

2. Leave the switch alone and put a relay in the truck and run the trailer reverse lights from that relay
Pro: very accessible,
Con: Put putting the relay in and making it look clean, only fixes the 12v for the trailer not the truck lighting.

3. Leave the switch alone and put a relay in the truck and run the all the reverse lights from that relay
Pro: very accessible, Fixes the 12v for ANY trailer AND the truck lighting, no additional work over #2 above.
Con: Put putting the relay in and making it look clean.

4. Leave the switch alone and put a relay in the tongue box and run the trailer reverse lights from that relay.
Pro: accessible, fixes the 12v for the trailer and allows the opportunity to put in a switch to allow the backup light to be on manually with out the truck being in reverse.
Con: Put putting the relay in and making it look clean. Additional wiring required. Only fixes the problem for THIS trailer.

I am leaning to #3 and that leaves me the opportunity to do #4 later if the switched backup light become a needed option.

What am I missing? What other options do you see?

 

[total newb]

Not super familiar with ford pu crossmembers but id bet if you pulled the cross you can get enough lean out of it to access the switch.

If theyre like eb nevermind.


Relays not a bad idea, but ive never tried to see if a relay would pull on 8v.
I made a tester power stick some odd years ago out of some pvc and caps, some springs and wires. Stack some aa in till you get the vol range you want (1.5v ea in series ) and apply leads.

 

[CarterKraft]

Assuming source voltage at truck is 12+ bolts?

If you want to find the "drop" put one meter lead at the rear most point of the wire and the other lead at the truck.

In a perfect world you would have 0 volts, but in the real world you might get up to close to .5 volts.

Based on your voltage reading at the bulb you are going to read 4.x volts on your meter.

To further isolate the problem keep the rearmost probe connected and move the front probe toward the rear to the next connection, splice etc.if you lose the previous voltage then the problem is between your new probe location to the previous location.

 

[Oddball]

So with the reverse lights connected youre reading 8.3 volts. So each light must be pulling 2ish volts right. Take the bulbs out and test at the socket. Put the bilb back in and test at the socket to frame. A led should be fine. They usually have a pretty good working range and draw much less current.

 

[71PA_Highboy]

Travelling and no time to answer.. will be back sunday...

 

[71PA_Highboy]

This is what I am seeing. Seems to me there shouldn't be a 3.6v drop across the b/u light switch.

Did electrons change their operating parameters in the 40 years since I went to school?

The 8.3v is under the load of the incandescent light bulbs in the factory taillight housings. Their could be additional load from the old wiring.

When I disconnect the incandescent lights I have the full 12v at the trailer connector with no load. I can try to run the LED load to see what happens later today when I get home (in Chicago now).