Engine/Transmission (1998.5 - 2002) electrical demons, please help.

[Jeepmullet]

Okay, I've been working on this project on how to wire in some reverse lights using a SPDT switch using both on terminals as power inputs and the middle pole as the accessory being powered. I'm using a fuse tap for power for the relay when the switch is in one position. I found that both sides of the reverse light fuse are above ground potential all the time. one side has 1. 5 volts and the other side has 12. 5 volts or so. Either side of the fust when I put the fuse tap on is switching on the relay whether the reverse lights are on or not. Why would both blades be above ground potential all the time? I understand that one of them needs to be.

 

[nps]

It means that the fuse is putting out constant 12V to the reverse light switch which then switches the 12V on or off to control the lites.

 

[WestTN]

Lights used to be grounded all the time and power was switched to them to turn them on. Now these trucks have the power hooked permanently and the ground is switched to activate the lights.

 

[2-ND TIME]

electrical problems

I read most of what you wrote but am too tired for much more. Sorry

First thing is the SPDT is probably the wrong switch to be using. What the spdt is ment to be used as is to feed the center and then feed out one of the other at a time. Such as if you wanted to use lights on one side of you truck to work on something or the other side you would use a SPDT. I would just us a SPST. Are you using the factory wiring if so run a wire to the coil of a relay and then to ground. Then all the extra load on the system is the approximatly 0. 150 amps extra to the system. Us a fuse to power your switches for the coils in the relays. Fuses as well for the power to the relays for your backup lights.

 

[Thomas]

... I found that both sides of the reverse light fuse are above ground potential all the time. one side has 1. 5 volts and the other side has 12. 5 volts or so... Why would both blades be above ground potential all the time?...

Both blades of the fuse are essentially the same point in the circuit, separated by a short length of wire - the fusible material in the fuse.

I question the two different voltage readings you claim for the fuse blades. I assume that the voltage was tested with the OEM backup lights off. Unless the fuse is defective, the voltage readings should be essentially identical.

The basic backup light circuit is:

+12v battery terminal > fuse 7 in terminal block > backup light switch on transmission > backup light filament > ground

If you wish to use the existing backup light switch to trigger the relay, you need to tap the violet/black wire between the backup light switch and the backup lights.

 

[PRout]

I question the two different voltage readings you claim for the fuse blades. I assume that the voltage was tested with the OEM backup lights off. Unless the fuse is defective, the voltage readings should be essentially identical.

Exactly. . It was sounding like the fuse was blown since one blade had different voltage than the other.

 

[Jeepmullet]

yeah I was being an idiot not remembering that there was a switch on the transmission. The fuse was not blown.



A SPDT switch is exactly what I need, with a center off. The center pole is attached to the relay switching power. The outer poles are the power sources. If it's in one position they are switched on by the OEM lights (when I get that wired) and in the opposite position they are forced on at all times.



I was told it does not matter if you use the SPDT switch backwards from how it was intended, that the voltage direction does not matter. I'll tap that wire on the transmission switch today and wire it into one of the outer poles of the switch.

 

[cojhl2]

Now that we are here,, why do some relays have a resistance across the primary circuit in the solenoid? The resistance is internal to the relay.



Does it make a difference which one I use? What is the criteria of selecting one over the other?



So much to know so little time!