Running Light Circuit Current Draw (1 Viewer)

[Wildcat Walker]

75' 40 Series, stock (virgin, no hacks or mods) electrical system. The circuit for the running lights is drawing about 5-5.5 amps with all bulbs operating (multimeter in series with one side of fuse panel and end of fuse). The fuse panel gets noticeably warm to the touch when the circuit is energized. I haven't added up the wattage of all the bulbs and done a calculation but 5-6 amps seems reasonable for this circuit. The rated value for the stock fuse is 15 amps and I have replaced it with a new one with no change. Am I paranoid or is this heat at the panel normal? I have cleaned the contacts on the fuse block and is visually in good working order. @Engineer8000 What are your thoughts on this?

 

[Wildcat Walker]

I cleaned those terminals during the diagnostic process. Heat is the energy generated by current flowing across a passive element in the circuit, as you know as an EE. Assuming source voltage and the other passive elements in the circuit remain unchanged, then increasing the resistance at the fuse panel due to corrosion will lower the current coming from the source. I believe I am thinking correctly. The fuse is designed as a sacrificial passive element, taking advantage of the properties of the filament inside to burn when a calculated amount of energy is dissipated through it, ie. burnt. I'm not sure what the "safety factor" is that's built into the circuit, but right now with my measured current draw, I'm at about 3. Even propagating the error associated with my measuring device, I'm still between 2 and 3 for SF. Full on Engineering breakdown discussion in 3, 2, 1, ...

 

[Living in the Past]

By pass the fuse and check you amp draw. Landing your inline meter at the wire feeding the the fuse panel and remove the parking light wire completely will remove the fuss panel from the test of the lights. Having a meter in the circuit you will be monitoring the amp draw while the fuse block is bypassed. The clips for the fuses are tempered sprig metal. Once it has gotten hot enough it will become a resistor in the circuit. Did HVAC as a career. If the clips for the fuse in a disconnect were soft and no longer acted like spring steel the disconnect was bad. Just pinching it together would not solve the problem. That section would always a resistor.

 

[Wildcat Walker]

By pass the fuse and check you amp draw. Landing your inline meter at the wire feeding the the fuse panel and remove the parking light wire completely will remove the fuss panel from the test of the lights. Having a meter in the circuit you will be monitoring the amp draw while the fuse block is bypassed. The clips for the fuses are tempered sprig metal. Once it has gotten hot enough it will become a resistor in the circuit. Did HVAC as a career. If the clips for the fuse in a disconnect were soft and no longer acted like spring steel the disconnect was bad. Just pinching it together would not solve the problem. That section would always a resistor.

I can do that, and their may be some corrosion that its causing this problem. I wanted more to understand if my measurements, and thought process was reasonable.

For example...

I am measuring a current draw of about 5.5 amps across the fuse holder terminals, this is the current that the fuse is experiencing under load, adding the fuse in series makes no measurable difference. Isolating the entire fuse block may yield a slightly lower current measurement, and one may easily place blame on the fuse block itself, but is this reasonable? Calculating the load on the circuit from the wattage of the various passive elements (bulbs) yields a current value very close to what I am measuring. This leads me to believe there isn't anything wrong at all. The values I used are...

• Front Marker #67, 7.97 W x2 = 15.94 W
• Rear Marker #67, 7.97 W x2 = 15.94 W
• Tail Lamp #1157, LOW 8.26 W x2 = 16.52 W
• Instrument #194, 3.78 W x3 = 11.34 W
• License #89, 7.54 W x2 = 15.08 W
• Dash #3022, 3W x2 = 6 W
• Total 80.82 W or 81 W

Assume 13.5 V and 5.5 A measured = 74.25 W
Assume 13.5 V and 81 W calculated = 6 A

Again, Maybe I am paranoid? I don't recall ever touching the fuse panel before so I can't say of the amount of heat there is normal or not. I could Isolate the fuse panel, measure the current, calculate delta I and then perform a thermal energy calculation and see if this result is reasonable. I could also measure delta T and work this same equation in reverse. @Engineer8000 Is my logic sound?

 

[Wildcat Walker]

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[Wildcat Walker]

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[Wildcat Walker]

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[Coolerman]

Your math is correct and what you are experiencing is normal. Most don't realize that the park light circuit is one of the larger current draws on the truck. Its the same as the original Kioto sealed beam headlights on HI beam! (modern H4 bulbs can pull up to 9.7 amps on HI) It amazes me that Toyota only used 18ga wire for the entire park circuit. I upgrade that to 16ga when building harnesses. (No I am NOT starting harness building again...)

I would highly suggest you do the following to ensure the fuse panel is operating with the least resistance possible.
1. Remove the fuse panel from the truck. Clean it with a tooth brush and hot soapy water to remove any dirt.
2. Get a container tall enough or deep enough to completely submerge the fuse panel in liquid.
3. Obtain some standard white vinegar and some good old table salt. Mix up about a pint of vinegar to 2 table spoons to table salt. Adjust for you container.
4. Pour this mixture over the fuse panel and let is soak for about 20 minutes. The mild acid will remove any corrosion or rust and breakdown that black coating.
5. Remove and throughly rinse with fresh clean water. Let dry or use a heat gun or blow dryer if you are impatient.
6. Use a Dremel type tool with a steel wire wheel and go over every single fuse contact inside and out and do the same for the terminals on the bottom. They should shine when you are done.
7. Install all NEW fuses into the now cleaned fuse panel and re-install it in the truck.

 

[Wildcat Walker]

@Coolerman YOU'RE ALIVE!!!! Glad to hear from you. Wow what about those monsoons in your neck of the woods! I didn't think I was ever gonna make it back home last time I came through there!

I do need to pull the fuse block and run it through the ultrasonic. I usually use a solution of dawn and citric for brassy stuffs. Will post before and after picks when I do. Thanks for the tip!

I sent you a pm about some connectors.

 

[Coolerman]

Reports of my demise have been greatly exaggerated! I'm still not building full harnesses, but still selling parts, and making sub-harnesses.
My day job had gotten really busy so I had to stay off MUD for a while. Things have slowed down considerably, so I have more time to come here and help out with electrical issues.