Amazon: Voltage drop with brake lights

[zzaletel]

General question - do most of you hear an audible decrease in blower fan speed when you engage the brake lights at a stop? I can it’s ‘normal’ at this point but sub optimal overall.

[142 Guy]

I am guessing that you are probably still running a generator? At idle, the generators generally have less capability (compared to an alternator) to maintain voltage while supplying an increasing current. Simple test - while pressing your brake lights speed up the engine. If the fan speeds up then the drop in blower speed is likely generator related and may just be a fact of life.

The brake light is off the switched +12 v supply that also supplies the blower, wipers and other stuff. The current draw associated with the brake lights can cause voltage drop in this circuit (which includes the ignition switch) which will cause the blower to slow down. Some voltage drop is unavoidable. Excessive voltage drop may be due to bad electrical connections. The only way to diagnose that is go in and measure all the voltage drops or just go in and clean all the contacts to see if that improves things.

Of course, your problem could be both generator and poor electrical connections. Besides cleaning the contacts, you can add an auxiliary switched relay to carry heavy electrical loads which helps reduce the voltage drops. Ron K discusses that type of relay on his web site along with fixing general electrical issues.

https://www.sw-em.com/

[Ron Kwas]

ZZ;

If you have not significantly increased the Brake Light circuit current, which might explain the increased load-caused Voltage Drop, I would say that your symptoms are not typical and suggest a VDrop due to a poor inline connection (Series Resistance!) which is causing the interaction of two only semi-related circuits...I have written about this here: https://www.sw-em.com/electical_circuit_interaction.htm

Thanks to 142 guy for the reference as always, but I don't think you're ready for modifications just yet... Find and remedy the VDrop! I'd start by assuring the System V is not collapsing due to a Charge Sys issue, and then checking for VDrops (when activating Brake Lights), at the Fuseblock Fuses 1 and 2 (Ref: http://www.sw-em.com/Wiring_Diagrams...Wiring_Diagram)
Fuse1 is Ign Power including Fan, and Fuse2 powers Brake Lights...both get power from Term 54 of Ign Sw.

Is there a lot of Green corrosion evident at Fuse1 and 2 and associated connection?...that's also a clue where to look and measure and check for snug connections...

Good Hunting and cheers from the lower fortyeight!

[Bed2492]

Ron are you certain that fuse 2 is for brake lights. My reference on this topic says fuse 1 for brake lights etc and fuse 2 for backup lights - reverse- and horns. Thanks Russ

[Derek UK]

The top and second fuse has a joiner bar on the right hand side so they are effectively the same supply.

[Ron Kwas]

Russ;

Derek has it right of course!...a closer look at either Wiring Diagram and/or Fuseblock would confirm that both F1 and F2 are (both) supplied with Ign pwr from the Ign Sw on the right side (because they are tied together), and loads are connected on the left side...what is your reference?

Cheers

[zzaletel]

Thanks for the feedback all - a few quick replies -

Running an alternator

Hadn’t touched the fuses in my 66 wagon since taking receipt of it in 2018 other than to remove the cover when trying to figure out where the windscreen fluid pump line had gone (eventually found it tucked behind some electrical tape in the main harness going to the front of the car).

Grabbed the dremel, some metal polish and some dielectric grease and got to work cleaning up fuse ends and terminals last eve after some bushings work swapping in a rear right bilstein (other three corners already swapped). The fuses were not looking their freshest. Afterward, don’t know that I’ve ever seen such shiny fuse ends before whether on an Amazon, 240 or Vanagon in my possession.

A brief test post-polish seemed improved but will test further in transit today.

Cheers from the last frontier -
Zach

[Ron Kwas]

Zach;

Polish is only for when you and a shiny surface...abrasive only is fine also to remove surface corrosion/contamination, but I recommend ACZP (zinc dust filled grease), over dielectric grease (which all greases are anyway at this low voltage)...see: https://www.sw-em.com/anti_corrosive_paste.htm

Cheers

Edit: ...and now that I'm thinking about it a bit more, in a Gen based Chg Sys at idle, the Gen is not putting out much at all, so the Batt is supplying most if not all vehicle power...so there will be some VDrop simply due to the current Load of the Brake Lights (not much admittedly), but with a few less-than-perfect connections in the current path, there could be enough VDrop from the Brake Lights, to make that slight drop in the V being supplied to the Blower, which in-turn could be noticeable...

In an Alt based Chg Sys, the Alt puts out more at idle, pulling up the sys V, but this does nothing for a VDrop resulting from a poor in-line connection...the trick is to locate where and which connection is passing both Brake Light and Blower currents, and that's Fuseblock F1 and F2! My recommendation remains unchanged...but now it's maybe explained a bit more...

Good Hunting!

[zzaletel]

Ron -

Couldn't put my hands on the oxgard last evening when I was doing the work, while the dielectric grease was in easy reach. Of course, found the new package of oxgard on the shelf as I was cleaning up after the work was done.

Most important lesson: clean the garage.

Tested the cleanup work from last weekend on the way into the office today - general improvement but still audible drop in fan speed when the brake lights engaged at idle. Additional cleanup planned.

[Derek UK]

It could be that your fan needs some lubrication. The spindles aren't well supplied with oil. Being stiff it would draw extra current and any extra load on the system would reduce what share of it the fan was taking. Result a small slow down. Worth a look.