Thread: Voltage drop when brakes applied

The meter should drop a little, but not as much as when you turn the head lights on.
You can:
Take the bulbs out of the tail lights and then push the brakes.
If you still have a voltage drop (with no load) then you have a short to earth somewhere.
Suspect areas:
the brake light switch (behind the brake MC)
The tail lights themselves
The hole where the loom enters the chassis in the engine bay.
The hole in the firewall behind the air filter where the loom exits the cab.
Look for chafing and broken wires.
Good luck

Originally Posted by Phil B

The meter should drop a little, but not as much as when you turn the head lights on.
You can:
Take the bulbs out of the tail lights and then push the brakes.
If you still have a voltage drop (with no load) then you have a short to earth somewhere.
Suspect areas:
the brake light switch (behind the brake MC)
The tail lights themselves
The hole where the loom enters the chassis in the engine bay.
The hole in the firewall behind the air filter where the loom exits the cab.
Look for chafing and broken wires.
Good luck

hmmmm. I was actually under the rear bumper yesterday and spent a few hours re-wrapping the loom which had chafed through the wrapping where it exits the chassis at the rear, and in several other places along the rear bumper.
I wonder If I missed a ground connection or something.
Thanks Phil, you've given me a few things to check!




suggestion for everyone, if you haven't already done so, check the condition of the wrapping where the looms passes through the chassis. Mine was rubbed through in several places, and what remained of the wrapping was completely dry and fell apart when I touched it.

Just did some testing.
Removed the globes and no voltage drop.

Checked out the ground line and it seemed good.
The ground clip on the bulb holder for the brake light was a little loose so i tighten that up - made the light much brighter !

Does this mean it's ok ? It may have been doing it the whole time I've had the car, I may have just not noticed???

Last thing is that the bolt that the lights ground wire runs to is a bit corroded; couldn't work out how to remove it. it looks like it is just sitting it the bracket, but a few reasonably heavy hits on the back of it didn't move it a bit. I need a bigger hammer or is there a trick?

Sounds like corrosion is the problem.
A bad earth will increase resistance (thus the dim glow) and that will show as a drop on the volt meter.
Best bet is to strip all the lights and holders and clean/renew all connections and earths.
I would do the head lights as well as they are also know to fail.
The wiring connections have been in the path of dirt and water for 20+ years and the resultant corrosion is your problem IMHO.
Regards,

Originally Posted by Phil B

Sounds like corrosion is the problem.
A bad earth will increase resistance (thus the dim glow) and that will show as a drop on the volt meter.
Best bet is to strip all the lights and holders and clean/renew all connections and earths.
I would do the head lights as well as they are also know to fail.
The wiring connections have been in the path of dirt and water for 20+ years and the resultant corrosion is your problem IMHO.
Regards,

On the list of things to do.
The whole bumper/chassis rail there was/is completely full of mud.
It's just a pain that the wires are all crimped in, rather than have a plug.
Mine was worked pretty hard in the Army 145000K and not rebuilt. To be expected I guess.

Headlights is on the list as well. when funds permit, I'm going to get that Traxride wiring kit and do it properly.

Cheers

Originally Posted by Phil B

Sounds like corrosion is the problem.
A bad earth will increase resistance (thus the dim glow) and that will show as a drop on the volt meter.
Best bet is to strip all the lights and holders and clean/renew all connections and earths.,

Hi
I think it's the opposite. High resistance = less current and less voltage drop.
My RFSV does the same thing. I checked out the wiring no problems found. My next job is to put my fluke on the battery to check the volt meter, just haven't got around to it yet.
Cheers Andrew

Everybody on this thread is more or less correct about resistances causing voltage drop, but let's think about it a bit more specifically for this case. Firstly, I suspect most Perenties do this - mine certainly does - and I don't think it is a problem.

A voltmeter measures the difference in electrical potential between two points - what we call the voltage. This measurement can be extremely useful or a complete waste of time depending on where those two points are in the system. In our Perenties, the measurement is taken between a spot off the ignition feed out of the ignition switch and the bulkhead earth, which is a vague term for a collection of corroded ring terminals screwed into points in the bulkhead which are now rusty.

Why is that not good? In between the battery (who's voltage you are actually interested in) and the point measured, you have:
- Battery terminal
- Battery cable to starter motor
- Ring terminal on starter motor
- Other ring terminal on starter motor
- Cable to ignition switch
- Ignition switch and connectors in and out
- Ignition feed cable
- Whatever terrible joint is made to tee off the ignition feed*
- ignition feed to voltmeter

Voltmeter

- Voltmeter earth cable
- Aforementioned "bulkhead earth"
- Whatever electrical connection is scavenged between bulkhead and chassis
- Main chassis earth connection point*
- Main earth cable
- Negative terminal connection to battery

Every one of the listed elements has a certain amount of resistance and a certain amount of current drawn through it depending on what is switched on or off. Resistance x current equals the voltage drop and means a discrepancy between battery voltage and measured voltage.

The brake lights are the problem here, so let's indicate where they hop off the path to the voltmeter and then when they rejoin (*s). Your problem is before it hops off, or after it hops back on.

Could it also be a bad earth? For interest's sake for the moment we'll say yes, BUT it wouldn't be the earth at the lights; it would have to be the main earth point on the chassis or after it. Why? Because the voltmeter runs through this earth too.

If the resistance was at the light's earth, the voltage the bulbs receive would be split up between the bulb itself - the "design" resistance - and the new bad earth resistance. The bulb would be dim because of that, but the total voltage between the ignition feed at the bulkhead and the main chassis earth point (and therefore the voltmeter reading) remains the same. The same argument can be applied to the brake switch, blackout light switch etc. These are not our culprit.

The reason we know it isn't the main chassis earth though is because this would cause the voltmeter to read a drop when anything with a big current draw is turned on i.e. the headlights. So not the main chassis earth then.

That leaves us with the elements before the brake light circuit jumps off the voltmeter circuit.

The brake lights draw their current through the ignition switch and I suspect the resistance of the ignition switch itself is what will cause this voltage drop. The biggest current draw through the ignition switch by a fair margin is the brake lights. It could also be other things causing resistance on that circuit - i.e. the things before the first asterisk - or a combination of all of them.

The headlights won't drag down the voltage on the circuit that the voltmeter is on because they are powered via a relay that means the current bypasses the ignition switch and most of the things on the list above.

If you wanted to wire up the voltmeter properly - i.e. to measure the battery voltage rather than the voltage at a arbitrary point in the loom - you could wire it directly to the battery via a relay switched by the ignition feed and the earth wired directly to the battery negative terminal... Or you could just accept that this is the voltmeter you have and learn it's foibles, in other words 'if it ain't broke don't fix it'.

If you grab a voltmeter and measure the battery voltage when the brake lights are on I guarantee it will remain constant; battery charging isn't being impeded, your alternator is not dying, it's just a foible of the vehicle.

Wow, that took a lot longer to type than it did to think! Hopefully that comes through as a reasonably coherent insight into circuit troubleshooting and not just me rambling.

Thanks for this information as I have the same thing happen on my Perentie when the breaks are applied, gives me some places to start looking