Q&A from webinar 187 - Body Electric Circuits

Hey guys sorry for the tech issues! We've got a long evening ahead doing a lot of testing now to ensure they get ironed out.

Amos Garcia: Question with the brake line switch. I have one on my pickup and sometimes if I don’t drive it in a few day the pressure in the lines goes up and triggers the switch. I do come from a very hot climate. Wondering if I did something wrong to make this happen.

Interesting. Does the brake pedal also get hard during this period? I'd wondering if its something to do with your brake booster? Maybe its slowly leaking internally somehow, pressing on the master cylinder and upping the pressure enough to trigger the switch.

Corona: I want to power the indicators in rear of car via small relay - should I be concerned about the longevity of the mechanical connection between relay pins 85/86? Are there other options other than using a low amperage solid state relay? Cheers

I'm thinking by small relay you mean an ISO 280 style? These need to be used with a proper holder for them to be reliable. A quick google will find a good holder and get you sorted on that front. To be honest though, your indicators are going to be a pretty low power draw in the scheme of things, a 21W filament (pretty common) is going to be around 1.5A, and the switch itself will be totally up to the task of passing this through.

David Hodges: Discussion on a dedicated main ground throughout car instead of depending on frame/unibody?

For power transmission applications (lights, defogger, etc) the body will be fine as a grounding point, the voltage drop between the connection point on the body and the engine block (Alternator negative) will be pretty minimal, so whatever the part is will still get plenty of power being delivered. Anything with a composite, or bonded aluminium chassis will need more investigation though, as they can be a little more complicated in my experience.

Dylan Lange: Are dp dt switches reverse polarity

Not 100% sure what you mean here Dylan. The switches simply connect pins, so they dont have a polarity as such. You can use them to switch 12V, or ground :-).

Once again, sorry about the tech issues guys! We're onto it though :-).

Hi Zac, my understanding is that a mechanical relay typically differs in how it makes/breaks contact of the circuit between 85/86 than that of flasher a unit which makes the more audible 'click'.

If I'm installing a relay downstream from my flasher unit (mine is digital with simple 12v output), can I expect that relay to fail [specifically on the switch side] early due to the frequency/usage of flasher unit being upstream?

Yeah, I was considering something like a cheap ISO Relay + Mini Blade fuse holder as the amperage rating over 30/87 is obviously more than sufficient.

Odd ball question, lol, thanks for your time.

Thanks for the followup Zac, Do you have a suggestion for sunroof wiring, I've got a aftermarket roof fitted by the dealer in my old supra and power goes straight to the switch. I'm thinking the switch drawing 7.5 amps either way (open or closed) is abit on the high side, the wiring looks about 16ga also. I was think of setting it up with a two way relay similar to the low/high beam setup as in the webinar?

Amos, about the only way you can get a pressure build up in a brake line is if the master cylinder relief/replentishment port isn't operating properly - this does two things a/ allow fluid to pass from the reservoir to in front of the master cylinder piston and b/ when the brake pedal is released it allows any line pressure to be released back to the reservoir. Sometimes there can be a small bit of debris that sometimes blockes this small hole and sometimes the brake pedal linkage is adjusted a little too much and the piston can't move back far enough to release the pressure. Have you recently replaced any brake components?

Dylan, do you mean "do the switches need to be connected a specific way, +ve to +ve and -ve to -ve?"?

No, but for some specific wiring circuits specific terminals will be used - EG, a on-off-on, as used for indicators, has a common centre terminal and it has to be used that way for that application.

J C - On a standard relay, pins 85 and 86 are connected to either side of an electromagnet (solenoid). When you power pin 86, and ground pin 85, current flows through the electromagnet, which pulls closed the connection between pins 30 and 87. If you have a look at webinar 183 I've got some video in there of a relay working with the upper housing removed, so you can see the internals move.

You indicators will flash at a slow enough rate that using the output from the flasher to power the relay coil, which then in turn powers the bulbs will be fine. Sure, it'll wear out faster than if it wasn't be actuated as much, but I'd still expect many years of work out of it. However your indicators shouldn't be drawing so much current that you need to power them by a relay.

Craig - It'll depend on the switch that's been used, if you can get to it, often they will have their current rating printed on the side. If its inside its rating, adding a relay wont make things any more reliable really, infact it'll be adding another possible point of failure.

Here is some information for understanding poles and throws and the many different configurations.

https://electronicsclub.info/switches.htm