how to bench test a 2 post brake light switch

[kdancy]

These are the type that screw into the brake line fitting+ and have two posts for wires. I have two old ones and one new one that I have hooked up an ohm meter to and then manually pushed in a plunger where hydralic fluid enters the switch. No ohmns when plunger is in or out.

I know all 3 are not bad, what might I be doing wrong?

[rodneybeauchamp]

Never tested these, but it should be infinite ohms when off and zero when on. Three positions to try the leads if it has two terminals.

 

position one- One leads on each terminals

position two - One lead on first terminal, Other on body of switch

position three - One lead on second terminal, other on body of switch.

 

This should check if working, it did on a Buick carburettor start switch.

 

Good luck!
Rodney 😀😀😀😀😀😀

[Oldtech]

should just be the 2 pins connected when pressure applied. might have to rig up something to screw it into and apply air pressure

 

[37_Roadmaster_C]

I have never tested this type of switch, but I think you would need to actually put brake line pressure on it to test. Compressed air MAY be enough pressure. The brake line pressure is very high. According to online source, manual brakes at full lock would be as much as 1000 psi and power brakes 1400 psi and up. With this in mind, compressed air is normally between 125 and 175 psi. The electrical switch connection for the switch you describe is most likely between the two terminals with OHM reading being low when brake applied. I say low because many ohm meters display something like 0L when probes are not connected. Some people refer to this as "0" ohms incorrectly. It actually means "Over Load", which in terms of an ohm meter is "more ohms than it can measure". To be sure, short the probes on your meter together and this will be low ohms (switch closed/on). When the probes are NOT shorted together is very high ohms (switch open/off). The other connection possibilities mentioned above are also possible, but unlikely. In any case measure all three possibilities to be sure! I hope this gives you some help.

 

[Frank DuVal]

1 hour ago, 37_Roadmaster_C said:

many ohm meters display something like 0L when probes are not connected. Some people refer to this as "0" ohms incorrectly. It actually means "Over Load", which in terms of an ohm meter is "more ohms than it can measure".

 

Or, Open Leads!👍

 

Or, Open Loop.😉

 

Back to OP's question, you need pressure to close the switch. That's the only test that shows if it would work in the hydraulic system.

[Jubilee]

 I have rigged up some fittings and used a grease gun to close similar normally open pressure switches 

[37_Roadmaster_C]

47 minutes ago, Frank DuVal said:

 

Or, Open Leads!👍

 

Or, Open Loop.😉

 

Back to OP's question, you need pressure to close the switch. That's the only test that shows if it would work in the hydraulic system.

@Frank DuVal fair enough .  @Jubilee has an excellent way to test these switches!!!

[m-mman]

21 hours ago, Jubilee said:

 I have rigged up some fittings and used a grease gun to close similar normally open pressure switches 

 

NOT ON A BRAKE SWITCH!   

 

If you squirt grease into a (mostly sealed) brake component how would you ever clean it out to avoid contaminating the brake fluid in the system??? 

 

The only 100% best method is to screw it into a known good system. With 2 wires one is always hot (from fuse box probably, I have never seen a brake light that was key operated)  The other wire leads to the rear bulbs. With a test light you can see if there is power to the switch and apply pressure and see if there is power on the other terminal. 

 

Maybe there is a application where grease contamination wont matter, but the brake system is not it. 

Edited August 31, 2020 by m-mman (see edit history)

[PFitz]

The switches are usually 1/8 pipe thread, so it's easy to connect them to compressor air supply hose and no risk of grease, or other pressuring method contamination.

 

I thread them into a compressor air line then slowly turn up pressure using an airline pressure regulator. Then I watch the pressure gauge reading and the volt/ohm meter reading to see if, and at what pressure, the gauge reads zero resistance. Sometimes I can see it's not consistent indicating that the contacts inside the switch are not making good contact.  

 

Many of the pressure switches from early non-boosted hydraulic brake systems make contact in the 40 to 45 PSI range. The newer switches are much higher pressure - often over 100 PSI.  Pushing in a rod to test, when the diaphragm inside the switch is meant to only make contact at over 100 pounds per square inch applied over it's entire surface, may not be enough total pressure to move the diaphragm against such a strong spring and activate the switch.

 

Paul

[Oldtech]

I'm thinking 100 lbs air pressure should trip it easily. don't put grease in your brake system.