Brakes, Vacuum Pump, Low Vacuum Switch Fails, Question

Maintenance/Repairs
1

Technician A says that an electric motor-driven ­vacuum pump may quit pumping if the low vacuum switch fails. Technician B says an engine-driven vacuum pump can quit pumping if the low vacuum switch fails. Who is right?

(A) A only.

(B) B only.

(C) Both A and B.

(D) Neither A nor B.

This question is from ASE brakes practice questions. My study guide doesn’t really describe this to well and there is no answer key. I couldn’t find the answer online.

I believe the answer is (A), at the very least, maybe even (C) Both A and B. Was wondering what others thought.

I believe the low vacuum switch is a normally open switch. When the switch closes, it lets the comptuer know that the there is a low vacuum, so the pump turns on. So if the switch fails, the comptuer will never know there is a low vacuum, so the pump will not turn on. The electric motor driven vacuum pump can turn off and on as desired by the comptuer. Where as the engine-driven vacuum pump by a belt I invision would always be on as the pulley would always be spinning if the engine is on, but I don’t know if such a pump could have a clutch, resulting in the pump running only when desired, kind of like a AC compressor? It also just says the switch fails. So we don’t know if the switch is stuck closed, so the comptuer thinks that the vacuum is always low, or if it is stuck open. So there is that to consider as well.

Thank you!

2

For the systems I am familiar with; mechanical vacuum pump driven directly from the camshaft (no belt), the low vacuum switch is for the warning light. The low vacuum switch does not control the vacuum pump.

3

Hey thanks. It looks like this question may have been placed in the wrong chapter. I found this

“Some vehicles equipped with a vacuum pump have a vacuum switch mounted on the power assist unit housing. The switch will activate the dashboard brake warning light if vacuum becomes excessively low.”

“The vacuum pump motor is usually controlled by a relay operated by a low vacuum switch. If vacuum is not sufficient, the switch energizes the relay, which turns on the pump. When vacuum reaches a preset value, the switch and relay turn off the pump.
A few older vehicles have a belt- or gear-driven vacuum pump. Operation and service of these pumps is similar to the operation of electric motor drive diaphragm pumps.”

Sounds like what you are saying for the mechanical ones, can be gear driven, by the camshaft, in which case are they always on and cannot be turned off, for the gear driven mechanical vacuum pumps.

Belt driven vacuum pumps, I think “can” be turned off if there is some sort of clutch system like an AC compressor, but I don’t know that such a thing exists.

For the electric vacuum pumps, sounds like it’s a normally open switch, when the vacuum is low, the switch closes, energizing a relay, and turns on the pump. Once the switch is no longer closed because vacuum has been restored and no longer low, the switch opens up, and the pump turns off.

So if the switch fails, well we don’t now how it fails, stuck closed (pump always on) or broke open (pump always off). Broke open is much more common, so I guess I have to assume that is what is being discussed. Then if I interpert the last sentence of the quote that I italized, as the operation is the same with a switch that controls if the pump is on and off, than the answer is (C) Both A and B. But I don’t understand how a gear driven vacuum pump, driven by a camshaft can ever be off, it will always spin?

4

Nevada reminded me that my diesel had a pump mounted where the distributor would be. It was on all the time the engine was running until it blew a hole in the housing. No switch. That’s why I’m not good at multiple choice questions.

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Thing with ASE T.est (redacted word lol) is you don’t answer it as a multiple choice question, at 1st.. You have to break it down and answer question 1 as Tech A is in Ohio working on a Ford or whatever, is he/she right or wrong.. Then question 2 as Tech B is in Main working on a Kia or whatever, is he/she right or wrong, then compare your answers and either answer Tech A, or Tech B or Both or Neither… Simple right??

BTW, you also need to understand that the questions are not always written by an ASE certified tech, but could be by someone that doesn’t even know how to check oil level in a vehicle… at least it was years ago…

6

I really hope that statement is incorrect. A safer failure mode is to fail-ON so the pump runs continuously so the vacuum brake booster always works. Safer failure mode. It would be an easier diagnostic because at idle, you could open the hood and just listen to the pump never shut off.

Same for a mechanical pump. If there is a bypass valve to reduce load on the mechanical pump, it should also fail so the pump always runs.

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