Vacuum pump circuit breaker

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Luis A
Posts: 86
Joined: Tue Oct 31, 2017 12:35 am
Dino: Fiat Dino 2.4 Coupe

Vacuum pump circuit breaker

Post by Luis A »

Hello everyone,

I have been going over my brake system vacuum pump system as I have observed sometimes that the pump will run for over the 10 seconds ( a lot longer) that it is specified as a maximum. For this to happen, either there is a vacuum leak or the vacuum sensor is not working.

My vacuum is maintained for over 24 hours so I know I don't have a system leak.

The motor will stop operating if either the vacuum sensor (Item 79 in the diagram below) senses the appropriate level of vacuum and de-energizes the relay, or if the electric breaker, item 75, interrupts the flow of power to the pump (through the closed relay contacts 30/51 to 87).
Pump circuit diagram.JPG
Pump circuit diagram.JPG (12.32 KiB) Viewed 3055 times
I took the circuit breaker off and directly connected the 2 wires so the pump will only stop if the vacuum sensor works correctly. Indeed, my vacuum sensor will make the pump stop when a level of vacuum is reached so it works.

This is what the original circuit breaker looks like. It sits on the top of the firewall with two thick white wires going to it. The back has a small nut that in principle allows the bimetallic disk to be adjusted to set the trigger point (time). I was unable to adjust mine (it never triggered) so I decided to look for an alternative.
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Inside, there is a bimetallic element through which the current travels, it then heats up and eventually it "clicks"; it goes from concave to convex and the contacts separate thus interrupting the power to the motor.
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While the pump is specified to consume 10 Amps, when I put a hall effect DC current sensor on it, It showed around 21-23 Amps while running with a surge of around 28 very briefly when it started up. This is with the car running and the alternator producing 14.2 Volts.
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There are commercially available (Amazon) 12 V, self-resetting circuit breakers. They do not trigger instantly when reaching the rated current but rather it varies as a function of the amount of current. As seen on the graph, the 10 second point would be around a 200% load. This also varies with temperature so if it is hot, it would be shorter, colder it would be longer.
breaker curve.JPG
breaker curve.JPG (40.76 KiB) Viewed 3055 times
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Since my vacuum sensor works, all I want is to prevent the pump from running "forever" and potentially damaging itself if I have a leak. So I selected a 15 Amp breaker and that has given me about a 10-12 second run time with the same time off, so a 50% duty cycle. The firs time it runs, when the breaker is cold, it runs longer, around 30 seconds but if it keeps running it settles into the 10 on 10 off mode.

Here is the breaker showing that it fits perfectly in the same location as the original one and is invisible thus maintaining an original look.
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dinoverde
Posts: 330
Joined: Wed Nov 27, 2013 5:56 am
Dino: Fiat Dino 2.0 Spider

Re: Vacuum pump circuit breaker

Post by dinoverde »

very ingenious indeed ..i have been looking for one of those bimetal sensors for a while..found a couple but was never able to test them of the car..how can one test these bimetal sensors? a12 volt battery will probably fry them?
Scalino
Posts: 179
Joined: Mon Jul 24, 2017 7:22 pm
Dino: Fiat Dino 2.4 Spider
Location: The Netherlands

Re: Vacuum pump circuit breaker

Post by Scalino »

Interesting post! I still need to have a look at the vacuum pump for the same reason, will keep this in mind.
Luis A
Posts: 86
Joined: Tue Oct 31, 2017 12:35 am
Dino: Fiat Dino 2.4 Coupe

Re: Vacuum pump circuit breaker

Post by Luis A »

dinoverde wrote: Mon Sep 28, 2020 6:49 pm very ingenious indeed ..i have been looking for one of those bimetal sensors for a while..found a couple but was never able to test them of the car..how can one test these bimetal sensors? a12 volt battery will probably fry them?
Easiest way to test them would be 'in situ' in the car, otherwise on the bench with a hefty 20 Amp, 12 V power supply and a suitable high power resistor so that ~20 Amps flow through. Or you can forget about testing the original bi-metallic ones and just get this one from Amazon. Your pump may be consuming less amperage than mine. Not sure why mine is taking in 20+ amps when the spec is only 10...
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