vacuum lines


Old 11-18-03, 09:06 PM
mower17's Avatar
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vacuum lines

Hi. I am currently a student at a vo-tech college for diesel mechanics and I, for some reason, could never get a good explanation of exactly what vacuum lines are for. I never really messed with much automotive stuff, so I am more used to tractors and other lower-tech designs. From what I have heard, the engine and the transmission both use vacuum lines, but that's all I know. What do these vacuum lines opperate? What are they connected too? If someone could please explain in detail about these things I would REALLY appreciate it. Thanks a bunch!!!!!!!
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Old 11-18-03, 10:17 PM
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Hello Mower17!

Vacuum lines run vacuum motors, switches, sensors, servos, diaphragms, etc...

Diesel engines don't really develop vacuum. In automotive applications they use a vacuum pump to provide vacuum. Gasoline engines develop vacuum in the intake. This vacuum is tapped into to power things. There are diaphragms that move linkages when vacuum is applied. One example is the brake booster on cars with power brakes. It is a large diaphragm that vacuum pulls on to aid in brake operation. Air conditioning blend doors move by vacuum motors in many move the flap from the heater core for hot air, to the evaporator core for cold air. Manifold vacuum also tells a lot about how an engine is running, and what kind of condition it is in. Fuel injected vehicles have a computer that uses a sensor that reads engine vacuum to tell it how much of a load the engine is under. It is called a MAP sensor. (Manifold Absolute Pressure).

Some transmissions use a vacuum modulator. It is also basically a diaphragm that moves a rod. This rod limits the travel of a valve in the transmission. This valve controls the shifting of the transmission. So...when you are giving the engine full throttle, there is low vacuum, and the transmission will hold in its' gears longer, but when you are lightly accelerating, it will shift through the gears a bit quicker.

The reason a diesel doesn't develop any vacuum to account for is because it has no throttle plates. A gas engine does. This limits the amount of air that can enter the intake manifold. As the piston draws air into the cylinder on the intake stroke, it creates a vacuum in the intake. The farther the throttle is opened, the quicker the air can enter, so the less vacuum you get.

Hope that covers it. ??
Old 11-18-03, 10:18 PM
mike from nj
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gas engines produce vaccum, it's high at idle and zero at wide open throttle. vacuum is also a strong force, plus it's free when the engine is running. certain things on the engine, need vac at certain times, to actuate them, and also need no vac at other times, which works out good for a vacuum advance on the distributor.

vac brake booster, vac cruise control servo, all the heat/a-c doors under the dash, 4X4 actuators---to name a few.

i think mercedes benz still uses it for their power locks, it's free, it's strong, and no noisy solenoids, no extra wires.

basically, a vacuum actuator and a corrosponding vac line is much cheaper than wires, a switch and a solenoid. plus it needs no fuses, never has a dirty 'ground' and almost always works.

guess which one the factory will use?
Old 11-18-03, 10:28 PM
Join Date: Dec 2001
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well there isnt that much use for vacum lines anymore but in some older models they ran a vacum line to the transmission vacum modulator and this controlled fluid pressures in the transmission high vacum at lower rpm resulted in lower pressures and low to no vacum at higher rpm or wider open throttle resulted in higher pressures same princible also used on fuel pressure regulators that are mounted on the engine.
vacum assist power brakes uses a check valve in the vacum line which prevents the loss of vacum when engine is in a low vacum condition such as higher rpm and wide open throttle conditions giving you good braking power even though the engine is not currently producing good vacum.
alot of models used vacum diaghrams to switch blend doors from hot and cold and defrost to floor or panel these also had a check valve in the vacum line and usually a resoiver to allow to work even when the engine was not producing good vacum.
generally they are used to apply vacum to a diaghram and may go through a solenoid that turns the vacum on and off in some cases.
two exceptions would be crancase and canister purge wich are just controlled vacum leaks that reburn the vapors to reduce emmissions.
on a new car most of the things listed above are now all electricaly controlled, vacum assisted brakes and canister purge is still used in most cases which eliminiated a bunch of vacum lines.
Old 11-19-03, 07:07 AM
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vacuum?...nah...negative pressure!

vacuum is negative pressure and is measured in inches of mercury...just like a barometer reading (barometric pressure is positive). it's not the negative pressure that does the work in all of the previous's positive pressure. here is an buy a soda and stick a straw into the liquid and then you create a low (negative) pressure area in your mouth and the positive pressure exerted on the surface of the liquid (called atmospheric pressure) "pushes" the liquid up the straw and into your mouth. high and low pressure want to be static with one another so they equalize...the same thing happens inside the engine, as the piston moves downward, it creates a low pressure area (or pulse) and atmospheric pressure moves air molecules past the throttle plate to take up the space vacated by the piston. keep doing this over and over again, faster and faster and you have a canstant low pressure see and engine is just a big air pump and in a gas engine we throttle air and in a diesel engine we throttle fuel so there's no where for low pressure to "collect". a diesel engine makes low pressure pulses just like a gas engine, there is just no restriction and the low pressure area is at the opening where air enter the indution system...ever had a shop rag pulled...oops!...pushed into a diesel engine? get the idea...i just love the scientific stuff about there's a different way to think of "vacuum". good luck...
Old 11-20-03, 12:23 AM
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Thank you all very much. That was exactly the kind of info I was hoping to get!!!!!!! That clears a lot of questions up. Thanks again!!!!!!

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