What does the vac line that goes to the ECU do?

[benebob]

Finished packing for going to Mich. Upper Peninsula Adventure Trail today and while adding some fuses to the tool kit I noticed the vac line to the ecu was not attached.  Guessing it hasn't been attached for months which got me thinking.  What the heck does it do?

[JoeDirt]

It measures barometric pressure. It will work inside or outside of the air filter. Indirectly, I think the computer uses it to adjust for altitude. I am sure that Mr. Triumph wanted to keep spiders and dirt getting in there and put it on the "clean" side of the filter.

[benebob]

Okay so it is pretty much useless then huh.  :) 

[Bixxer Bob]

 No it isn't.  It's vital for getting the correct fuel air mix. If the ECU can't measure the depression in the air box when you open the throttle hard it can't respond with the correct fuel mix for a given throttle position.you need to hook it up, and make sure it's not blocked or pinched by blowing down it toward the air box.  Don't blow toward the ECU, the sensor can be damaged.

[benebob]

No it isn't.  It's vital for getting the correct fuel air mix. If the ECU can't measure the depression in the air box when you open the throttle hard it can't respond with the correct fuel mix for a given throttle position.you need to hook it up, and make sure it's not blocked or pinched by blowing down it toward the air box.  Don't blow toward the ECU, the sensor can be damaged.

I was kind of kidding but it isn't so vital if my bike has been running just as poorly as any other Triumph without it being attached for months. LOL

[JoeDirt]

I am in no way saying it's useless. It can only measure pressure... it doesn't sniff the fuel. I am only saying that "yes" the machine will run without it, "yes" you didn't break your ECU and "yes" you need to hook it up.

[Bixxer Bob]

No-one suggested it "sniffs" the fuel Joe.  If you read my post again you'll understand I was talking about the ECU, not the sensor.  For Benebob's benefit, the ECU opens the injectors to deliver a squirt of fuel for a pre-determined time based on the amount of oxygen available, determined by the air temp and pressure; the pressure is measured by the sensor in question.  To give extremes as an example, cold pressurised air contains more oxygen than a hot depression.


Of course the calculations are more complex than that - throttle position, revs etc - but you get my drift.

[JoeDirt]

I am literally staying on point... the subject is "vac line"... it can only be measuring one part of the equation. That line can only be used for measuring the pressure inside the airbox. My point is that it really isn't different from ambient air pressure. Hell, everyone tunes their throttle bodies without it hooked up.

[Sin_Tiger]

Gents, the line reads Manifold Air Pressure, which includes a negative pressure. As BB says it feeds the ECU with the change in demand by the variation in throttle position.

Any variation in ambient pressure will affect the pressure in the manifold, so the adjustment for ambient/altitude is relative as the MAP sensor only sees what's in the air box.

It'll run without it but nowhere near as good as with it attached and intact.

[benebob]

Well then mine is pretty useless as it runs pretty much the same with it on as when it is off.  Thanks guys, hooked it back up even before posting.  Just curious what it was for and why it never seemed to run differently or throw a code.  I am UP of Mich bound here in about an hour.  That is if I don't melt first.  2 days of 90+ and humidity that reminds me of when I was in school down near Joe.  :) 

[JoeDirt]

The computer isn't smart enough to know that what you are feeding it is correct or not. I think the execution of the "vac hose" should have been a sensor separate to the ECU and signal sent via the wire harness. That's probably what that spare connector on the wire harness was intended for... 

[Bixxer Bob]

In most car systems, as you'll know Joe, it is.  But I think Triumph realised that with the oil breather venting into the airbox as well, and both that and the sensor needing to be on the throttle body side of the air filter in order to measure the vacuum created when you open the throttle or suck the fumes from the casing, the sensor wouldn't last five minutes in that oil mist environment.