Vacuum source in 1980 SC?

clnilsen · clnilsen's Garage

This came up in the Cruise control thread, and I was wondering if anyone knows of a good constant vacuum source on an 1980 SC engine for cruise control. the only line I could find was the master cylinder line, and I don't think that's the best source.

Anyone?

Thanks!

Chuck

tsuter · 06-05-2003, 07:16 AM

Any connection below the Throttle Body butterfly is a good vacuum source. Here are a couple choices. Also the front where the vacuum retard connects.

MotoSook · 06-05-2003, 07:43 AM

There is also a vacuum port on the front (pulley side) of the TB. There should be two branches. The top one is for atmosphere pressure to the WUR (not that it works anyhow), and the bottom one is the vacuum.

As Tsuter said, any port downstream of or below the TB butterfly is a vacuum source.

Thad, did you get the Alternator drawings?

clnilsen · clnilsen's Garage

Question:

on an 80 sc, would the deceletation air valve have reletively constant vacuum from the throttle body?

I'm not very familiar with how vacuum works from the throttle body, my understanding is that vacuum retard and advace are both NOT constant vacuum. I'm not sure what would be and would not be constant vacuum.

Thanks!

Chuck

clnilsen · clnilsen's Garage

Bump anyone???

Bueller, Bueller..?

targa80 · targa80's Garage

On the 1980SC 3.0L engine there is a vacuum port specifically for the cruise control. It is located on the back of the throttle body on the left side. This location would be if you were looking at the engine from the rear of the car.

clnilsen · clnilsen's Garage

THANKS!!!

I'll look for it tonight!

Chuck

MotoSook · 06-06-2003, 09:17 AM

for a 78-79, but a better view of the TB from the flywheel end.

Are you looking for a constant vacuum source for all RPM range?? I don't think there is one on our motors.

clnilsen · clnilsen's Garage

Souk:

Yes, I am looking for a constant vaccum in all the RPM range. After many hours of looking, I thought that there was not one that was constant throughout the RPM range, except the Brake Master cylinder.

But I think there has to be for cruise WAS a factory option in 80.

I'd love to know where that would have gone in.

Thanks!

Chuck

MotoSook · 10:04 AM

You only need vacuum when you are crusing right? So why worry about vaccum when you are at rest (idling) or accelerating? I have not studied cruise control installations, but I would think a nearly constant vacuum source when you need it (crusing speed) is what matters.

Tee-off the brake booster for your installation. Then again, how is the constant vacuum created for the brake booster? In the 80+ vacuum diagram, booster vacuum appears to come from the TB as does all the other vacuum points.

How does the vacuum booster asssist work? Me thinks that the vacuum assist is minimal at rest or non at all. So perhaps the vacuum to the brake booster is NOT constant.

I think it's time to break out the books.

targa80 · targa80's Garage

I down sizes a picture from another posting on what this vacuum line is for.

MotoSook · 06-06-2003, 10:58 AM

That appears to be on the upstream side of the throttle plate, Pat.

Judging from that, I don't think you are suppose to have a constant vacuum to the cruise control at all RPM, or rather, all throttle plate opening.

Someone just put a vacuum gauge on their factory cruise control vacuum source already! and open the throttle. You will find that the port the port that is in the picture Pat posted, will not give you constant vacuum. It should be relatively constant at a static throttle position though.

clnilsen · clnilsen's Garage

Right now I'm tapping the brake booster, which I though would not be good from a safety standpoint, though it works excellently for cruise control function.

Yeah, If someone can put a vccuum gauge on their 1980 car, that would be awesome! Inquiring minds want to know!!!

Chuck

targa80 · targa80's Garage

Ok, I don't have a clue about what vacuum pressure has to be availiable for the cruise control. But I do own a 1980SC with the engine out of the car and have taken a picture of the port that the cruise control connects to.

MotoSook · 06-06-2003, 05:15 PM

That port looks like it is in the throttle by-pass stream, downstream of the idle adjustment screw.

I would still put a vacuum guage there to test and end the speculation.

BUT! One last speculation from moi. Since that port is downstream of the adjustment screw, there is likely to be"vacuum" at that point during flow. Internal flow through an orifice (idle screw opening in this case) will create a zone of lower pressure just downstream of that orifice. The pressure recovers as it continues down the flow path. (If you are interested run a search on vena contracta on the web...not exactly what we are talking about, but you'll get the idea).

The lower pressure should be less than atmospheric pressure, hence the "vacuum." The pressure differential is likely enough to control the cruise control at very small throttle opening. At speed and with a larger throttle opening, the "vacuum" should increase as the port IS below the throttle body.

clnilsen · clnilsen's Garage

Ok guys, last night I went and got a vacuum gauge and tested the line that Targa80 posted.

That line actually connects to the Aux. Air Regulator on a 1980 car. Disconnecting the line and testing vacuum, here's the readings:

Idle - 1-2 lbs
1500 RPM - 15 lbs
2000 RPM - 15 lbs
2500 RPM - 16 lbs
3000 RPM - 17 lbs
3500 RPM - 18 lbs

So, it seems to have vacuum at speed, but not idle.

In the "proof is in the pudding" I connected the cruise control to the line with a "T" fitting (car ran rough w/o the Aux air regulator disconnected, particularly the pop-off valve kept popping), and it hold speed great. The cuise works as I would expect, hold speed, slows down, and accellerates perfectly.

Therefore, I would conclude this is a good place to add in the vacuum line. Don't know why the rough running issue with the Aux Air regulator existed, maybe someone could explain this to me.

Thanks everyone for the help!

Chuck