Nicholbry

After reading a few threads here on Rennlist about diagnosing hard shifts I thought I would try my hand at vacuum testing the line from the transmission at the modulator (that green thingy w/ the black cap). The cap appears to be OK w/o any cracks to speak of. I tried chasing the line into the engine compartment and there appears to be two lines coming over the bell housing into the engine bay for me to follow. One is larger than the other. If I understand correctly, the smaller diameter line is the one I am working with.

I was initially looking for a spider-shaped connection where many of the vacuum lines connect based upon some reading here. I removed my air box (not the MAS yet) and the line became clear for me to see. However, the line that I am following doesn't connect to a spider-like connection. Maybe this is on older cars (mine is a '90 S4 Auto). The line runs along the driver's side fuel rail to the front/top of the motor where it connects into the top of one of those silver semi-domed shaped vacuum thingys (??sorry). Am I following the proper line for this vacuum check or did I get off track somewhere? Or should I be testing the other, larger line instead? After pulling and plugging the line off the modulator at the transmission and then pulling and testing the line at the top/front of engine, it yields 0 vacuum making me question whether or not I'm testing the correct thing.

Thanks

Jascd

sounds like you have found the vacuum line to the front fuel damper rather than the AT vacuum line.

On my 89s4 the spider you refer to can only be easily seen by actually lying on top of the motor. The line running back over the bell housing should be clear from that view point.

here is a pic with the MAS removed so you can see where to look.

Jascd

whoops, wo

Jascd

Ok, this time.

Nicholbry

In the pic you provided are you referring to the smaller diameter line (red) or the larger diameter (green)? In this image, are you looking at the back side of the engine w/ the MAS removed and viewing from directly overhead? I just don't see this spider thing everyone is referring to.

I can clearly see the bell housing and the only two lines that I see coming from underneath is one that goes to the throttle (cable in a plastic shroud) and a smaller line that I mentioned in the previous post. Can't source the transmission line although I can feel it on top of the torque tube towards the rear where I can get my hands in.

Thanks for the pic. It is always great to have.

Nicholbry

[IMG]vacuum[/IMG]

Chris Prack

Why not just check for vacuum at the modulator with the engine running at idle? That is how I do it. If the lines are good you will have manifold vac at the modulator connection. Then put a vac pump on the modulator itself and see if it will pull a vac.

Simple test. You then only need to worry about where the line goes if you dont have vac to the valve at the trans.

That's my professional opinion.

Nicholbry

I can (w/ the engine running....) after I wrap up some other ongoing things. My car is balanced at four corners on jack stands while I wrap up a Motor Mount and Oil Pan gasket exchange w/ exhaust removed so I was hoping to be able to check it out while I have this stuff apart. I'm kinda uneasy about cranking the car (engine rocking) while it is elevated 1.5 ft.; hence this method w/o cranking.

Admittedly, I just purchased this MityVac and have never done this before. I'm a rookie trying to learn. For example, I don't fully understand this whole vacuum business and how it relates to mechanical performances, but I'm trying to learn. I was following a couple threads here on Rennlist that had me performing the vac test in the manner in which I did. Without that sort of guidance (step-by-step) I'm a bit lost w/ the limited ability that I have; so I apologize if I seem a bit dense on the subject. I can talk to you all day about mortgages and music composition though.

So, if I understand you correctly, pull the vac hose off the green thing on the driver's side of the transmission and put the MityVac directly onto that connection w/ engine at idle and test for vacuum there; right? Is this what you mean when you say "modulator connection"?

Additionally you make reference to putting it on the "modulator itself"; where exactly are you referring as both descriptions are seemingly the same?

So let's assume that all turns up fine there; I would then need to resolve whether or not the line has a leak. Given the image above, which is the line in question; red or green and it's descriptive location at the engine compartment?

I greatly appreciate the help.

Nicholbry

Anyone?

jpitman2

Modulator is a thing in the gearbox that has engine vacuum on it. Low vacuum is present when at wide throttle openings, box changes shift points (higher). High vacuum is present at very low throttle openings , box changes earlier . If you have a mityvac, you dont need to run the engine. Connect it to line going to box, pump and see if it can hold vacuum. If not, there is a leak somewhere - find point where vac line goes into box, fit mtyvac there, pump and test for holding - if not, problem in box; if yes, problem in line to engine. Frequently easiest fix is to run a new hard nylon line, as its difficult to replace original metal line on same route. If your gear changes points are ok , and not harsh, your system is probably ok. When you are back on the road, try disconnecting the vacuum line (plug it too), and see how box behaviour changes......it should be immediately apparent.
jp 83 Euro S AT 50k

Nicholbry

Thanks for explaining it simply/clearly. Another issue is identifying the vac line at the engine. The image provided, although appreciated, isn't specific on which line in the circle is the subject. Do you happen to know? Also, can yo provide a descriptive location of this line (driver's firewall, side of engine, front of engine, etc.)

The only two lines that I can see over the bellhousing into the engine compartment doesn't seem related to the trans. One goes alongside the driver's fuel rail to the front of the engine and clamps onto a domed shaped vacuum device up there. The other line turns out to be a cable associated w/ the throttle. I've got my air box off, but haven't removed the MAS yet since I can't seem to locate the line anyway.

I appreciate you trying to help.

jpitman2

Dont have an S4 so cant help directly.Domed thing will be a fuel pressure regulator. There should be a metal line ~1/8" dia going down towards the torque tube from somehwere up there. On my 83 there is a metal line from up front of the plenum body going back, so its easy to find. At the back there is a short length of fabric coated rubbe rtube going from the metal line to the modulator...but 3 spd is different from 4 spd.
jp 83 Euro S AT 50k

Nicholbry

Thanks for trying to help; Ill get in there and look again. I see the line coming off the modulator so that isn't a problem identifying. I must admit that I wasn't looking for a metal line; however, as I thought is went from metal to plastic again near the bellhousing into the engine bay off of the torque tube.

Nicholbry

I got a test directly on the stem at the modulator. I took it up to 15 lbs. My line that came w/ my MityVac seemed a bit looser on the stem than I would have preferred so that may be the cause, but my results showed a slight leak that would drop 1 lb every 2.0min. Again this was directly on the box at the vac. line connection w/ a less-than-tight fit and with the engine off.

I still cannot locate that vac line at the engine. I can clearly see behind the engine down onto the top of the bellhousing. Again, there is a small, rigid, plastic line that makes a slight curve at the passenger side firewall then curves towards the driver's side at the firewall where it seems to join a rubber carrier that also supports another identical sized line.

Nicholbry

".......there is a small, rigid, plastic line that makes a slight curve at the passenger side firewall then curves towards the driver's side at the firewall where it seems to join a rubber carrier that also supports another identical sized line."

Like the one provided in the image w/ the red lines!