What's this thing?

[theholycow]

What is this vacuum doohickey and why does it have a rubber thingie between it and the vacuum lines? Googling the part number stamped on the plastic part tells me it is the Distributor System/ Vacuum Modulator Valve (DS/ VMV) PN- 560694...but I don't know what that means.

It looks like the vacuum lines could easily connect directly to the DS/VMV but instead there's a rubber thing between the lines and their connections on the DS/VMV. What's the rubber thing for?

[Jay2TheRescue]

The lines can connect directly to it, but the way it is, it prevents you from screwing up which hose goes where. Kind of like a connector on a wiring harness.

[theholycow]

I'll have to look for markings on the rubber thing, that might be what that is.

So what is the DS/VMV?

[Jay2TheRescue]

it basically controls emissions equipment... but that stuff became problematic when the vehicles got older.I have something similar on my Buick that was a PITA to find a replacement for when it failed.

[theholycow]

I now know exactly what it is.

From the 1980 Buick Advance Chassis Manual:
The DS-VMV limits the distributor spark vacuum to a calibrated value until ported vacuum applied to the control port of the DS-VMV is greater than the calibration value. When the ported vacuum is greater than the calibration value, the distributor spark vacuum is equal to the ported vacuum signal.

In short, it provides whichever vacuum source is greater (assuming that "calibrated value" means "value of the other vacuum source") to its output (the distributor vacuum advance).

[theholycow]

Update to the update:

On further reading I've found out that the original info I found calling it a DS-VMV is incorrect. It is actually the SAVM (Spark Advance Vacuum Modulator) Non Linear Valve.

There are a few different components that connect exactly the same way but differ by application. The smaller manual I read first didn't describe as many as the larger manual I have since read.

I'm more sure of this identification for these reasons:

1. The DS-VMV was described in the manual for a V8 and my engine is a 4.1 V6. The SAVM description does not specify an engine.
2. The SAVM description visually matches one thing that the DS-VMV description failed to describe: "green spot". BTW, the green spot identifies the calibration of the SAVM.
3. The manual says "Non linear valve" for the SAVM but not for the DS-VMV. The thingy is labeled "Non linear valve" on the vacuum diagram label under my hood.

From the manual:

Originally Posted by 1980 Buick Chassis Service Manual

SPARK ADVANCE VACUUM MODULATOR SYSTEM (SAVM) (NON LINEAR VALVE)

Operation

The spark advance vacuum modulator, controls vacuum to the distributor vacuum advance. This is a dual diaphragm regulating valve with ports to the distributor, manifold vacuum, and ported vacuum.

Three operating conditions of the valve are possible:

1. When both manifold and ported vacuum are below the regulation point (7 inches, green spot coded valve; 5.7 inches, blue spot coded valve, +- .5 in.) output of the valve to the distributor vacuum advance manifold is manifold vacuum.

2. If manifold vacuum increases, but ported vacuum is lower than the regulation point, output is (7 inches, green spot coded valve; 5.7 inches, blue spot coded valve, +- .5 in.) (the point of regulation of the valve).

3. When both manifold and ported vacuum are above the regulation point (7 inches, green spot coded valve; 5.7 inches, blue spot coded valve, +- .5 in.) the vacuum to the distributor is ported vacuum.

The valve is responsive only to engine load and is independent of car speed, transmission gear or temperature.

Functional Test

1. Connect a vacuum gage to the "distributor port" then with a vacuum pump apply 7 inches of vacuum to the "intake manifold port" green spot code valve, or 5.7 inches blue spot coded valve. The vacuum in the gage should follow the pump vacuum up to 7 inches green spot code valve, or 5.7 inches, blue spot coded valve += .5 in., then remain constant.

2. Leave the vacuum gage on the "distributor port" then connect the vacuum pump to the "carburetor port". Pump the vacuum pump. The gage should remain at zero until the vacuum pump output reaches 7 inches green spot code valve, or 5.7 inches, blue spot coded valve. As pump output goes above 7 inches green spot coded valve, or 5.7 inches blue spot coded valve, the gage should follow the pump level.

3. With vacuum pump applied to the "distributor port" and the vacuum gauge connected to the "carburetor port" of the valve, pump up several inches of vacuum. The vacuum gage should remain at zero.

If any of these functional checks fail, replace the SAVM valve.

Here's two other formats (geek speak and plain English) for Operation:

1. if "mani" < 7in and if "carb" < 7in then output="mani"
2. if "mani" > 7in and if "carb" < 7in then output=7in
3. if "mani" > 7in and if "carb" > 7in then output="carb"

1. Heavy throttle: Low vacuum at both inputs: manifold vacuum controls distributor
2. Idle: High vacuum at "mani" input but low vacuum at "carb" input: distributor gets 7 inches
3. Part throttle: High vacuum at both inputs: ported vacuum controls distributor

Testing in plain English:

1. Apply vacuum to "mani" input. Output should follow "mani" input and stop at 7in.
2. Apply vacuum to "carb" input. Output should be 0 until input reaches 7in, then output should follow input.
3. Connect gauge to "carb" input. Apply a little bit of vacuum to "dist" output. Gauge should remain at 0.