Troubleshooting Resonance Flap

[aston@ultrasw.c]

I have a 95 3.6 motor (non V) in an 86 bodyshell.

I suspect the resonance flap is not working correctly. There may be more than one issue.

Here's what I observed and my guesses at status!

Turning on ignition sends 12V to the vacuum valve - OK

After starting the engine, at idle, the 12V remains - not OK? shouldn't the voltage drop to zero to open the flap?

Connecting a cheap vac gauge to the circuit gives 13.5"Hg with the engine running - OK

Vac doesn't drop after 3 mins - ok

With vacuum stored in the system, turning on the ignition, does nothing. Bad vacuum valve?

Connecting vac directly to flap actuates the motor nicely - ok. If the engine is now turned off, vac bleeds away in a few seconds - not OK?

Any insight is appreciated!

[ischmitz]

I am speaking for the 964 motor here but am almost 100% positive it is the same for the early 993:

When you turn the ignition on without starting the engine the vacuum flap gets actuated once. You can hear that. Especially, when there is enough residual vacuum left in the storage canister. On my engine that vacuum stays there for a long time. I have never measured it but I am talking hours. And I can do the cycling several times before the vacuum get too low to do anything.

The pneumatic valve (like all the other items such as injectors, tank venting valve, CEL) get permanent +12V on one side once the ignition is on and the DME provides switched GND via an internal low-side switch.

In your case it sounds like the pneumatic valve might be the culprit. And vacuum bleeding away is not right, either.

For troubleshooting you could use an old-fashioned test light (or a 12V light bulb with two wires) and connect that to the pig tail to see if the pulse comes through when cycling the ignition. Then, you need to figure why the residual vacuum disappears so quickly. Finally, connect +12V and GND to the pneumatic valve to see if it switches.

Let us know how things go,
Ingo

[aston@ultrasw.c]

Thanks Ingo,

I disconnected the wiring from the valve and connected my multimeter to the pig tail. I get 12V when I switch on the ignition. But after starting the engine the 12 V remains. The manual says that when the DME detects the engine is being started the flap is opened, which is the no-current state.

So is my measurement incorrect or is there something else wrong?

I will be replacing the valve and the vacuum motor.

[ischmitz]

A Multimeter isn't the best tool to do the test since you need to have a real consumer that can draw a real current. Remember, one pin of that connector is permanent +12V (once the ignition is on) and the other pin is either GND or floating. If I understand correctly you measure +12V connected to the pig tail. That would be correct, however keep in mind the Multimeter doesn't draw any current to speak of. So the test is half-valid at best.

A conclusive test is to use an old relay with two wires or a light bulb and connect that to the pig tail.

On the 964 the flap valve gets turned on once you turn the ignition to ON (but not start the engine). And that only happens if the DME is happy. A dead DME will not do that.

With the valve connected can you hear it click when you turn the ignition to ON? If not take the multimeter in Ohm setting and measure the resistance of the valve itself. It should be something in the 100 Ohm range give or take. If it is open, it is bad.

Ingo

[ischmitz]

I just read through the 993 WSM. You should observe the following:

- when turning the ignition to ON the valve needs to audible click (current flowing, flap closes if you have enough residual vacuum)
- once you start the engine and it idles the valve needs to switch to open the flap (no current flowing)

If for some reason the current remains on all the time after the engine started (flap remains closed) I would highly suspect the DME to be faulty.

If unsure connect a light bulb and do the test: Ignition ON and the bulb needs to light up. Start the engine and the light needs to go off. If that works the electrical tests are complete.

Then in the next steps are to check if the vacuum solenoid works and if the vacuum is properly maintained and if it properly actuates the flapper itself.

Ingo

[aston@ultrasw.c]

ah HAH

you were right

with a lamp and flying leads connected to the wiring harness the lamp goes on and off as expected

OK now i'm just waiting for new vac parts

[aston@ultrasw.c]

new vac valve and vac motor installed

everything seems to work correctly with hand vac pump and gauge

also flap opens when the ignition is on and closes when the engine starts

so far, so good

but just to get some positive feedback that all is working correctly at speed I installed a micro switch that is tripped when the flap is fully actuated i.e. completely open

switch wired to an LED that sits on the dash

so off to the track

start of the straight, WOT in third around 3,000 r

car accelerates and LED lights up around 4,500 r

btw tach is not so accurate w/3.6 motor

car continues to accelerate smoothly but LED goes out around 6,000 maybe a little before

i was expecting LED to stay on all the way to the rev limiter

all i know is that the flap is not 100% open: it may be 90% open or completely closed

so, the ubiquitous question, is this normal?

for several laps behavior is the same, then i get tired of watching the LED and start chasing down a GT3

i do have the smaller vac reservoir installed and the check valve: black side towards the manifold

[Bill Verburg]

Originally Posted by aston@ultrasw.c

new vac valve and vac motor installed

everything seems to work correctly with hand vac pump and gauge

also flap opens when the ignition is on and closes when the engine starts

so far, so good

but just to get some positive feedback that all is working correctly at speed I installed a micro switch that is tripped when the flap is fully actuated i.e. completely open

switch wired to an LED that sits on the dash

so off to the track

start of the straight, WOT in third around 3,000 r

car accelerates and LED lights up around 4,500 r

btw tach is not so accurate w/3.6 motor

car continues to accelerate smoothly but LED goes out around 6,000 maybe a little before

i was expecting LED to stay on all the way to the rev limiter

all i know is that the flap is not 100% open: it may be 90% open or completely closed

so, the ubiquitous question, is this normal?

for several laps behavior is the same, then i get tired of watching the LED and start chasing down a GT3

i do have the smaller vac reservoir installed and the check valve: black side towards the manifold

I'll have to double check but I believe that the stock tipping point for a 964 or 993 is ~5500 below that it is closed above that it is open

the vrams have 3 modes for the resonance flap, closed open then closed again

it's possible that there is a vacuum leak or that the vacuum reservoir is too small for extended wot(or some combination of both)

[aston@ultrasw.c]

Thanks bill. I had a similar thought. I suppose im gonna have to hook up a vac gauge so i can see what the pressure is doing at wot.

Also ill post a sketch of the vac circuit to see if there are any improvements to be made.

Non vario btw.

[Bill Verburg]

Here's the layout for a '95 993 non vram


I checked and both 964 and 993 non vram are 2 stage systems that operate as described above

[aston@ultrasw.c]

OK here's a third option to consider:

If I understand correctly throttle valves are designed such that when air is drawn through them they tend to close. It's a fail safe mechanism so that if the return spring fails the engine will tend to shut itself down.

Is this the case on the 3.6 intake?

If so, is there a chance that the closing force can overpower the vacuum motor?

As I said my micro-switch is actuated at 100% open or close to it.

It would only take a small movement of the flap to extinguish the LED.

[aston@ultrasw.c]

To answer my own question - vac leaks all the way.

Every fitting, hose, adapter had to be checked and sealed.

Initially flap would stay open for about 15 secs with my (POS) hand vacuum pump.

After fixing all the leaks, the flap was still open after 17 minutes.

[Bill Verburg]

I did a little more research and it appears that the 1995 non vram has 3 modes
1) open from idle to 3000
2) resonance flap closed from 3000 to 5500 and throttle greater that 60°
3) open from 5500 to redline

and for reference

[aston@ultrasw.c]

Worked on my resonance flap (again) this am....

Here's vid #1

at about 4 secs the ignition is switched on, the residual vac opens the flap. at 11 secs the engine is started and the flap closes for low rpm operation



All is well. Note the micro switch I installed to trigger a light on the dash when the flap is 100% open.

Now things get interesting.

This is a WOT run in second gear.



Based on my LM1 data, here's the approximate operation:

At about 8 seconds the flap opens, which is about 5,200 rpm.

At about 11 seconds it closes about half way, 6,000 rpm

Finally at 12 seconds we hit the limiter and the flap closes completely, 6,500 rpm


It doesn't match Bill's modes exactly, but it is not an exact measurement either.

Do i have anything to work on or is this close enough?

Thanks for taking a look.