Duty Cycle

Before doing anything, let's discuss what duty cycle is:

Duty cycle is the pulsed ground signal sent from the oxygen sensor control unit (Jetronic box under the dash) to the frequency valve that determines the amount of time the valve is open, which adjusts the air-fuel mixture for various conditions in order to maintain the ideal 14:1 ratio (courtesy of tolusina of VWvortex.com):

1. Cold-running enrichment (forced during warm-up).
2. Open-loop (just hangin' at 50% after cold enrichment is over, waiting for oxygen sensor activity).
3. Closed-loop (oxygen sensor has finally kicked in).

4. Full-throttle enrichment (driver has floored the accelerator).

The pulsed signal sent from the oxygen sensor control unit can be measured in duty cycle on a 0-100% scale, or in dwell on a 0-90° scale; this information is used to adjust the air/fuel mixture on a CIS car.

Here's how the system works (provided everything is functioning as it should):

If you start the car when the engine temperature is cold, the oxygen sensor thermoswitch closes and sends a signal to the control unit.  The control unit, in turn, sends a fixed 72° dwell/80% duty cycle (58.5° dwell/65% duty cycle for 1987-1989) signal to the frequency valve.  That fixed cold-running enrichment signal (72°/80%) remains in place until the coolant reaches a specific temperature (25°C or 45°C, depending on the thermoswitch installed in your car); the thermoswitch then opens, cutting its signal to the control unit.  If the oxygen sensor is not at its operating temperature when the thermoswitch closes, the system reverts to open-loop, or "limp-home" mode (45° dwell/50% duty cycle).  Open-loop occurs only between the end of cold-running enrichment and the start of closed-loop with a (finally) warmed-up and functioning oxygen sensor. Once the oxygen sensor is heated to its proper temperature, it sends a signal to the control unit and the system enters closed-loop, or its normal regulating function (45°±7° dwell/50%±8% duty cycle).  Should the full-throttle switch be activated, closed-loop is overridden for forced full-throttle enrichment (58.5° dwell/65% duty cycle).

Up through and including the 1986 model year, cold-running enrichment (72° dwell/80% duty cycle) overrides full-throttle enrichment (58.5° dwell/65% duty cycle).  From the 1987 model year onward, cold-running enrichment does not override full-throttle enrichment because the two values are the same (58.5° dwell/65% duty cycle).

"Open-loop will also happen when the car has been parked briefly, the oxygen sensor has cooled off, and the thermoswitch hasn't closed; upon restarting, it takes a minute or more before the oxygen sensor heats enough to produce a signal and send the system into closed-loop. That's why oxygen sensors are now heated; they start working faster, systems go into closed-loop sooner.

"There isn't always an open-loop stage during warm-up.  If the engine is cold (real cold, like winter in Montana cold), and the oxygen sensor is in great shape, the oxygen sensor's heater and the exhaust heat may get the oxygen sensor up to operating temperature before the thermoswitch warms enough from from the coolant to open. In that sort of circumstance, the system will go straight from cold-running enrichment to closed-loop; no open-loop transition at all.  Same car, same cold weather: The oxygen sensor will cool quickly when the car is parked, while the coolant will retain heat much longer; on restart, there will be no cold-running enrichment -- the system will be in open-loop until the oxygen sensor warms up enough to put the system into closed-loop." ~tolusina of VWvortex.com

Likewise, if the car is in a warm climate (or during warm seasonal months), the system may revert to open-loop upon initial start-up, skipping cold-running enrichment due to the coolant already being at or above the thermoswitch's pre-set temperature.

Before Making Adjustments

Requirements:

• Car should be otherwise well-tuned (see below for further information)
• No vacuum leaks
• Distributor advance mechanism(s) should be operating correctly
• Timing should be correctly set with a closed throttle plate
• Air/fuel mixture plug removed and mixture screw free of gunk

Tools Needed:

• Digital volt-ohm multimeter (DVOM) that measures dwell or duty cycle

• If using a duty cycle meter:  CIS produces a negative slope duty cycle.  Some meters will only measure in a positive slope range, while others can be changed to read in a negative slope range; know what your meter reads before beginning (if it can't be determined ahead of time, print out the testing table so your meter's slope can be revealed while testing).  If it reads only in a positive slope range, no worries: Simply subtract your readings from 100 and you've got your car's duty cycle readings.

• 7mm open-end wrench

• Long 3mm Allen wrench

Start with a complete tune up (if not recently conducted):

• Replace ignition components: spark plugs, cap, rotor, wires (use Bosch ignition parts and use NGK plugs)
• Replace air and fuel filters
• Replace injector seals and idle screw O-ring
• Tighten the injector holders; replace the injector holders if needed
• Check for vacuum leaks using carburetor cleaner (spray around hoses, gaskets, injector O-rings); if idle goes up, leak found (replace faulty part -- use a bit of RTV when replacing gaskets)
• Check your K-Jetronic grounds
• Test your oxygen sensor and replace if need be
• Check your throttle switches and idle boost valves
• Check your CIS system and control pressures
• Check your distributor's centrifugal and vacuum advance mechanisms

• Set your timing, if need be

To adjust the idle without a DVOM, click here.

Once you've made sure all the basic stuff is working as designed, should you still find improper idle conditions and/or drivability problems, post your findings/troubles on the Cabriolet forums listed on the Links Page so that it can be discussed to a conclusion, or more likely, several conclusions.

Measuring the Duty Cycle & Adjusting the Air/Fuel Mixture and Idle

Step 1: Run the engine

Warm up the car to operating temp (80°C or higher), cooling fan should cycle on/off at least once.  Be sure all electrical accessories are off (A/C, lights, cooling fan, etc.).

• Note: If using duty cycle, be aware that Cabriolets produce a negative slope reading.  Many DVOMs read only in positive slope.  Easy test: Connect the test leads as described above. With the engine running, actuate the full-throttle switch.  If the reading is 35%, reverse the test leads: red (+) to brown wire, black (-) to blue/white wire.  Actuate the full-throttle switch once more; the reading should be 65%.

Step 2: Connect your meter  

Connect the red (+) DVOM wire to the blue/white wire's female terminal in the two-pin test connector by the cold-start valve. Connect the black (-) DVOM wire to any convenient ground*.  

• If using the dwell setting, set meter to "4 cyl".
• If using the duty cycle setting, set meter to "frequency %"
• Read your meter's instructions; some meters require an additional step, such as pressing a button or two.

*If you have installed disconnects in place of the OEM connector, simply attach the red (+) probe’s alligator clip to the blue/white wire disconnect and the black (-) probe’s alligator clip to the brown wires’ disconnect.

Step 3: Read your meter  

Your target reading is 45° ±7° dwell / 50% ±8% duty cycle.  If it is not within this range, keep reading.  If it is within this range, skip to Step 5.

   Higher than 45°/50% = running lean

   Lower than 45°/50% = running rich

The picture linked above shows an initial dwell reading bouncing between 17.1° and 20.9° (this Cabriolet is running rather rich).  If the dwell bounce is higher than a range of 10 (a bounce between 35 and 50, for example), there are other issues that need resolving before continuing.

Step 4: Adjust the air-fuel mixture  

Get out your long 3mm Allen wrench to adjust the air/fuel mixture (next to the fuel distributor on top of the air filter housing; see photo). The weight of the wrench will affect the mixture! Turn the wrench a little, just enough to feel it move, then lift the wrench out of the adjustment screw. Repeat this process until 45° ±7° dwell / 50% ±8% duty cycle is achieved.

   Higher than 45° / 50%: turn wrench clockwise (this richens up a lean mixture)

   Lower than 45° / 50%: turn wrench counterclockwise (this leans out a rich mixture)

Step 5: Adjust the idle  

Have your 7mm open-end wrench handy; you may have to tweak the idle bypass screw (backside of the throttle body) if the idle changes too much.  You need to stay below about 900 RPM while adjusting the idle.  Use the tach on the dash and your ears, you know what sounds right and normal. If you go over 900 RPM, the centrifugal advance starts to kick in, changing the timing and confusing all your readings.  Set the idle RPM at 900± but above the threshold of the idle boost valve.

Notes

If the idle boost valve kicks in while you are adjusting, you've dropped too low on idle RPM. You can unplug the idle boost valve electrical connector, or better, pinch off either of the hoses going to it. When the idle boost valve (or hose) is reconnected, idle should not change if you've adjusted the idle correctly.

Switch on the A/C (if installed) to make sure that the A/C idle boost valve works; idle should stay about the same with the A/C compressor running.

Should you find questionable meter operation while setting frequency valve duty cycle (ex., the reading never changes), connect your test lead to the Hall sender green/white wire and test your meter. It should read a very steady 36-37° on the 4 cylinder scale, 40.5-40.6% duty cycle. If the meter proves to be good, conduct the tests in the following table.

above information provided by "tolusina" of VWvortex

Testing the Lambda System Components

If you have out-of-the-ordinary readings during the mixture adjustment: After verifying that your meter is good, print this table, conduct the tests listed (warm-up and operating temp are the same measurements you took initially; they're included in the table for convenience), write your readings in the "Your Reading" column and compare them to what the readings should be to help determine what component could be faulty.

Testing the Oxygen Sensor and Jetronic/Lambda Controller

Tools Needed:

• High impedance volt/ohm meter (DVOM) like the one shown above
• New D-cell 1.5 volt DC battery

OXS locations: Early Cabriolets (Rabbit Convertibles) have the oxygen sensor located in the catalytic converter; all others are located in the exhaust manifolds.

Step 1: Testing the cold-start enrichment function

1. Disconnect the leads from the oxygen sensor thermoswitch (underside of the cylinder head coolant flange) and bridge the terminals using a jumper wire with spade terminals.
2. Run the engine.
3. Duty cycle should be a constant 80% ±2% (or 72° ±2° dwell).

Step 2: Testing the open-loop (aka, limp-home) function

1. Disconnect the leads from the oxygen sensor thermoswitch (do not bridge them).

2. Run the engine.
3. Duty cycle should be a constant 50% ±2% (or 45° ±2° dwell).

Step 3: Testing the rich correction function

1. Disconnect the leads from the oxygen sensor thermoswitch (do not bridge them).

2. Disconnect the oxygen sensor wire (black) from the oxygen sensor control unit wire (green).
3. Run the engine.
4. Ground the oxygen sensor control unit wire's spade terminal on bare metal (extension used in the picture at right due the wire being too short to reach a good ground).
5. Duty cycle should be 87% and higher (dwell should be 78° and higher).

Step 4: Testing the lean correction function

1. Disconnect the leads from the oxygen sensor thermoswitch (do not bridge them).

2. Disconnect the oxygen sensor wire (black) from the oxygen sensor control unit wire (green).
3. Run the engine.
4. Ground the negative end of the D-cell battery while touching the oxygen sensor control unit wire's spade terminal to the positive end of the D-cell battery.
5. Duty cycle should be 20% and lower (dwell should be 18° and lower).  Engine may stall; if the reading is within spec, stalling is normal.

Step 5: Testing the oxygen sensor

• Method 1:

1. Start the engine and let it run until it reaches operating temperature (80°C or higher); if it's already warm, let it run for two minutes.

2. Clamp off the hose from the idle speed boost valve (white valve, if your car has A/C).
3. Disconnect the crankcase ventilation hose.
4. Make note of the duty cycle reading.
5. Plug the crankcase ventilation hose.  The duty cycle should drop, then rise and fluctuate.
6. If the duty cycle does not drop, then rise and fluctuate, the oxygen sensor is faulty.

• Method 2:

1. Start the engine and let it run until it reaches operating temperature (80°C or higher); if it's already warm, let it run for two minutes.

2. Disconnect the oxygen sensor wire (black) from the oxygen sensor control unit wire (green).

• If you have two DVOMs, you can connect the second one as shown in the "Measuring Duty Cycle" section above to read the duty cycle/dwell at the same time.

3. Connect the black DVOM lead to any convenient ground and connect the red DVOM lead to the oxygen sensor wire (black).
4. Set the DVOM dial to 2 DCV (or 4 DCV, if that's the lowest setting on your meter).
5. Ground the oxygen sensor control unit lead wire to bare metal to simulate a lean condition.  The oxygen sensor's voltage should go high, 0.8 to 1.0 DCV (duty cycle/dwell should go high, 85%/75° or higher).
6. Connect the 1.5 volt battery's positive end to the oxygen control unit lead wire and ground the negative end of the battery against bare metal; this simulates a very rich condition. The oxygen sensor's voltage should go low, 0.15 DCV or lower (duty cycle/dwell should go very low, 11%/25°).  Note: The system may go so lean that the engine stalls; this is normal.

Notes

A sluggish oxygen sensor may cause a failed smog inspection while exhibiting absolutely no other drivability issues.

Should you see an operating range at the oxygen sensor ranging from -0.5 DCV to +0.5 DCV (instead of the normal +0.1 DCV to +0.9 DCV), your sensor has been permanently damaged by chemical contamination and needs replacing.

Cabriolets built from July 1987 through 1989 have heated (3-wire) oxygen sensors. Should you see 12 DCV (or charging voltage) at the oxygen sensor wire, replace the oxygen sensor immediately -- the heater has shorted to the sensor.

Should the oxygen sensor control unit happen to be faulty, it will fail to compensate for the simulated lean/rich conditions described above.  The oxygen sensor control unit rarely goes bad, but is does happen.  Before condemning the control unit as being faulty, verify that the control unit is receiving power and that all ground wires/connections are good.  Additionally, if the duty cycle stays at 65% or 80%, disconnect the cold running enrichment switch (if installed) and the full-throttle switch, one at a time, then both together if need be.

above test methods supplied by "Thomas_covenant" of Yahoo! Groups and "tolusina" of VWvortex

Testing the Frequency Valve & Thermoswitch

Coming soon

DIY Timing

The proper method for setting your car's timing requires the use of a timing light.  If you're without a timing light, use the Bad Habit Rabbit's static method until you can set the timing properly (you can also download the DIY Timing Guide):

"All of your VWs have marks on the rotating parts to denote 0° TDC. The cam has a dot that lines up with the valve cover. The flywheel has a notch that lines up with the center of the hole on top of the bell housing. The distributor has a slash that lines up with the rotor. Line all of these up and your car is at 0. No advance. No retard.

In a pinch, I've put the car in 5th gear (with the car off!) loosened the distributor and pulled the car forward until the advance mark on the flywheel appeared in the little timing hole. I then turned the distributor until the hash mark lined up with the rotor and tightened her down. If your flywheel notch is at 3° or 5° or whatever, now your distributor is at 3° or 5°. It's probably good for keeps or at least until you can get your hands on a timing light, but it DOES work accurately!"

Removing the Anti-tamper Plug

It’s very possible that your air/fuel mixture adjustment hole still has its anti-tamper plug installed.  If so, it will need to be removed, or tampered with, in order to adjust the air/fuel mixture as described in the measuring and adjustment section.

The following tampering instructions are courtesy of tolusina of VWvortex.com:

“The easiest way to [remove the anti-tamper plug] is to flip the complete sensor plate assembly over and punch the plug out with a long thin punch.

The other tamper method requires drilling a centered hole in the plug, screwing in a sheet metal screw and pulling the screw up. You've got to be sure in advance that the drill bit used is small enough that the sheet metal screw you choose will have plenty to grip. Also, grind the point off the tip of the sheet metal screw so it doesn't bottom out.

Oh, yeah; you can't drill all the way through the aluminum plug [because] there's a floating steel disc in the bottom of the plug that will just spin when the drill bit hits it. The steel disc is to prevent drill chips from dropping inside. The steel disc is also what a pointed sheet metal screw will bottom out on before the rest of the screw gets a good grip.

If you booger things up with a drill and sheet metal screw, no worries; flip the assembly and punch it out, bottom-up.”

K-Jetronic Pin-outs

2: Oxygen sensor (green wire)

4: To oxygen sensor wire (black wire)

5: Ground (brown/white wire)

7: Fuel enrichment switch (gray wire)

8: Power from Lambda relay (green/yellow wire)

12: Oxygen sensor thermoswitch (brown/red wire)

15: Frequency valve (brown/blue wire)

16: Ground (brown wire)

17: Duty cycle test connector (blue/white wire)

26: To pin 33 (white wire)

33: To pin 26 (white wire)