Frequency Valve

Have you tested the duty cycle with the O2 disconected? If so what did you get?
With the motor warmed up it should be a rock solid 50%.

 

Never replaced one but I have a funny feeling that it will not show 50%. Make sure the O2 is disconnected.

 

You say that the fuel injection system is going into closed loop, this would mean that the oxygen sensor is modifying the fuel mixture, which doesn't sound like it is happening. If you hear the frequency valve buzzing, then it is ok. If the frequency of the buzzing doesn't change, then the problem is not the valve, but the output of the computer.

Disconnect the oxygen sensor and start the engine and measure the voltage from the Jetronic control unit (Lambda computer), it should be .5-.6V.
Now measure the voltage from the oxygen sensor (still disconnected), if the mixture is lean it will less than .5v, rich greater than .5v.

Now ground the connector from the Jetronic control unit (this simulates a lean condition and forces the mixture to go rich). You should hear the frequency valve change pitch, the engine will run differently and the voltage reading from the oxygen sensor will read .8-.9v.

Now apply your 1.5v to the connector from the Jetronic control unit (This simulates a rich condition and forces the mixture to go lean), once again the frequency valve should change pitch, the engine should lean out and the oxygen sensor voltage will be .1-.2v.

If the Lambda system doesn't respond, then it is staying in open loop. There are two temperature switch inputs (one oil and one coolant switch). If either of these is closed then the system may stay in open loop. The throttle position switch also has a full throttle input to the Jetronic control unit, if it is bad it will also cause the same condition. There is a wiring diagram of the Lambda system in the emission control section of your owner's manual. With the engine at operating temperature, check the to see if the pins of the sensor inputs at the Jetronic control unit are grounded.

Jetronic computer failures are rare, but I have replaced a couple in my lifetime.

Brian Brown
Patrick Ottis Co.

 

with the engine running and everything connected.....

monitor O2 voltage with a meter from O2 connector and ground (O2 output voltage.)

monitor dwell to frequency valve at ECU pin

monitor actual CO reading at manifold sniffer

If the basic setting is WAY off, nothing will change the "note" of the engine. You can also check the duty cycle without running the engine by unclipping the plate position sensor. If there is a signal out and the valve is buzzing, the inputs are very few, H2O temp, oil temp., throttle position, and O2 sensor, if they check out, then the ECU is bad, very uncommon though.

 

What I think important to know at this point is what was the duty cycle reading with the O2 disconnected.

Data is important, without it no diagnostics can take place.

 

That needs to be converted to %.

What is the full range of the 8 cyl scale on your dwell meter?

If it is 90, 44 is close enough to 50% to say it is OK.

If however it is like most and the range is 45, 44 is for our purposes 100% and that is a very important piece of information.

 

Thats twice what it should be.

Do you have the warm and cold control pressures handy?

 

Warm pressure is too low.

Just for laughs I would take the back off the warm up regulator and take a peek inside.

The interior of many I see these days look like an object brought up from the Titanic. So full of rust they cannot possibly work properly.

Sorry I do not remember much from the first thread. Did this start all at once?

 

Did you check your airflow sensor plate height? Just pull the air filter off and take a picture of the plate and post it. I can tell if it is in the right spot. also when you try to rev it up if you push the sensor plate with your finger does it rev good then?
Larry Fletcher

 

Larry.. Have you done any engine mechanical tests, such as a leak down or compression test of the engine? Have you checked cam timing? CIS system are "engine sensitive". Low engine vacuum can cause a lean conditon due to the sensor plate not being "pulled" down far enough to allow enough fuel in for proper airfuel mixture. Put a vacuum gauge on one of the ports the come off the side of the upper intake plenum and see what kind of reading you get at idle.

 

Larry,
The best place to check the resistance of the two switches is at the connector for the Jetronic control unit under the right rear access panel. That way you are not disturbing the connectors at the switches and are checking all of the harness leading to the switches. The oil temperature switch is in the banjo fitting where the oil return from the vapor separator leads back into the sump on the front of the motor. The water temperature switch is in the coolant expansion tank. I believe that both switches are normally closed and open with temperature. The oil switch opens at 77 deg F and is pin #12 on the Jetronic connector, the full throttle input from the throttle position switch also shares the same pin. The coolant switch opens at 135 deg F and is pin #7 on the Jetronic connector. Be aware that both of these switches also connect to the relay for the electrovalve for the air injection system, so if this is plugged in, it will affect your Ohmmeter readings.
Check your owner's manual, there is a good wiring diagram in the air injection system section.

Brian

 

Larry,
It sounds like all temperature switches are functioning normally.

.095v at the O2 sensor wire is not good, either you have no power from the power protection relay (the one with the fuse), the wire to the O2 sensor has an internal short, or the Jetronic control unit has died.

If the frequency valve is still buzzing happily away, then the power to the Jetronic control unit is probably ok, but it doesn't hurt to check it.

The large green wire to the oxygen sensor is actually a shielded wire, with the signal going down the center and a braided sheath that is grounded around the outside. If there is any damage to the wire, the outer shield could short to the inner wire and cause the voltage to read low. With the control unit and O2 sensor disconnected, check resistance between pins 2 and 4 on the Jectronic control unit connector, it should be infinite.

If you have good power and no short on the sensor wire, then it's time to look for a new control unit. I think I saw one on ebay recently. Take the Bosch number off of it and check with Bosch, it may be used on other vehicles that are more common.

Brian

 

Larry,
There should always be roughly .5V at the oxygen sensor connector at all times, if there isn't then your brain is bad.

The only thing that keeps the computer from going into closed loop are the two temperature switch inputs. If they are open then the computer will go into closed loop as soon as the oxygen sensor heats up and starts working.

Brian