Dwell meter hook-up for checking Lambda control?

Yes, the "round" plug is just a place where some of the ignition system signals can be sampled -- no injection system signals there.

To measure the dwell of the frequency valve signal, just connect the two leads of the dwell meter to the two terminals of the frequency valve (while they are still plugged in). Or follow those wires on your schematic to other locations where they maybe can be more easily accessed (when plugged in) -- for example, the PN (beige/black) wire can be found at the protection relay terminals, and the AG (light blue/yellow) wire goes to the K-Jetronic ECU connector.

The tricky bit is being able to "read" your dwell meter correctly. Even if it reads dwell directly, the same signal could read 10% or 90% depending on the polarity of the lead connections and/or how the meter defines dwell. Additionally, as Probst describes in the last paragrapgh on page 5.22, if you are using a points-type dwell meter, you need to do a conversion from degrees to percent.

 

While the frequency valve could correctly be described as an "electrovalve", IME F is pretty consistent in using the terms frequency valve or metering valve for that gizmo, and the term "electrovalve" is more often used to describe the small solenoid electro-pneumatic valve (as used to control the cutoff valve in your air injection system):
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An example electrovalve (and differential pressure switch) from another US 308QV:
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If you can give the 307/84 OM page number where the term "electrovalve" is used, I'd be glad to comment.

 

Had a feeling that it might be that one because (as you noted) they mangled the English descriptions in that figure by omission. You need to use the Italiano side descriptions -- "O" is the frequency valve, and "I" is the air electrovalve in the air injection system. No direct functional relationship between them, but:

When cold = coolant thermoswitch "L" closed = air electrovalve "I" activated (if oil thermoswitch "G" not too cold/closed, and WOT microswitch "M", the SR portion, not closed = leaves relay "C" unactivated) = intake vacuum reaches the cutoff valve = opens the cutoff valve = air injected (sucked in) at the exhaust ports = signal from O2 sensor would be meaningless. The injection ecu gets the same signal from the coolant thermoswitch "L", and knows to ignore the O2 sensor signal and the FV duty cycle defaults to something like a constant ~60% = FV more open = lowers control pressure = lowers upper chamber pressure = increases the pressure drop across the slits in the FD = adds a little richness for the same size slit opening.

When warm = coolant thermoswitch "L" open = air electrovalve "I" unactivated = intake vacuum does not reach the cutoff valve = cutoff valve closed = no air injected (sucked in) at exhaust ports = signal from O2 sensor is valid. Again, the injection ecu gets the same (now open) signal from the coolant thermoswitch "L", and uses the (now valid) O2 sensor signal and tweaks the average FV duty cycle trying to keep the average O2 sensor signal at 0.5V (even though the FV duty cycle is changing in this mode, the average FV duty cycle should be ~50%). If the O2 sensor is unplugged during warm running, the FV duty cycle should default to a constant ~50%.

 

Need to correct a typo and use some better language -- should've been:

When cold = coolant thermoswitch "L" closed = air electrovalve "I" activated (if oil thermoswitch "G" not too cold/closed, and WOT microswitch "M", the SR portion, not closed = leaves relay "C" unactivated) = intake vacuum reaches the cutoff valve = opens the cutoff valve = air injected (sucked in) at the exhaust ports = signal from O2 sensor would be meaningless. The injection ecu gets the same signal from the coolant thermoswitch "L", and knows to ignore the O2 sensor signal and the FV duty cycle defaults to something like a constant ~60% = FV more open = reduces lower chamber pressure = reduces upper chamber pressure = increases the pressure drop across the slits in the FD = adds a little richness for the same size slit opening

 

Hi Steve,

I am wondering this as well... I have a Euro '85 which has a Lambda installed (I'm guessing it was done back in the day when it was federalized). I don't think it is the same as a US car (maybe) but it looks like the one in this description from Bruce Watson's book: "How To Tune & Modify Bosch Fuel Injection" It notes that the Dwell in the four cylinder position should read 45deg. Seems simple enough... There are two wires on my frequency valve are are red-ish if I remember correctly. I'll look more carefully.

I'm thinking this will let me know if that old O2 sensor is actually working (I think it is actually welded in place) and if the frequency valve is doing its thing.

Do you see any danger in hooking up my analog Dwell meter? I don't want to blow anything up. I would probably have a shop check it/set it for reals.

Thanks so much,

Franny
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No, it isn't. On a US version K-Jet with Lambda, the frequency valve changes the lower chamber pressure; whereas, on a euro K-Jet without Lambda that has had Lambda functionality added, the added frequency valve changes the control pressure.

No, a dwell meter is just essentially a slightly smarter voltmeter, but rather than hooking it up like in the figure, it's easier to just hook the two leads from the dwell meter to the two wires on the frequency valve (while still plugged in to the frequency valve). (If hooked up like in the figure, one wire on the frequency valve will just have a constant 0% or 100% dwell relative to ground, while the other wire on the frequency valve relative to ground will give a dwell reading -- if things are working ). Alternatively, if the frequency valve is actually working, you should be able to just touch it with a fingertip and determine if it is "buzzing"/vibrating (working) or not vibrating (not working).

 

Great! Thanks so much Steve.

I'll do a couple simple checks, but I think I'll let the shop take a look at it and do the adjusting. The owner's manual actually has the CO: (1 +/- 0.2) and HC: (<300ppm) (at idle) and I know from my emissions test I'm double those numbers. The injectors have been replaced and the plugs are new - new air filter too. Something is a bit off.

I always want to, at least sort of have an idea of what is going on before bring it in

Again, thanks so much for your reply,

Franny

 

I had problems trying to measure my freq valve duty cycle on my US 84 308 until I read this:
1980 to 1985 ECU Mod for Correct Duty Cycle Measurement - Mercedes-Benz Forum
Turns out my car already had a factory test lead to pin 17 of the UCU and no modification was needed.