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Walt Fricke's Avatar
 
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Location: Boulder, Colorado
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SC Frequency Valve issues

I've been having problems with the air/fuel ratios in my 83 US SC. This car is retired from the street (still licensed, but no insurance because of headers with no provision for a muffler, and no passenger seat, etc)and now is DEd and raced. Several problems I caused myself when trying to figure out other problems. I got intrigued by a modification to the WUR which lets you not only adjust the cold (starting) pressure, but also adjust the warm pressure.



I fabricated these things, and I raised the big plug (the one with the fittings)so far that the cold warm-up piece was still pressing on the spring even when warm, so up it came. Each time at each track, from Utah to Colorado to Nebraska to Oklahoma to Minnesota seemed to call for much fiddling to avoid either being way too lean, or way too rich. I was aiming for about 13.2 at WOT, but consistency for that eluded me.

At times the frequency valve seemed to be working (could feel it buzz), and sometimes it did not. Lately, no dwell meter readings from the plug, and no useful information on my inexpensive digital scopes.

So I took the oxygen sensor control unit (the big box) and the acceleration enrichment unit (smaller box), and the (US NAPA) relay for these apart, to see if anything looked amiss inside. Didn't see anything obvious, but I added an LED to the power and ground leads for both so I could tell by looking if they did or did not at least have power.

1) Does anyone know if a schematic is available for either or both of these? The parts look replaceable, the few ICs are still available (or can be substituted), and so on. And it would help trying to figure out issue 2).

2) Last year I got a nice 70Hz square wave from the test pin, and I could even see it change the pulse width. This year, nada. While I had the PC board exposed I added a wire to Pin 15 (provides the ground for the FV) and Pin 17 (goes to the test plug) so I could check both conveniently in the cockpit.

Jim Williams posted this picture, showing what the waveform at pin 15 looks like - lots or overshoot and ringing - as opposed to pin 17.



After fixing various goofs (one of which was a spade connector with the power for the relay having pulled off its companion where it runs up to the trunk, and the other was miswiring the 12 pin plug back in the engine compartment)the FV is now buzzing nicely.

And here is the pin 15 trace:



I'm unsure if this looks like it should, but I think so.

But the pin 17 trace doesn't look good at all - a few millivolts and not a hint of a pulse width really. Certainly not enough for a dwell reading, and my cheap frequency meter won't read it either.



I don't know why this is. I think I saw some discussion to the effect that while pin 15 does the work, extra circuitry cleaned up the signal so you could use a dwell meter on the test port. Maybe just that circuitry has been damaged? Otherwise, how would the FV be buzzing at a nice low frequency?

Any suggestions?
Old 08-11-2016, 12:55 AM
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Oscilloscope..........

Walt,

I will be following this thread with a lot of interest because someone gave me a professional grade oscilloscope which I have no idea how it works. I am still trying to learn and understand how to set the knobs and be able to read the square wave signals from the FV. Will contact you when I get back home and renew my interest in learning how to use my new inherited tool. It even came with a rolling cart.

Tony
Old 08-11-2016, 04:25 AM
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Tony - I'm hardly the guy to learn from with these tools, as I don't understand very well the function of several of the controls. Some are easy, like how many volts or fractions of a second are displayed on the X and Y axes. Trickier are AC and DC coupling, plus there is a Ground coupling, which never gives me any reading - maybe it is for measuring just voltages with no time component? Then there is what triggers things, and at what level. Edge triggering is simple, but there are others. I suspect that Googling how to use an oscilloscope would set you up just fine, as opposed to my sort of hacker approach.

I wonder if there is a practical way to hook a pressure gauge to the lower chambers of the differential chamber system? If so, perhaps one could observe the effects of simulated idle or WOT enrichment mechanically? If it was happening, then probably the FV is doing something.
Old 08-11-2016, 10:48 AM
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Well, my Google search failed to turn up a schematic for the SC K-Jetronic Frequency Valve system printed circuit boards. What I did find were some interesting discussions and essays from Mercedes guys. It turns out that the 1980 MB 450 uses a very similar frequency valve thing. The Bosch multi pin connector, and the cast housing to which the PC board is attached are the same part as on the SC, or so close as not to matter. The large PCB is very similar, although it is not the same part. It uses an external Darlington transistor, while the SC does not have this external, but has two power transistors with heat sinks attached to the board.

The MB does not have pin 17 connected to an external test wire/connection. But its pin 15 serves the same function as the SC board 15 pin does - grounds the FV valve circuit. The author of one of these pieces concluded that the FV valve was basically just a solenoid operated fuel injection valve, and that the electrical signature is the same, with the coil collapse spike, and also with the "ringing" which I saw on my pin 15.





Since my pin 15 looks like this, I am assuming that the mini brain is sending the right signals to the FV (which buzzes merrily), and the fact that the pin 17 output no longer looks like this:

is an indication that something has failed in the circuitry used to filter and convert the pin 15 signal to something a common dwell meter can read. While I'd like to be able to fix that, as long as I can use my inexpensive pocket scope to read pin 15 I guess I don't need to make that a high priority. Perhaps someone knows why the CRT scope display looks rather different - straight line, mostly - from what inexpensive digital scopes show, which is the spike, but those close packed up and down bits showing the FV on modulated width of the pulse.

I did see that someone had created a schematic for the 914 ECU. But, alas, so far no such thing for the SC oxygen sensor unit.
Old 08-16-2016, 08:43 PM
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Walt,

The FV is an EFI injector and another option is to drive it with a programmable controller like this :

https://splitsec.com/product-category/additional-injector-controllers/

Or something like a used Haltech F9A, which would also allow closed loop control. Many of these are sitting retired on a shelf in favor of ign+fuel ECU's

Or you could step off a cliff and design and build a wide band, self learning, closed loop injector controller and build the ultimate CIS lambda machine.

Paul
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Old 08-17-2016, 06:35 AM
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I'd wonder if there is a Bosch archive (online) somewhere that documents the testing and repair of those circuit boards. That must exist somewhere at least in paper form.
Old 08-20-2016, 11:11 AM
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Paul - YOU could do all those things, but I race this car in a class which does not allow such modifications. Running open loop is OK, because we can de-smog the cars. I suppose that with wide-band stuff you could use a closed loop system to keep the AFT at 13.2 or whatever you wanted? But not in this class.

So I have to work with what Porsche and Bosch came up with, pretty much. There is a fellow who rebuilds these "min-brains" for the FV Lambda system, but it's not cheap, and of course I hate to have someone else do anything if I just possibly might be able to do it myself.

I sure would like to find a Bosch archive. I'm not sure I've ever seen a schematic from Bosch on anything, though, out in public so to speak.
Old 08-23-2016, 09:25 PM
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Hi Walt,

I had a bit of a look around and found this on rennlist:

The ECU

It's D-Jetronic but hope that helps just a little bit.

G.
Old 08-24-2016, 06:36 PM
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Right - I had found that too, and copied the schematics, but they don't really relate to the SC K-Lambda. I fear that no one with Mr. Anders' electronic skills and determination has undertaken a similar project with the K-Lambda. Jim Williams measured the wave forms from pins 15 and 17.
Old 08-24-2016, 09:55 PM
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Yep that's a shame I haven't found anything either. We need an ex Bosch guy .
Old 08-28-2016, 06:43 PM
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Osciloscope........

Quote:
Originally Posted by Walt Fricke View Post
Right - I had found that too, and copied the schematics, but they don't really relate to the SC K-Lambda. I fear that no one with Mr. Anders' electronic skills and determination has undertaken a similar project with the K-Lambda. Jim Williams measured the wave forms from pins 15 and 17.


Walt,

The lack of electronic skills is something we have to overcome but for sheer determination and ability to analyze problems your are indeed very good. Keep up the good work and with some help from others you will get to where you want to be. Right now, if I could get to learn and understand how my new toy is used to test the frequency valve in a lambda system I will participate in this discussion. But that will be a long uphill battle. I have zero background in electronics and just getting into pre-K electronic education.





Tony
Old 08-28-2016, 07:53 PM
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A 1970s Tectronix - looking it up, it runs up to 2.5 MHz. That is slow, but nothing we are interested in runs anywhere near that fast, so this looks like a great tool to have. And you have room for it! You can learn a lot just dinking around. I bet you could build a test jig for the K-Jet Lambda. All the inputs except the O2 sensor are just switches, so a bunch of labeled toggles on a board would let you simulate all the open loop conditions the mini-brain sees. You could look up or measure the resistance of the FV, and substitute a suitable high Watt resistor for that. The O2 sensor is a voltage generator, but you should be able to rig up something to simulate that. Shouldn't be too hard to breadboard a small circuit which would, at a rate you could vary, switch the voltage from say 0.6V to 1.2V or something - a voltage above Stoich and one below it. Something like a 555 might be able to do most of this work, with a variable resistor as part of the frequency circuit to vary that, or just a resistor and capacitor substitution box with its leads clipped on.

But you should get quality scope traces.
Old 08-30-2016, 09:21 PM
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