Modifying a 80's Bosch CIS module to accept a 6cyl tach signal instead of 4cyl?

lookforjoe

lookforjoe

True Classic
So, I have all the Bosch Constant Idle Speed components that I had on my X1/9 at one time. It is a standalone system that Volvo used on K-Jet cars.
I'd like to use it on my 280Z, however the module is looking for a 4cyl tach signal. I looked at just finding a 6cyl version of the module, however even used ones are $$$.

If I dismantle the 4cyl one & take pics of the circuitry, would one of the electronic gurus on here have any idea as to what I would need to change to register the 6cly pulse instead? I'm assuming it comes down to resistor values?

I'll add some pics later

Thanks in advance :)

EDIT - original X1/9 install thread
 
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lookforjoe said:
So, I have all the Bosch Constant Idle Speed components that I had on my X1/9 at one time. It is a standalone system that Volvo used on K-Jet cars.
I'd like to use it on my 280Z, however the module is looking for a 4cyl tach signal. I looked at just finding a 6cyl version of the module, however even used ones are $$$.

If I dismantle the 4cyl one & take pics of the circuitry, would one of the electronic gurus on here have any idea as to what I would need to change to register the 6cly pulse instead? I'm assuming it comes down to resistor values?

I'll add some pics later

Thanks in advance :)
Click to expand...
A schematic would be most useful.
 
dllubin said:
A schematic would be most useful.
Click to expand...
Unfortunately there won't be a schematic for the module circuit board. Only schematic I have is the system wiring. I'll find a module & remove the board & take pics of it, at least.

PXL_20230713_230254636.jpg

PXL_20230713_230349776.jpg
 
I was able to download several versions of the manual but no schematic. If it is using a capacitor for integrating the waveform and using a comparator to trigger an action when it reaches a particular voltage, you might be able to fool it by using a larger capacitor which will require more charge to reach a given voltage. That would also work if it is using an amplifier to drive something based on the capacitor voltage which should be proportional to rpm. If that is how it works, a 50% larger capacitor may take care of 50% more cylinder. Of course, it could use a different technique that would require a different solution. If you can get some good photos of the inside, perhaps we can create a schematic. The key is to figure out what it is doing with the signal on pin 12 of the module.
 
Interesting they used a separate module to control the aux air valve, but those sorts of designs were common back then.

Given this controls the idle RPM, I'm thinking having a accurate input from the tach (e.g. RPM speed) is critical to it working correctly. That might make modifying it for a different number of cylinders a bit impractical, but worth a try. ;)
 
Dr.Jeff said:
Interesting they used a separate module to control the aux air valve, but those sorts of designs were common back then.

Given this controls the idle RPM, I'm thinking having a accurate input from the tach (e.g. RPM speed) is critical to it working correctly. That might make modifying it for a different number of cylinders a bit impractical, but worth a try. ;)
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It controls a (cyclic?) Idle Control Valve (Bosch 0 280 140 520) - not an aux air. The signal comes straight off the coil - the same as the signal to the tach :)
 
I took apart the module I could find -
PXL_20230714_234205153.jpg

PXL_20230714_234157774.jpg

then I noticed the cover.... this one is a 6 cylinder version - 0 280 220 008 😜

PXL_20230714_234317056.jpg
 
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lookforjoe said:
It controls a (cyclic?) Idle Control Valve (Bosch 0 280 140 520) - not an aux air. The signal comes straight off the coil - the same as the signal to the tach :)
Click to expand...
Sorry, I meant a idle control valve rather than aux air valve. My point being most of the CIS systems I've dealt with a idle control valve have the ECU control it instead of a standalone module. But those were likely later versions of CIS....there were many and each vehicle manufacturer had variations at times.

But more important was my comment about the need for some accuracy when converting from a 4-cyl to 6-cyl signal to that module. Fortunately you don't need to do so now that you have a 6-cyl version. :)
 
Dr.Jeff said:
Sorry, I meant a idle control valve rather than aux air valve. My point being most of the CIS systems I've dealt with a idle control valve have the ECU control it instead of a standalone module. But those were likely later versions of CIS....there were many and each vehicle manufacturer had variations at times.

But more important was my comment about the need for some accuracy when converting from a 4-cyl to 6-cyl signal to that module. Fortunately you don't need to do so now that you have a 6-cyl version. :)
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Different CIS: You are probably thinking of CIS, as in Continuous Injection System (aka K-Jetronic). Hussein is referring to CIS system as in Constant Idle Speed system. I believe the latter was fitted to LH-Jetronic cars.
 
ng_randolph said:
Different CIS: You are probably thinking of CIS, as in Continuous Injection System (aka K-Jetronic). Hussein is referring to CIS system as in Constant Idle Speed system. I believe the latter was fitted to LH-Jetronic cars.
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Ha, once again I've misspoken. Thank you for pointing that out. ;) I actually was referring to Jetronic systems, not CIS-injection. Man, my mind must really be going bad. However I have also dealt with many of the CIS-injection systems....but in this case yes, we are talking about the electronically controlled Jetronic design - hince my reference to a ECU (which CIS did not have) to control the idle valve. Once again, sorry for any confusion. :)
 
ng_randolph said:
You could try using a 6-cylinder to 4 cylinder tachometer adaptor between the coil primary and the CIS module, Something like this, perhaps.
Click to expand...
I think Bjorn is on the money. It is probably looking at the pulses to detect the RPM. You can create a frequency multiplier with a phase locked loop, but detecting the ignition spike is tricky. The product Bjorn links looks like it will do the job nicely.
 
Note also that the way that the pulses from the ignition are processed is quite specific to how they are generated: in the case of the L-Jetronic (similar vintage to your CIS) the input circuit is actually looking for the "spikes" in the primary winding of the ignition coil, caused by the collapsing magnetic field when the points open:

1689594022856.png


As a result, if you add any intermediate processing, the output must be compatible with what the CIS is expecting to see.

Cheers,
Dom.

PS: In addition to the product Bjorn linked, I also found this which is a bit closer to home.
 
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dcioccarelli said:
Note also that the way that the pulses from the ignition are processed is quite specific to how they are generated: in the case of the L-Jetronic (similar vintage to your CIS) the input circuit is actually looking for the "spikes" in the primary winding of the ignition coil, caused by the collapsing magnetic field when the points open:

View attachment 75466

As a result, if you add any intermediate processing, the output must be compatible with what the CIS is expecting to see.

Cheers,
Dom.

PS: In addition to the product Bjorn linked, I also found this which is a bit closer to home.
Click to expand...

That you for all the additional research.
In my case, all I need is the alternate TPS operation you helped me with, as the module I have left is actually the correct 6 cylinder version :)
 
Forgot to wrap this one up.

System functions as designed, (including AC-on idle bump) the only alteration was to add a diode on the factory trigger wire from the TPS to the CIS relay (above). Without it, the engine was confused when rpms were held are 2K 🤪

PXL_20231118_175648223.jpg
 
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