CDI install instructions for Jetronic buses

[metahacker]

Can anyone share some detailed CDI installation instructions for the Jetronic equipped Late Bay Bus? In my case, a 78.

I have been looking all over (and, yes, searched here a bunch, too) and I mostly just find stories and people griping about failed installations where they provide technical details... or bragging about how wonderful their system functions without any details.

The site referenced by Ratwell (shagadelic.org) is long gone, and the pictures are not kept on the archive.org site...

I run one on my Type 1 early Bay (where I also have a CB fuel injection ECU, wideband O2 etc) .. and it was very easy... but on these late buses there are a number of wires going to the coil. The biggest concern being the fuel injection trigger.

I've heard of some people needing tach adapters, some people not needing them, and even one guy say it only worked properly when the motor was warmed up, and that they actually toggle the wiring back and forth (crazy right? lol...)... and seen lots of overall issues people had installing them on Type 4 Jetronics (both Bus and Porsche 914)...

I've also heard a similar amount of people raving about how great they were on their Bus or 914, as I've said before... but those people didn't provide any installation details.

[Wildthings]

The wiring diagram shows everything that should be connected to the coil excepting the wire for the tach if present.


Double click to enlarge:

The ignition system is in Current Tracks #66 and 67




.

[SGKent]

the only caveat is that when the CDI changes the waveform of the points that it then sends to the coil, the L-jet ECU may have trouble with that waveform which is is the basis for everything the ECU does. That is why some folks have no problems and others do, even with the same brand electronic ignition. These ECU's are 40 - 45 years old and there is no guarantee they still hold the tolerances they were made with.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[aeromech]

What the fuck is a CDI?
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[SGKent]

[Wildthings]

One needs to make sure their unit isn't one that supplies multiple sparks or that will send the FI crazy.

[aeromech]

Hmmm,
In over 30 years I’ve never needed one
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[SGKent]

[metahacker]

you know, ignition box...typically multi-spark.....Mallory HyFire, MSD, Jacobs, Crane, Accel, Pertronix, etc.

in this case I was planning to install a Compufire Digital HP Mobile ... with a Mallory Unilite vacuum advance distributor ...

I guess the real question is whether you

(1) keep all the wires the way they are - relative to each other - and you just connect the CDI trigger input to that bundle....and keep the +12V stuff for the backup/fan/coil + together and add the CDI box ignition 12V input to that... and remove the coil itself, e.g. just keeping the +/- connections all together on their own junction independent of the coil. and have the CDI as the only thing connected to +/- on the coil. which i believe means the the distributor trigger wire is feeding the ECM the way it's already wired stock, and the CDI is just also "listening"

Or,
(2) whether you go and try to insert the tach output of the CDI box into the equation ... which I believe is between the white and green. I'm not sure why people do this but this is what I read people complaining about...tach adapters and such.

It's night time at the moment and I'm not actively doing the install while I'm here armchair mechanicing from my computer. Maybe it will be intuitive and easy once I get started (perhaps with a little trial/error required) but I wanted to gather some information ahead of time before I dove in.

[metahacker]

i know we can all enjoy a nice debate on the merits / needlessness of this particular bit of technology

i will quote this bit from a source that seems to be well-known, trusted and loved in the bay window community

[metahacker]

and referencing my own post ... i would assume #1 is the only logical way to do it , but i read so many posts in forums with people talking about tach adapters and tach wires feeding the ECM (approach #2) .. which doesn't make sense to me (why would they wire it this way) am i missing something?

basically just attach the bundle of wires on the coil + to the CDI box +IGN and the coil - wires to the CDI trigger wire...and connect the box to the coil and battery... and that's it

but this is not what i see people talking about doing...which is what confuses me.

does anyone have any first hand experience?

[aeromech]

You’ll have trouble finding your answer because you’re in the wrong forum. Go to the performance forum
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[metahacker]

ok i posted it there as well thx

[SGKent]

you might consider reading up on L-jet before you make your final conclusions. Mayor Atwell is a great guy and there is no doubt a CDI would be fine on a carbed bus.

L-Jet ECU takes the baseline pulse from the coil, reshapes it, and uses it as the trigger for the L-jet injectors. The ECU adds for cold TS1 (cold air coming into the AFM, adds for TS2 cold engine, adds for door position, adds for the door swinging open quickly as in like an accelerator pump would on a carb, and adds for higher RPM until it saturates the injectors somewhere around redline. Anything that messes up the baseline pulse will affect how the engine performs.

The length of the tach wire seems to cause a reflection that causes the tach to bounce in some cases. Adding a diode seems to help in that case. If the CDI has a tach output that might solve the need for a diode.

I think most of the performance guys use carbs, but some use megasquirt for their FI system.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[metahacker]

ok i am starting to understand, i believe its the high voltage ignition coil ground spike output that is key for the ECU (not the trigger firing square wave signal)? hence the need for the tach adapter to simulate this behavior rather than just simply allowing the ECM to see ground pulses to fire. the coil negative wire can't be connected to anymore with the crazy multi spark stuff going on, hence the tach adapter simulates the old coil behavior.

[Wildthings]

I would still question how the ECU can handle having multiple sparks if it is not designed to somehow ignore them. No experience here to draw on.

[metahacker]

the ecm would not be fed the actual coil signal

[metahacker]

yeah this makes sense now - conceptually, i think the answer is now clear:

the stock fuel injection is looking at the coil negative wire. this has voltage spikes as the system fires. this is different than watching the coil trigger itself which would be a simple square wave on/off signal. it is not this typical simple "tach output" square wave that the ECM expects.

the tach adapter / dedicated tach wire from the cdi are needed though - but it needs to support a coil ground voltage spike mode, not just a square wave on/off trigger ..

apparently, there are 3 common modes of tach output - square wave (normal 'tach wire' output, the trigger opening and closing), voltage trigger aka voltage spike (e.g. simulates coil ground wire), and current sensing (coil positive wire)

i believe some boxes support these with no special tricks (like the new pertronix), some require adapters, some don't support it at all.

i think that's where everyone hits all the problems. i don't believe any boxes will work as-is and have a default of 12v square tach output. so the tach adapter comes into play, and i think some people choose the wrong one.

so, on a pertronix digital HP/HP mobile this would mean you put it into "voltage spike tach mode" by putting +12V on the red/black wire and use the grey tach wire to the ECM. i believe on MSD this would be done with tach adapter PN 8910EIS. and with mallory this would be PN 29074.

so, literally, that means on the Bus that it should be as follows:
green trigger wire from distributor on coil- instead connected to trigger wire input to cdi box
white wire also on coil- (this is feeding ecm) should now be fed the voltage trigger/spike mode output of the modified tach output from the box. (not the standard tach wire, which is where people hung up)

thereotically, it should provide a more accurate input to the ECM since it will be simulated/controlled as it's synthetic, and not require so much tolerance on the part of the ECM having to read what's happening at the coil, which could be variable

that is just a hypothesis at this point but it seems logical

i guess now that i installed it in my mind i should give it a shot on the car

[SGKent]

[metahacker]

that makes sense.

i assume that the ECM is just trying to simply derive RPM from the coil negative spikes?

and simultaneously uses the AFM board to provide variable resistance input (e.g. the ohm relative fuel map, so to speak) as the wiper arm is moving around the little laser trimmed tuned resistance paths....

and the product of these two inputs (and whatever else might be in the mix, altitude switch, 79 CA Smog stuff, etc.) to run fuel injector duty cycle?