ICU/Tach wiring after Pertronix conversion - PeachParts Mercedes-Benz Forum

Captain Scott · #1 12-03-2006, 12:17 AM

I'm not sure how to wire the Ignition Control Unit after putting in the Pertronix kit? I know it isn't needed, but without certain connections, the tach does not get its signal.

She runs great now, but without ICU being in the loop, I have no tach.

I have seen posts on folks who have successfully made this conversion, but there has been no mention of having to give up their tach. So I assume they have figured out how to wire it up.

Does anyone here know how?

GaryF · #2 12-04-2006, 05:35 AM

I attached my tach to the -ve side of the coil primary after the conversion, assuming your using and -ve switch coil.

Captain Scott · #3 12-05-2006, 11:54 PM

OK. That did bring the tach back to life and it reacts to speed changes. But is is reading about double what it should and then it also is jumpy - not smooth the way it used to be. Both voltage and current vary with engine speed. Any ideas on what I might have wrong?

Captain Scott · #4 12-10-2006, 09:11 PM

Hey Gary,

Was your coil (-) tied to ground before you made the conversion? If I read my 107 schematics right, the points wire was tied to the tach-sense wire. And the coil (-) went to ground. But I dont think that is how it was rigged before I pulled wires out and started the Pert install. And of course now I wish I had really had documented the pre-install condition... so I cannot prove it.

My tach green wire shows 12v(+) so that seems to be why hooking the Pert black (per their tech support guy) didn't allow her to fire at all. The tach sense wire seems to want to see the breaking of the ground circuit that the points did.

So I am left trying to figure out how to send 12+ to the Pert red wire and also send a breaking ground signal to the Pert black (and therefore to the ICU green wire) to pick up the RPM cycle.

How far off am I???

Quote:

Originally Posted by GaryF

I attached my tach to the -ve side of the coil primary after the conversion, assuming your using and -ve switch coil.

Captain Scott · #5 12-14-2006, 12:05 AM

Quote:

Originally Posted by GaryF

I attached my tach to the -ve side of the coil primary after the conversion, assuming your using and -ve switch coil.

I just realized (by good guidance from this forum) that my ignition system switches the positive side of the coil and not the ground side which seems to be the way this Pertronix is set up for.

Do you have your wiring diagram handy? See mine, the attached PDF and let me know if you have any ideas about what I might be doing wrong here... or a better way to hook it up.

Captain Scott · #6 12-14-2006, 12:06 AM

Oh, here's the attachment....

Quote:

Originally Posted by Captain Scott

I just realized (by good guidance from this forum) that my ignition system switches the positive side of the coil and not the ground side which seems to be the way this Pertronix is set up for.

Do you have your wiring diagram handy? See mine, the attached PDF and let me know if you have any ideas about what I might be doing wrong here... or a better way to hook it up.

Captain Scott · #7 12-14-2006, 12:11 AM

Well, I'm done for the evening. I thought it said it was attached, but don't see it after the post posted. I must be out of it. Good nite...

Quote:

Originally Posted by Captain Scott

Oh, here's the attachment....

GaryF · #8 12-14-2006, 05:22 AM

Scott,

Here's a wiring diagram I marked up before I did the conversion just in case I ever had to reverse out my changes. If you can make it out, the Green/Black whire goes off to the Tacho and this was connected directly to the points. This is normally at 12v and is pulled to ground when the points are closed.
To carry out the conversion I left all of the existing wiring in place but:
1) Removed the coil and replaced with a Pert Flame Thrower, it was compatible and I coulnt see how the pert switching unit would work with the internal wiring of the original blue coil. As you can see in the diagram the coil (60) has its ground side of the primary and secondary windings joined. My theory was that if you tried to switch the ground side of this coil you are also directly making and breaking the secondary coil circuit so that would put the secondary voltage through the pert and give a limited spark, if any.
2) Connected the red pert wire to the red/black wire which was removed from the left hand side of resistor 59 in the diagram and attched both of these to the +ve side of the coil
3) Connected the black pert wire to the -ve side of the coil and to the tach wire green/black.
4) Disconnected the red/purple wire from the right hand side of 59

Then physically removed the points and installed the pert into the distributor, and the old ignition box is left in place but with no power.

Hope this helps,
Gary

Captain Scott · #9 12-19-2006, 02:57 PM

Well, after connecting the wires per the third Pert tech support guy (black to the green tach wire from the ICU and the red to 12v+), it works fine! Tach and all. While I thought I had originally tried it that way, I must have been mistaken. Too bad this isn't the first time it's gone this way...