7MGTE swap Tach modfication

[johnb7]

Some of you are lucky and have had no issues with your MSD-8620 to convert your tach signal. For the rest of us who have had multiple failures, fires, and experienced obnoxiously loud ticking, here are the instructions to do away with the box full of witchcraft.

A couple of notes before we get started. I have found this to be off slightly at idle. It can also be slightly sluggish in my high horsepower 7MGTE in the low gears. I think these are just side affects of using the tach for a motor that it was never designed to handle. The stock tach is also not known as a precision device to begin with, but mine is dead on when not in the situations described above. This has never bothered me much, and only requires me to remember to shift slightly early in the low gears.

So first you must remove the gauge cluster. I am not going to detail this. Please refer to the TSRM if you do not know how to remove it.

Now lay it on a table and give yourself some room to work. Remove the 2 knobs for the clock and trip odometer. One just screws off and the other requires a small screwdriver. Also remove the two screws show in the picture.



Now you can pop off the clear plastic cover. There are several black clips that need to be released to do so. I suggest getting the bottom ones first and then the top ones should just slide out.

Now flip it over and unscrew the following screws:



The black display piece should literall fall out of the white plastic backing as you undo the last couple screws. Now focus on the back of this, specifically the tach electronics. You will notice the green circuit board on the back with two metal prongs holding it in place. Take a screwdriver and pry the prongs out of the plastic holder until it is free.



Now get your soldering skills and supplies. Remove the 30K resistor (orange black orange) pointed to by the clip that is holding the circuit board.



For the 7MGTE, we need to replace this with a 15K resistor (brown green orange).
SIDE NOTE: for a 5V square wave, you would just bridge this connection. I have tested this with a MegaSquirt 5V square wave, but not a JZ motor (which is that). It "should" work.

Here is mine after soldering in the 15K resistor.



Now just assemble in the reverse order. Make sure you do not get and dust/fingerprints inside your cluster!

Just connect the wires that went to the white and purple wires on your MSD 8620 together. Or connect the black wire from the diagnostic port on your 7MGTE harness to the black wire with the single pin connector that used to go the the 5M ignitor (both should be in front of the drivers shock tower).

Hope this helps. Please post here if you have success doing other things using this information.


Sources for resistors:
15K is a common resistor, so anywhere that has resistors should carry it. Start with your local Radio Shack. If you have no luck, order it online from Frys. Shipping is only $2. http://www.frys.com/product/1005243?...H:MAIN_RSLT_PG

-John

[flygfsh1]

John,

Thanks a lot for posting this mod, as I am one that is quickly approaching double digit replaced MSD boxes. Only question I have is the
15k resistor easily available at radio shack? Thanks again, Dave

[Suprasaurus]

Thanks John! I'm sure this will be getting done this winter!

[Donn29]

So what is the difference on the tach signals? 12v non square vs 5v non square?

[johnb7]

So what is the difference on the tach signals? 12v non square vs 5v non square?

5M/7MGE = direct coil voltage
7MGTE = 12V wave
Most JZ and standalones = 5V wave (but may require pull-up or pull-down)

Most of these are square waves but some are more curvy. Should not matter as long as it hits a min and max consistently.

I updated the post to include sources for resistors.

-John

[BillyM]

Free beer for JohnB7!

--billyM

[Suprasaurus]

Free beer for JohnB7!

--billyM

[Donn29]

5M/7MGE = direct coil voltage
7MGTE = 12V wave
Most JZ and standalones = 5V wave (but may require pull-up or pull-down)

Most of these are square waves but some are more curvy. Should not matter as long as it hits a min and max consistently.

I updated the post to include sources for resistors.

-John

Nice, Thanks! Does Direct coil Voltage mean the Secondary voltage? (kV?)

[johnb7]

Nice, Thanks! Does Direct coil Voltage mean the Secondary voltage? (kV?)

I think it is hooked to the negative on the coil. So it gets a nasty voltage spike every time it fires.

-John

[flygfsh1]

Went to local Radio Shack here in Indy last nigh...no luck on the resistor John mentioned. Will try another back home this weekend, if not we all may just need to order them online.