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Let’s say you wanted to swap from EFI to carb, and you’re gonna scrap the trusty TFI ignition system for an HEI distributor. Your personal feelings on the subject aside, how would you wire up the tach to keep the factory tachometer in place and functional?
there seems to be a small discrepancy between the wiring diagrams and the troubleshooting chart in two Ford Books I own. The troubleshooting chart says I should have 1ohm of resistance between coil negative and the instrument cluster connector pin 6 (tach signal from coil negative)
but in the wiring diagrams there are two resistors (for 5.0 and 5.8) 1.8k ohm for the tach and 22k ohm for the computer (not the TFI module)
The 4.9 has a different wiring diagram which would explain why the troubleshooting chart would say you should see 1ohm in this circuit and not 1800ohms.
pictures below.
I haven’t done any testing yet and frankly don’t want to, but eventually I’ll have to. Can anyone set the story straight on this? How would you wire up the original Ford tach with an HEI distributor on a 5.8L
It would not surprise me if the 1990 V8 diagram still shows that buffer resistor in the tachometer circuit versus the 4.9L trucks without. I suspect the V8 diagram is wrong. The test procedure clearly states the resistance between C250 Pin 6 and the coil negative should be less than 1 ohms. I would retain the existing wiring for the tachometer as-is.
I will not ask why you want to downgrade to a carb./HEI ignition.
Easiest job ever
Just hook the tach lead that comes out of the HEI cap to your existing tach wire at your existing coil
The PIP signal coming from circuit 11 is your wire
Good on 'ya for having the EVTM for your truck
Ford books are not always "spot on" correct
That dropping resistor and the tach have nothing to do with one another
In other words, the system voltage has nothing to do with tach operation, and the pip signal from the stator will still be there at 9 volts
Note the "REVISED" print across the top of the 1990 service manual
I have TSB references in some of my EVTMs
Easiest job ever
Just hook the tach lead that comes out of the HEI cap to your existing tach wire at your existing coil
The PIP signal coming from circuit 11 is your wire
Good on 'ya for having the EVTM for your truck
Ford books are not always "spot on" correct
That dropping resistor and the tach have nothing to do with one another
In other words, the system voltage has nothing to do with tach operation, and the pip signal from the stator will still be there at 9 volts
Note the "REVISED" print across the top of the 1990 service manual
I have TSB references in some of my EVTMs
Interesting that they would put that resistor there. I’ve seen HEI swaps on the internet, everyone seems to run a custom Tach mounted to their dash cause they can’t figure out how to wire the original tach. I just wanted to be sure it was a don’t know how to wire it kinda deal and not that the ford tach was somehow incompatible with HEI. When I do this swap I’ll just tap into the factory harness at the ignition coil negative, and disregard the resistor.
