So I got my 393 stroker back in my truck and running well.....


....other than a jumpy tach that seems to indicate an ignition coil wiring problem.


I have run jumper wires to the coil and it will run excellent with no tach flucuations or irregular readings.

When I plug it in as stock, it seems to have about 11.3ish for voltage at the coil with the ignition on. Battery voltage is about 12.6ish. You run the motor and the tach moves irratically and will sometimes jump a little. It does not work as it should. While it moves, the motor misses with the movement of the tach, indicating the coil is not getting the right power and it's affecting the spark.


So far I have replaced and checked both the pos and neg wires of the coil to the firewall, ign control module, and distribution box. I didn't find anything wrong, and as such replacing the wires didn't fix it. I guess the problem is deeper. Are any of you familiar with the wiring on a 95' like mine in regards to this? I don't have my hands on a good wiring manual (haynes is useless to me) and I need some help getting this figured out. Theres a diode I want to replace, but I guess the 22k ohm resistor is under the dash somewhere and because I didn't find it when looking through the harnesses.

I need to look through the IDM circuit and check that 22k ohm resistor and the wiring associated with it. I also need to put the voltmeter on both of the coil wires and see what I'm getting for voltages. I guess that would be a good place to start....

Any tips on the wiring or maybe you've had this problem before....? Any help would be great, thanks.

 

subscribing

 

Nobody...?

 

I can't believe no one chimed in. I just thought this was interesting, and that I might encounter the same problem at some point.

The only tach problem that I know of is the sticking tach issue that seems to be somewhat common on the late 80s to mid/late 90s Ford Mustang, Ranger, F150, etc.

You are referring to the factory tach right, or do you have an aftermarket tach as well?

 

Factory tach.

I think there might just not be enough people around here that know this specific info I guess.


Thought maybe someone would atleast chime in with a wiring diagram or something.

 

I have the factory wiring guide and the electrical troubleshooting guide for the '89 year.

There are two resistors "near the ignition coil T/O" on the far side of a splice near the coil, before the connector to the in-cab dash harness.

The schematics show a resistor, with no stated value, on the DG/Y wire leading to the tach.

The e troubleshooting guide shows two resistors. The first a 22K, on a DG/Y wire in and out, leading to the EEC computer. The second is a 1.8K resistor. It has a DG/Y input. It has an R/LB output leading to the tach. Both are in the same "box".

Good luck. Two conflicting sets of data from the Ford manuals. I hope this helps.

 

The 89's have a different wiring setup, although related.

I've got a 94' wiring manual I'm trying to follow but it doesn't help a lot because I need the powertrain control/emissions diagnosis manual.

I'm gonna do some more work on it today, according to my manual the 22k ohm resistor is in the main engine harness, so we'll see.