85 TC wiring harness: start diode?

[fastmonkeywheel]

I'm using the wiring harness form an 85 TC and I noticed that in the start circuit, the start wire runs to the TFI module, between the ignition switch and TFI module it splices off and goes to a resister then to a (start) diode then to ground. This part of the circuit must have been in the passenger compartment and I didn't get it with the engine harness. Is it required, I'm not even sure what it does.

So I guess I'm looking for what this part of the circuit does, knowing that I should be able to determine if it's required or not.

If it is required where can I get this resister and diode or what are the values so I can use regular ones and just solder them into the wires.

Thanks

 

[joebford]

When a coil, such as the one in the starter relay, is de-energized it creates a reverse magnetic field called "back electromotive force (EMF). The collapsing magnetic field causes current flow thru the coil windings of the relay in the reverse direction to the original field when the coil is energized. The current is in the form of a voltage spike. This spike can be damaging to other electronic equipment. To suppress the spike, the resistor and diode are added to the circuit to "bleed off" the reverse current.

The diode is not used in the Ranger, or in later tc start circuits. I doubt it would cause a problem if it were eliminated, but if you want insurance, use a common power diode and a resistor of about 10 ohms. Be sure an put the diode in correctly. On the diode there will be a band that encircles the diode at one end. This is the cathode side and goes to power. joe

 

[fastmonkeywheel]

Thanks for the explination. I found the information elsewhere saying something similar. I also learned that newer starter relay's have a diode built in. The relay I have has a sticker on it that says "diode suppressed." However, I used my DVOM and checked the circuit continuitity through the "starter" pole and the ground of the relay. First I placed the positive lead on the "starter" pole and the negative lead on the ground of the relay and got about 3.6 ohm's. Which I expected, however when I reversed the polarity by putting the positive lead on the ground and the negative on the "starter" pole I got the same resistance. I would assume that if it was corrected suppressed I would get no reading this way. Unless the internal coil is both grounded directly to the grounding plate and through the diode. If that's true then their's no way of testing if it's truly suppressed since I can't isolate the part of the circuit that has the diode in it.

I have tons of diodes and resisters but they're the kind I use in my electronic circuits and not really designed for automotive use. What I mean is, the diodes and resisters normally used in automotive applications are surounded in a thick rubber protecting. I know if I soldered the resister and diode to some wires I'd get the same result but I'm worried about the durability of these components. Is there a common method of protecting the leads from excessive bending or pulling like the rubber boxes provide, or is there a supplier of these pieces?

On a side note, since you seem to have the electrical side understood. Do you know if the tach signal form a TFI coil would damage the circuitry of a tach from an older Duraspark II system? The signal should be the same it's the level of current that I'm afraid of, I believe the TFI coils are possibly higher voltage on the output side, I just want to make sure that that wouldn't spike through to my tach, kind of like the starter relay thing.

 

[joebford]

If the relay is diode suppressed, you don't have to worry about adding another one. The reason you can't measure the diode is because it is in parallel with the coil winding. You can only measure the winding resistance in this application.

I will see what I can find out about the tach. I don't know right off hand.

Usually, you don't have to worry about current in devices as they only draw the current they need to operate. It is the applied voltage that is a concern whether directly applied, or pulsed, as is the case with the tach. It counts pulses from the TFI top operate. joe