I'm fighting a rough running 90 460. Using the haynes manual I think I have it narrowed down to stray voltage on the distrubotor harness...I Think. According to the manual, on pin 2 of the main harness on the distributor, states no stray voltage should be present. I'm getting a reading of 2.1 volts. I traced the wires, and it goes to the coil then threw a 2.2k resistor, splits, then one wire goes to the Tach in the instrument cluster, and the other wire goes to the IDM. I disconnected the harness and found that on the tach harness I'm getting .179 volts. not a real big deal, not sure where it would be getting the juice from. On the harness that goes to the IDM, I'm reading 4.4 volts!!! That is where my voltage is coming from. So the big question is why is the IDM sending voltage?? Do you think the computer is bad?? Thanks

 

That's supposed to be a 22K ohm resistor. Red-Red-Orange if you can read resistor color codes.

It does not matter what voltage is on the computer side of IDM. Through that 22K (or 2.2K ohm resistor) there won't be enough current flow to disturb the function of the ignition coil.

The input conditioning logic inside the computer probably pulls IDM up to 4.4 volts through a high impedance. This is normal.


According to your posts you have a 1990 vehicle. This means it should have a distributor mounted TFI module.

Pull out the gray connector from the TFI. Connect everything else. Turn the key on. Measure the voltage at both sides of the coil. It should be battery voltage -- about 12 volts. Measure the "tach" lead on the TFI harness connector. This should be the same as the negative side of the coil (12 volts). If not, try measuring "pin 4". If that one is 12 volts, remove the gray spout jumper. If pin 4 is still 12 volts, then you probably have the connector upside down and have switched pin 4 vs. pin 2.

 

Thanks FE. There is a shrink tubing over the resistor, so I was going off the wire diagram in the Haynes.

I did as you mention, and everything is showing battery voltage.

Here is a little more background. The truck was running, but very poor! Won't hardly idle, backfires, stumbles, and is extremly rich! I got a couple code KOEO codes, but replaced the EGR, and those went away. I won't idle so it's hard to get the KOER test completed. We tried messing with timing, but got it to far out, and now it won start. No matter where we set it. We pulled a plug and looks like weak spark. I hate throwing parts at it, but I'm about ready too. Do you think the coil is bad, or maybe the TFI? ANy help is appreciated, I'm about to pull my hair out! lol Thanks

 

Make sure your battery is fully charged. Jumper cables alone aren't good enough unless you let the helper vehicle charge up the battery for 30 minutes or more.

Check the grounds. Battery posts, frame, engine block, the two ring-shank connectors on the radiator support in front of the battery, the PCM ground at the firewall.

Plug the gray cable back in to the TFI. Measure the "TACH" lead on the coil. It should be either 12 volts or near zero volts. If it is 2.5 volts or 4 volts or whatever, the TFI could well be bad. This assumes you've confirmed the grounds are good. If the tach lead off the coil is 0 volts, bump the engine a little with the starter. It should go to 12 volts.

The coil can be tested by pulling out the TFI connector. Turn the key on. Ground and unground the "TACH" lead off the coil, with an old spark plug plugged into the high-tension lead and the electrode grounded. If you can throw a snappy spark this way, the coil is fine. Or check the primary and secondary resistances with an ohmmeter if you have one.

Check your fuel pressure. Correct PSI and no more than 5 PSI bleed down in 30 seconds.

Use the timing light to get the timing back at least close. Or static time it by bringing the engine to 10BTDC, pop off the distributor cap, and turning the distributor so that the vane of the reluctor is just entering the gap in the Hall effect sensor.

The coils don't usually fail. A TFI problem will usually be a catastrophic "no spark", rather than a weak spark. Same with the pickup coil.

 

Okay, after playing with it some more, we finnally have good spark. Still not sure exactly what we did. We pulled and cleaned all the plugs, and have gotten the timing close like you mentioned above. I'm letting the battery charge as we speak. We will giver her a shot and see what happens.

Oh yes I have check for fuel pressure and the bleed off. Everything is great. Thanks for all your help FE!!