Checked continuity from MAP ground to 46 pin and its ok. How do i check pin 46 while i got it on the table? Do i need pin 46 to work?

 

Check for continuity between as many power wires as you can, and check continuity between all the other locations of sig rtn. It sounds like there is a break in it somewhere, and it also sounds like there's a short to power somewhere too if you have 7 volts on it. Also, go through all your grounds - one on the fender near the computer, and all other ground straps you can find, as well as the negative battery terminal to make sure everything is tight and has very very low resistance.

 

I was just thinking I cant check for continuity of course there will be, the wire is not broken. What are the chances that the ecc is sending volts threw a ground wire? Since I heard a click when i was doing the self test and touched a wire to the frame. The sound came from that direction.

 

unplug the harness from the computer and test the sig rtn wire for voltage again. Plug it back in, and probe the back of the plug to see if there is voltage present at the computer.

 

I checked for voltage at the Map SIG RTN to ground on battery, key was on. no volts.Is that how to check it? Also did the same with pin 46 SIG RTN to ground on battery no volts. Can I just connect the ECC to the plug under the hood or do I need to put it back in place to back probe it for voltage?

 

Just plugged in the ECC and back probed the sig ret to neg on battery with key on, no volts. I was thinking maybe in one of the plugs to the sensors if there is a pos and neg touching??? rechecked Sig RET on MAP and it has 7 volts. Let me know if i'm testing correctly. Thanks

 

Do you only get the 7 volts when the MAP sensor is plugged in? What is your voltage on the sig rtn (black wire w/ white stripe) at the MAP with the sensor unplugged? That wire should have continuity between all the other places the SIG RTN wire goes, and should show virtually no resistance between all points. I'm still a little confused as to where the 7 volts is showing.

 

Sorry, when the ECC is unplugged I get no volts, to the 7 sensors ground wire. The battery was almost dead thats why I was getting 7 volts. Now with all sensors unplugged, I get 12 volts to all black wire with white stripe, SIG RET's. When I plug in all the 7 sensors I still get 12 voltage to each sensors ground wire. I open the wiring harness and i get volts up to the last splice before the wire goes into the ECC. With the ECC pluged in i back probed the ground and i get no voltage. The way i checked for voltage was ground from meter to battery negitive, and used the poss on the meter to all SIG RET wires. By the way when i unplug all sensors and silenoids i can start the truck sometimes for 5 mins at a time. You should see the black smoke at the exhaust.

 

The "ECC" aka PCM aka EEC-IV provides the connection from sigrtn back to battery ground. So if you do not plug in the EEC, there will be no connection to ground and the wire will "float" up to some voltage as the various sensors supply current into SIGRTN. For it to make it clear up to 12 volts requires that at least one sensor have one side of the sensor tied to 12 volts.

SIGRTN goes to one more place, and that is to the 6 pin self test connector. If you pretend this connector is a "face", with two eyes and a wide smile, the right hand "eye" is the SIGRTN wire. (This is also where you plug in your jumper for forcing the KOEO self test). Measure the voltage between this point and battery ground, with everything plugged in, the key on, but the engine not running, and no self test jumper installed.

 

I get 12 volts at the self test connector at the SIGN return pin . I used my wire diagram and checked everywheres the Black-white wire goes to, except the TPS and knock sensor. The tps doesn't have a black-white wire but i need to open the harness to see for sure, i saw a fuse link that changed it to a black wire. No voltage at black wire when unpluged. when pluged in i get 5 volts between the red and black wire. The manual says the SIG RET goes to the tps too. Wheres the knock sensor, and what does it look like on a straight six?

 

There should never be 12V on the black wire - ever. If the computer is unplugged, the sensors will get no voltage from the computer either. Even if they did, the most voltage you'd see would be 5V. There is a short to power somewhere in the SIG RTN wire if you have 12 volts at all its locations.

I don't think there is a knock sensor on a 300, but I don't know that for sure - check your wiring diagram. Test continuity between the sig rtn and the VPWR wires on the computer (there should be two VPWR at the computer plug). Also check for continuity between the injector pins on the computer plug and sig rtn, and to ground.

 

The closest diagram I have is for a 1988 F-series 4.9L six. It shows that the sensors that tie to SIGRTN have all of their other wires going back to the ECM. So if Mike L removed the 60 pin connector then SIGRTN should have been left without voltage at all. There is also no obvious source of +12 volts as the sensors all run off VREF.
So it appears that my prior post about "floating to +12" was off base and that EPNCSU2006 is correct.

One long shot -- the self test connector gets +12 volts through the check engine light. Sigrtn is on the same connector. So if these two wires are shorted, you will see +12 volts on sigrtn without the PCM plugged in. Once you plug in the PCM, the ground connection in the PCM should pull sigrtn down to zero volts (the MIL light doesn't source enough current to fry the PCM).

Mike L's posts are inconsistent. He says he back-probes SIGRTN with the PCM connected and gets 0 volts. He says there is connectivity between pin 46 and MAP sigrtn. Then he says MAP sigrtn is at 12 volts. How can this be? If there is continuity between the two points they must be at the samve voltage.

It isn't clear whether his latest measurement (+12 volts at SIGRTN at the diagnostic connector) was made with the PCM connected or the PCM disconnected.

I guess I'd ask that Mike L double check a couple of measurements. Are you SURE you are connected to the negative battery post and not the positive post? What is the voltage between sigrtn and (say) the case of the distributor? Disconnect the computer from the 60 pin connector and use an ohmeter to measure from sigrtn on the self-test connector to pin 46 on the 60 pin connector. This should be ~0 ohms. I am a little suspicious of a statement that there is "continuity" as this could be anything from the correct 0 ohms to a 1000 ohm false leakage path depending on how the measurement was taken.

 

My wire diagram 87 to 89 4.9 f series says i have a knock sensor connected to pin 23, but i could not find it. I'll follow it from the pin. VPWR are pin 37 and 57, will check.

 

Your right i knew there was voltage there but didn't know how far to put the paper clip into the EEC, pin 46. Battery is almost dead ,i get 6 volts at pin 46 to battery ground. Also got 6 volts from pin 46 to distributor base. Got 1 volt at map ground to battery ground. Do i need a new EEC? Why cant i move the wire from pin 46 to the frame? Did i damage the EEC when doing a self test for codes, with a volt meter?I had to jump the SIG RET????

 

There is a short to power somewhere in the SIG RTN wire. That is the only reason you are getting voltage on that wire. Also check continuity between STO and SIG RTN to see if you're getting 12V from the check engine light like fefarms said. I really think the problem is in the wiring, and not in the computer.