If your voltmeter has an "ohms" function (it probably does), measure the resistance from SIGRTN on the self-test connector to ground on the battery. Do this with the battery positive post disconnected and the computer connected. This will establish whether the computer is still providing a ground path for SIGRTN. Also touch the two probes of the voltmeter/ohmeter together on the same ohms setting and report the resistance you measure (as a check on the meter itself).

It is possible that Ford put a resistor in series with SIGRTN, in order to prevent the computer from being fried by a short between VPWR and SIGRTN. Or it is possible that there is just a printed circuit trace, in which case your computer may be dead with the SIGRTN connection burned off the board.

With a short between 12 volts and a ground, or pseudo-ground, like SIGRTN, you have the classic "irresistable force" and "immovable object" situation.

 

There's no resistor in the SIG RTN. I do agree that the circuit could have been shorted out inside the computer, and to check resistance between SIG RTN and battery ground.

 

I'm new to reading ohms here goes i set my meter to X1o and touch the two probes to together and got 0 ohms which is infinity, or a complete loop. I removed the possitive off the battery and tested the sig ret on the self tester to battery ground and got no reading. I did move over the the Fuel Pump lead on the self tester connector and got 4 ohms. Called a ford technition add said he thinks theres a dieode that would keep my ecc from burning out ???

 

There is, indeed, no resistor in the HARNESS for SIGRTN. But there may be a resistor, fuse, or diode on the circuit board INSIDE THE COMPUTER which limits the current that can flow into pin 46. This cannot be too high an ohmic value, or the sensor readings will interact with each other in bad ways. But a 10 ohm resistor would be workable and "not a bad idea" for robustness in the face of short circuits, particularly for a signal that goes to so many places.

The alternative is a simple PC-board trace on the computer board tieing pin 46 to ground. In this case, your computer protects the wiring harness by frying first (somewhat tongue in cheek here).

Holding the leads of an ohmeter apart should give a reading of "infinity" or open circuit. Touching the leads of an ohmeter together should give a reading of "about" zero ohms, or a short circuit.

If you measured from SIGRTN on the self test connector to ground on the battery, and the ECM was connected, and the battery ground was connected, you should have gotten some reading less than infinity, indicating that SIGRTN is connected to ground through the ECM. Try connecting the ohmeter with the red to sigrtn and black to battery ground. Then try switching the ohmeter probes with black to sigrtn and red to battery ground. Also, state whether any of the sensors are connected to SIGRTN at the time you make this measurement.

In the meantime, I will check the resistance on my own truck.

 

Ford technition said he thinkds there a diode that protest the ecc from being damage while doing a seft test?? I told him i was using a volt meter and touched the possitve lead to the fender and could not get anymore readings. All sensors are connected, ecc is in. Neg probe on meter went to battery and poss probe went to sign ret (blk-white wire) on connector. I did get a reading when i moved the red probe to the fp lead wire on the same connector, so my negitive probe to the battery ground was good??? I pulled out the SIG RET wire out of the harness, I can see my SIG RET wire from the seft tester pin to the ecc which runs about 4 feet. It comes out of pin 46 then goes up to the valve cover and splices ( factory splice, its solidack to the self tester connector. Where the splice is at the valve cover , it continues on to the EGR, TPS, then to the MAP, and ECT sensor. Cant find the Knock and the ACT sensor as shown on my wire diagram, for only 4.9, 1987 to 1989 f series/ Bronco ??? I have a 1987 F150 4.9 straight six

 

Fefarms, I see what you are saying now, and I would agree with your speculation. I have no idea what goes on inside the computer.

Mike, I wouldn't worry about the splices in the sigrtn - the problem is not in the splice. Somehow you are getting a short to power into the sig rtn wire, and you need to find that - which wire the power is coming from, i.e. pin 37 and 57 for VPWR, which is 12V. That is likely why you can't get into self test mode - there is power on the sigrtn wire, so it's not grounding out the STI wire like it should.

 

Found the knock sensor its by the distributor cap.

 

You have to be careful to sanity check any ohm reading of "infinite" and any volt reading of "zero". Such readings can be true readings, or indicative of not making contact to what you are supposed to be measuring.

When you "touched the positive lead to the fender", was that to bare metal or to a painted surface.

A convenient place to pick up ground on my truck is near the left hood hinge. Two wires come up from the PCM connector -- one black ground wire and one bare stranded wire. These are crimped into a STA-KON lug connector that is bolted to the hood hinge support. This is a very convenient "PCM ground".

The first sanity check you should do is measure the resistance from this point to the battery ground post. Should be about 1 ohm or less. Now measure the resistance from this point to various metal parts (alternator case, distributor body, master cylinder). Should be a nice low resistance. Not infinite or "no reading". Having done all this, you can have confidence in having a good ground reference.

Now measure from SIGRTN to this point. On my truck, with everything connected and the ignition off, I get 0.6 ohms from SIGRTN on the self-test connector to the above mentioned convenient ground reference. Going back to the battery post it is a little more -- maybe 1.5 ohms. Reversing the probes makes little difference in the readings.

When you want to run a self test, you jumper STI to SIGRTN. You do not need a diode to protect the computer to run a self test as the current out the STI pin is limited. If you inadvertantly jumper a non-current limited +12 volts to SIGRTN you will probably fry the internal connection on the computer. Same thing if the harness shorts +12 volts to SIGRTN, which may have been what happened to you, assuming your ohms readings are accurate.

The FP lead on the self-test connector on my truck measures about 640 ohms to ground with the key off. This is a reasonable value for the resistance of the relay coil.

 

"When you "touched the positive lead to the fender", was that to bare metal or to a painted surface." I did the self test just like the picture in the manual. I had a long wire from the positive on the battery to my positive on my probe. I dropped the positive probe on the bare spot of metal on the fender and heard a load click from the area of the ECC and then I could not longer get any readings. Volt meter is ok. I was hoping there was a fuse or relay that went bad, but I don't know.

 

Do you still need voltage reading from ground on battery to pin 37, then to pin 57 then to to sto on seft tester and then to sig ret on tester also?

 

[QUOTE=fefarms]
A convenient place to pick up ground on my truck is near the left hood hinge. Two wires come up from the PCM connector -- one black ground wire and one bare stranded wire. These are crimped into a STA-KON lug connector that is bolted to the hood hinge support. This is a very convenient "PCM ground".

The first sanity check you should do is measure the resistance from this point to the battery ground post. Should be about 1 ohm or less. Now measure the resistance from this point to various metal parts (alternator case, distributor body, master cylinder). Should be a nice low resistance. Not infinite or "no reading". Having done all this, you can have confidence in having a good ground reference.
QUOTE] Ok what do you call it when you set your volt meter to x1 ohms and touch the two probes together and the needle pegs to the end where it says zero?
I removed the positive from the battery, and check for ground. Meter ground to battery ground and poss on meter to the hood hinge and got o ohms . Then went from the hood hinge to the distributor case. Then from hinge to the alt case . then from hinge to intake manifold all have the same reading as when I touch my two probes together on my analog meter, set to X10 ohms.

 

When I run a self test, I jumper STI to SIGRTN and watch the MIL light in the cab of the truck. The positive post on the battery never gets involved.

Some of the manuals show a way to use a voltmeter or test light instead of the MIL light. In this case, you connect STO through the meter to the positive battery post, or some other handy source of +12.

Any of these points (STI, SIGRTN, STO) can be shorted to ground with no permanent ill effects. +12volts shorted to ground creates the obvious sparks but shouldn't do anything else.

If, however, you ever inadvertantly short the positive battery post to SIGRTN you will probably fry the PCM. If you short +12volts to STO, you may fry the PCM depending on whether the designers installed a current limiting resistor.

Your last set of readings suggest that you may have shorted the battery positive post to SIGRTN and burned out the circuit trace that is supposed to go to ground inside the PCM. I'm not sure I trust your recounting of the readings after all these posts through the internet -- sometimes things get confused in the translation.

 

The needle swinging all the way to the right indicates zero ohms. This is a good thing and you have established "sanity".

Now take a measurement between SIGRTN on the self test connector and the ground near the hood hinge. This should also swing all the way to the right. If it doesn't, your PCM is probably damaged. If it does, try "X1 ohms", and make sure it still swings all the way to the right.

 

I get 20 ohms from hood hindge and 50 ohms from intake manafold and 50 ohms from battery ground to sig ret on seft test plug. Volt meter set to X10, sorry i dont have a X1 setting. Sorry for giving you guys incorrect readings, i think my battery went dead a few times before i checked for voltage.

 

Mike, pin 37 and 57 (VREF) should show 12V (negative on voltmeter to battery/chassis ground) when the key is on and the PCM relay is energized.

FeFarm, for some reason '87 trucks didn't have the check engine light hooked up in the dash, so that's why the analog voltmeter was used.