Hello,

I am new to the forum and in desperate need of some help. I have an early 99 Ford F350 with the 7.3 and 4r100. The code I am pulling is saying OSS failure - open circuit. My symptoms are the same as most members on here with an OSS code. OD light starts blinking, truck shift points are incorrect, truck goes into safety mode with associated shift defaults and drives sluggish. This problem has been present for 2 years. This truck is not my daily driver, but it is about to be and now I need to seriously try and fix it. My main question is "IS THERE A SPECIFIC PROCESS (step by step) AND ASSOCIATED ELECTRICAL VALUES FOR TESTING THE OSS SYSTEM?" ......and I am by no means super knowledgeable on ELECTRICAL issues.

What I have done so far:
1. Checked and cleaned all ground connections on truck chassis, under dash, and in the engine bay.
2. Ensured both batteries have a good ground and the terminals were clean.
3. Pulled and tested the OSS sensor on the rear tail shaft of the tranny. Waved pliers in front of the sensor, with my meter attached and ensured the magnetic properties of the sensor were working properly. Ohmed the sensor out and it was within range at 1.2. Concluded sensor is good.
4. Ran truck with alternator disconnected hoping to eliminate any possible interference it could be producing. No change noticed and OD light continues to blink and no improvements on the tranny.
5. Checked resistance on both wires from sensor pigtail (pigtail disconnected from sensor) to PCM fuse block with the harness disconnected from the PCM. The resistance on the signal and the ground wire was .3-.5 ohms which tells me there is no break in them. When I check the resistance between the signal wire and ground wire I get infinite. Which to me makes sense (they are two different wires).........Now with my lack of electrical knowledge here is where I become extremely confused. When I connect the harness back to the PCM and retest, both wires show .3-.5 ohms again. BUT WHEN I CHECK RESISTANCE FROM SIGNAL TO GROUND (Pigtail is still disconnected from the sensor) IT SHOWS ME 4.6 ohms. Why would I not continue to get "INFINITE"? They are different wires and the circuit isn't closed because the pigtail is not connected to the sensor. What is this telling me ?? Do I have a bad PCM? How do you test the PCM?

I am utterly lost and frustrated and I have googled and read about OSS problems with no help to my situation. Not knowing the correct sequence / process to test the system and what electrical values I should be getting at each test step.

I KNOW THIS IS SUPER LONG WINDED ..... BUT ANY HELP WOULD BE GREATLY APPRECIATED!

Thanks,
Zack

 

Not a bad thing to do, but won't have any effect on this problem.

Not a bad thing to do, but won't have any effect on this problem.

Probably a good assumption.

Not at all related. For other OSS codes that would have been a valid concern, but P0720 says the electrical circuit is bad.

I don't know what a PCM fuse block is. I do know the OSS wiring does not go to a fuse block, so I'm unsure where you were testing.

There are two wires to the OSS. One is the signal wire and the other is called signal return. It is a common ground circuit for sensors. Each of these wires needs to be check for continuity from the PCM connector (is that what you mean by PCM fuse block?) to the sensor connector. Each wire also needs to be checked for continuity to harness power and to harness ground. Checking with the PCM connected is meaningless.

 

Mark,

You are correct. When I said "PCM fuse block", I meant PCM Connector. I did not know the proper terminology. Basically, where the harness connects to PCM, above the drivers side fender well, in the engine bay. The 2 wires I traced and checked came off of the sensor on the tail shaft of the transmission, ran across to the driver's side frame rail, down the frame rail, and terminated at the PCM connector. Are these the correct wires to test? Am I correct in assuming the wires are good with .5ohms resistance on both? I believe what I did was check the continuity of the "signal wire" and the "signal return wire" from the PCM connector to the sensor connector.

What I haven't done and I need to do is "check continuity to harness power and harness ground." How is this done?

Thanks Again for your help!

 

You're checking for an accidental short to ground or vehicle power for the signal line.


"Signal return" should show continuity to to chassis ground anyway if everything is connected.

What is the specific DTC?

 

The specific DTC is P0720. Thanks for the help.

 

With both ends of the harness unplugged, check continuity from the signal wire to the power feed wire in the harness. If I had a wiring diagram for your truck I could tell you which wire that is, but I don't have one. Repeat with the signal return wire to power.

Now repeat the above paragraph, but substitute "ground wire" for "power feed wire."

And you do know to NEVER put a probe into the end of the harness that plugs into the next connector, right? Most probes are large enough that they will spread open the female pins so that they won't make good contact to their mating pins. ALWAYS probe the back side of the connector.

 

So my next step is to try and locate a wiring diagram for my truck. This should be fun.

I was probing from the back....I guess dumb luck. Thanks for the tip though.

 

[QUOTE=Mark Kovalsky;15188478]With both ends of the harness unplugged, check continuity from the signal wire to the power feed wire in the harness. If I had a wiring diagram for your truck I could tell you which wire that is, but I don't have one. Repeat with the signal return wire to power.

Now repeat the above paragraph, but substitute "ground wire" for "power feed wire."


Is the power feed wire and the ground feed wire that I am looking for the actual power and ground to PCM?

 

Yes it is.

 

Also, am I correct in assuming if the continuity check is good on the power feed wire and ground wire that my PCM is shot?

 

It is very rare for a PCM to fail. I've seen it happen, but not often. 99% of the time it's the wiring or the sensor.

 

No.

Insufficient evidence.

Maybe this video will help to troubleshoot the circuit: Note: This is NOT your circuit, but is an example for troubleshooting.

 

So, is there a logical test sequence I can do to eliminate the pcm as the problem?

 

Yes there is. If the wiring checks out in all regards, you've replaced the OSS itself, and you have inspected the tone ring inside the trans, then the only thing left is the PCM. And that's a rare failure, but it can happen.

 

Mark and David thanks for all of your help!