Hey all.
98 ram 1500 with the 5.2 and auto.

Been having a few issues lately. First issue I had a few weeks ago was the code for Injector number 3 circuit malfunction. Tried cleaning all the contacts, checked the wires etc. finally just bought a new injector and replaced it. Problem solved. Ran like a champ.

When I got back from the test drive after replacing the injector the check engine light came on again. Ran the code and it came up as P0720. Now I have had this code before about a year or two ago. And I am seeing the same symptoms, at least I was at first. Speedo reading rather erratically, let off the gas and the speedo spikes up to 120 mph, needle basically bounces around. Last time I had this issue I replaced both speed sensors (kinda by mistake) didn't realize there were two at first. As I recall I replaced the transmission sensor first. It didn't fix it then I discovered the existence of the rear axle sensor. Replaced that and problem solved.

So today I was out running errands with plans to stop for a new sensor on my way home as it was only doing the odd Speedo bounce thing. Well on my way home it got way worse. Sitting at a stop light I watched the Speedo needle doing a full sweep back and forth like a windshield wiper. Then going to take off from a stop it bogged way down like it was in second or third, shifted rough, wouldn't up shift until about 4500 RPMs, etc. even stalled on me a few times taking off from a dead stop.

So I stopped at Oriely, grab a new sensor for the rear axle first. Replace it in the lot. Clear the code, go to pull out and the speedo goes nuts again and MIL comes on. Pull right back into the spot, go back in for the transmission sensor. Replace that in the lot. This time I disconnected the computer for a few minutes. Hop back in, go to pull out. Same thing. speedo goes ballistic and MIL comes on. At this point I don't know what else to replace so I just limped it home. It stalled three times taking off from a stop, bogged way down, etc the whole way home.

So far since getting home I have cleaned every electrical connector from both sensors all the way up to the PCM. Disconnected the PCM for over thirty minutes to reset it (I did this by unplugging the three connectors on the front, not disconnecting the battery terminals) and still it's not running right. The MIL hasn't come back on but the Speedo needle is still going crazy and I can feel it just not running right.

Does anyone have any input? I'm at a loss here.

 

P0720 Low Output SPEED Sens RPM, Above 15 MPH

Ok, that makes a bit more sense. (although, I could have just edited the title.....)

Check what the input voltage to the VSS is. Low input voltage gives low output voltage.

Check connections at PCM as well. They like to corrode.

 

I know the connectors are all good. I cleaned every connector from the axle sensor up to the PCM with electrical contact cleaner.

I just tried to reset the PCM again this time following the below procedure I found in a previous thread:


Remove the negative terminal of your battery (10mm wrench).
Turn your ignition key to START and hold it there for about 15 seconds.
Turn your ignition key to OFF and remove it.
Connect the negative terminal of your battery.
Start but do not hit the throttle. Idle for about 10/15 seconds then turn off.
Restart the truck and drive as usual. It will take at least 12 starts and 48 miles to fully retune the computer. Hard Codes leave a shaddow for 40 full cold starts.


Went around the block and the speedometer is still acting erratic. However it feels like its driving correctly but it's hard to tell from a short drive.

I will go check the voltage going into the sensor and post back the results in a few

 

Ok. So with the key in the run position, but the engine off and truck in park I got the following

2.6volts going in to the sensor on the transmission
4.8 volts on the rear axle sensor

 

2.6volts going in to the sensor on the transmission

There is at least part of the problem.

Diagram shows only two wires for the trans output speed sensor.

Lite Green with black trace from PCM connector 2, pin 28. Should be 5 volts there.
Dark Blue with black trace goes back to PCM connector 2, pin 25.

Check the lite green wire for high resistance, bad connection, etc. If that all checks out, could be bad PCM.

Although, I thought on the 98 and up trucks, speedometer was driven from the rear wheel speed sensor.....

 

That was my understanding as well. Last time I had this issue I replaced the trans sensor first and still had the erratic Speedometer. Then I replaced the rear axle sensor and that solved it. This time it did not.

And I showed 2.6 volts on both wires going into the transmission sensor. Grounded the meter to the frame and checked both wires. I will re test and check at the PCM as well

 

Ok, so made the huge mistake of disassembling the connector at the PCM. Everything looked good in there though. Put it all back together.

Checked voltage again at the sensor
2.6 volts on green with black trace
2.6 volts on blue with black trace

Checked the wires coming off the PCM
2.6 on both right out of the PCM

 

Unplug the connector, and test again.

 

Unplug which connector and test where? The pin itself out of the PCM?

 

Ok so I checked the voltage directly at the pins on the PCM with the same readings. Is there anything else that could be causing this or is my PCM going bad?