I have a 1996 Dodge Ram 1500 SLT with an automatic with the 318 CI engine. The truck now has about 128k miles on it. I don't tow with it. The previous owner had the transmission rebuilt at 105k miles. The truck, when it gets cold outside (40degF or below, I'd say) and during the first few minutes of driving from a cold start, will sometimes not upshift during normal acceleration. It will also sometimes act like it's hunting for the right gear. Like I said it doesn't do it sometimes, sometimes it does, but when it's well warmed up, it always shifts perfectly.

I had a shop look at it a year and half ago and one of the things they did was supposedly replace the vehicle speed sensor, but the problem didn't go away. I say supposedly cuz the shop owner told me later that he had 2 mechanics that he had to fire (this was in response to some bad water pump work they had done on my '02 Merc Mountaineer) Any way, this past Spring, I found a kink in the throttle valve cable and so I replaced it and thought I fixed it but when it turned really cold recently it started to do it again but seemingly to a lesser degree.

Recently, I got a code reader, and it's showing a P0720 output speed sensor circuit code, but the check engine light hasn't been on. I reset the code a couple of months ago, but it came back. I started looking at speed sensors on Rockauto.com but it shows 2 different sensors available for my truck. I do know that my truck has the one that bolts to a speedometer adapter which mates to the pinion inside the trannie.

Any ideas would be appreciated. Thanks.

 

OK, according to my Haynes Manual, I just did a check of the voltage on the wires going to the vehicle speed sensor at the tail end of the transmission. It said that I should get a "reference voltage" of about 8 volts on the solid orange wire. However, my voltmeter read about 3 volts shy of that at 5.13 volts. Then, on the white/orange striped wire, the manual said I should get a "signal voltage" of about 5 volts. My voltmeter for that one also read as 5.13 volts. I did these checks with the ignition switch on but the engine not running.

Does this indicate a problem? Haynes manual just says that if there is no reference or signal voltage available, have the PCM diagnosed by a dealership service dept. Then it says if the voltages are available, then remove the speedometer adapter and check the speedometer pinion for damaged teeth.

 

Ok, according to theory, there are two sensors on the trans. The Output speed sensor, which your truck is whining about, and the vehicle speed sensor, which is on the output shaft of the trans (2wd) or t-case(4wd.) If your speedo is reading reliably, the VSS isn't the problem.

I *think* the output sensor is on the drivers side of the trans, in the trans itself, before the overdrive section. I think.

 

Thanks for the response. The speedometer seems to be behaving fine and is fairly accurate. I've checked it in the past with my GPS. It's about 2 or 3 mph off around 35 to 40 mph but becomes more accurate at 50 mph and faster. Also, the other day my code reader which is actually a blue tooth adapter which I use in conjunction with my android tablet indicated 43 mph when my speedo said 40 mph.

Well, I was hoping not to drop the pan again. I changed the transmission fluid back in the spring. Do you think I can replace that sensor (if the sensor's the problem) by only dropping the pan?

 

Should just screw into the side of the trans. Don't have to drop the pan at all.

Of course, you will have to look, and see if there actually ARE two sensors there. I would go look at mine, but, it is friggin' COLD out there at the moment.

 

OK, you're right. I thought you said it was inside the transmission. I had already gone to check cuz I figured if it was a sensor it had to have some wires going to it. It has a green/black wire going to it and a black/blue wire going to it. I'll go get one tomorrow and replace it and see if that fixes it, unless there's a way to troubleshoot it with my multimeter.

 

That would be the one. Might wanna pull it, and see if you can just clean it up. Not sure if that is a hall-effect sensor, or gear driven though.

 

I just changed the OSS out today. It is a hall effect sensor. I didn't want any doubt...this has been a sporadic problem, really grating on my nerves. I'm still not sure why it would only occur mainly in the first few minutes of driving during cooler weather. There were a few times where it would occur after the truck had reached operating temp, been shut off for a few minutes and then cranked up again. The sensor I took out did have a little bit of sludge on it but wasn't completely covered, so maybe that explains it.

HeyYou, I am really grateful for your help. I didn't know there was another speed sensor in the trannie, and I had already purchased the VSS and was about to open the package to change it out when I saw your response. You saved me a good bit of time and more headache and about $30. Thanks

 

Now lets just see if it solves the problem.

Electrical sensors have a variety of failure modes.... some can be quite fun to diagnose.... You just had one of them.

Good Luck, hope that solves it.

 

HeyYou,
I could have sworn I posted in this thread a big thanks for your help on this one and to follow-up so that I might help anyone else who might experience the same problem.

Anyway, thanks and my P0720 problem was solved thanks to you. But, now I'm about to create another post for a new problem that just started on Friday.