On the way to work last night I got a CEL. My code puller was at the house and of course, I've got a `98 so I can't check it until I get home. This morning I get up and pull the code and get a P0720 - Output Speed Sensor Circuit Malfunction. Truck idles smoothly and makes plenty of power. All gage indications normal. Just the CEL. I know it's not the rear sensor (on the rear diff) because that one throws an ABS light and kills the spedo but doesn't pop a CEL. Any ideas?

 

not positive, but I think it's on the tranny.

 

Thanks for the quick reply.

Just looked at the tranny. Looks like there's 3 connectors. 1 farthest back has 2 wires, farthest forward has about 7 (way up high), one in the middle has 3 (looks like it's near a kick down or something). My money's on the two wire. It's back by the output shaft. Think I'll take that connector off and drive a ways. See if the codes change.

 

That's the one. Popped the two wire off. Tied it up. Cleared the code. Started her up and ran her around the block. Hadn't gone 100 feet when the CEL lit up. Got back and pulled the code ... P0720 ... same as before. Guess I need to order me one of those sensor thingies.

Thanks for your help.

 

how expensive is that sensor ? reckon there is any way to test it ? just for fun. take a good look at the connector. scrape/sand the pins in the connector, spray a little electonics cleaner, mild carb cleaner, or wd40 into the connector. DONT use brake cleaner. might help, might not.

 

Been researching it on the net. Found vehicle speed sensors for dodge trucks at Wholesale Mopar ... no idea which one it is (the one on the rear diff or the one I need ... there's no illustration). It's gonna cost me $31.25 to find out. So I was toying with doing just what you were suggesting. Even considered putting a meter to it. I suppose it's just a coil. Should be able to read through it if it's any good. I've been running around all day with the cable off and tied up with no additional error codes so I'm guessing that's where the problem is. Thanks for the nudge.

 

Problem solved.

Researched part on Advanced Auto to get good part number (they got pictures). Plugged number into WholesaleMopar and ordered part. Came in today (Priority Mail).

CEL with error code P0720 needs part #4800879 (Transmission Speed Sensor - covers many applications). Cost me $15.69 (+6.85 shipping). The one that cost $31.25 is the wrong one ... the OTHER Speed Sensor.

It's a 5 minute job. Very little leakage from the tranny (I parked it ~10 degrees nose down anyway). Takes a 1" deep well socket.

Cleared the code ... ran it ... didn't come back.

Next job ... rear brake cylinder upgrade ... along with a good bleeding.