Speedometer erratic and OD intermittently quits

[GPulliam]

My 4WD 92 Dodge (219K miles) has had two problems pop up simultaneously, so I think they are related. The speedometer has gotten increasingly erratic, especially at speeds over 40mph where it flutters between 40 and 90 mph regardless of my actual speed. The overdrive also started to lag at speeds of 40-55mph (it would take longer than usual to spin up and shift into overdrive unless I kept accelerating to 60 or faster. Over about 65mph it seems to stay in overdrive with no problem. Both problems have gone from intermittent and mild to very noticable and fairly constant. The speed sensing unit on the transmission was replaced in May and when it failed the speedometer failed to register any speed at all, therefore the OD didn't ever engage. Given that it flutters now, it seems like the speed sensing unit isn't the most likely culprit, but I'm not sure what else could be causing this. I suspect that the overdrive problem is being caused by the speedometer not reading correctly. Any ideas on where to start troubleshooting?

 

[PumpChatter]

Try checking the voltage from the throttle position sensor that sits on top of the fuel injection pump as this directly relates to auto. trans. shifts. With a volt meter and a sharpened paper cip, turn ignition on ,insert the paper down the center wire on the rear of the contector till it makes contact. The voltage shoulld be 1 volt. if not, take a 10mm wrench I believe to the underside of the sensor and turn it slightly in either direction till 1 volt is obtained. Now slowly increase the throttle and the voltage should rise up to approximately three volts at wide open throttle. Also the voltage shouldn't bounce or drop out which would indicate a bad sensor. This cured my erratic speedo and dropping out of overdrive. Definetley not fun being stuck a 65 with a semi on your but.

 

[dpuckett]

Check the connections from the speed sensor to the pigtail. If they arent dead on round, it wont give you a consistent signal. Though it does sound like the speed sensor. If it was erratic, but dropping now and then, I'd look into mechanical problems- speedo drive gear in transmission, driven gear coming out of it, and the adaptor if you have one. But since it bounces up to 90 and beyond, I'd suspect the speed sensor. The VSS is only slightly better quality than the TPS. I've had to put a second one in mine, last one was in March04.



Daniel

 

[dpuckett]

Pump Chatter-

That is weird, how adjusting the TPS fixed your speedo. I'd think the 2 were toatlly independent of one another.



-DP

 

[CGauthier]

I'm with Daniel on the speed sensor. The speed sensor is a bunch of reed switches that just contact on and off and they can stick giving you erratic behaviour. When I think of it I'm pretty surprised they would have a system set up like that with mechanical switches.

 

[cerberusiam]

Don't the 4x4's have a gear reduction box the VSS hooks to? I seem to remeber a discussion about the gears in it going bad and causing fluctuations in the speedo and other problems. I know the older ones had the box and they were problematic when they got worn.

 

[PumpChatter]

I had this same problem about 2 yeras ago, I did take the pig tail off the speed sensor and cleaned it and put a little dieltric grease on it. I checked the wires going up to the speedometer and everything seemed Ok. Unfortunatley the problem didn't go away, so before I bought a speed sensor I figured I would check the TPS voltage. It was about . 06 volts at idle and 2. 7 wide open. Adjusted the TPS to specs and the problem dissappeared and hasn't returned. It could be a coincedence I'm not sure, but the speedometer has worked flawlessly without dropping to zero and the trans has stayed in overdrive when it should.

Why the speedometer would drop to zero under a low voltage condition beats the heck out of me. But its something that doesn't cost to much to check.

 

[smokin-bear]

My 4WD 92 Dodge (219K miles) has had two problems pop up simultaneously, so I think they are related. The speedometer has gotten increasingly erratic, especially at speeds over 40mph where it flutters between 40 and 90 mph regardless of my actual speed. The overdrive also started to lag at speeds of 40-55mph (it would take longer than usual to spin up and shift into overdrive unless I kept accelerating to 60 or faster. Over about 65mph it seems to stay in overdrive with no problem. Both problems have gone from intermittent and mild to very noticable and fairly constant. The speed sensing unit on the transmission was replaced in May and when it failed the speedometer failed to register any speed at all, therefore the OD didn't ever engage. Given that it flutters now, it seems like the speed sensing unit isn't the most likely culprit, but I'm not sure what else could be causing this. I suspect that the overdrive problem is being caused by the speedometer not reading correctly. Any ideas on where to start troubleshooting?

Try a Fonzie on your dash, that seem to fix my intermittant speedo problems.

 

[GPulliam]

I'll check the voltage and clean the connection, if one of those don't do the trick then it sounds like the gearing is the next place to start. Hopefully it's the simple fix!

 

[GPulliam]

Checked the voltage on the TPS and it was low and erratic, couldn't tweak it to get it within range or constant so I am replacing it. Hopefully that clears it, part won't be in until mid-week.

 

[GPulliam]

Well, when we removed the TPS to install the new one, we noticed the plastic linkage between the sensor and the the throttle was worn. Tried the new TPS just for kicks, the speedometer was less erratic, but still not quite right. Currently checking with the dealer to see if they carry that little plastic piece. I'm pretty sure the TPS was bad given the low voltage, but the play from the plastic being worn might account for the improved but not fully corrected speedometer reading. The OD lagging has cleared up at least... Thanks for all the help...



Jenny