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Discussion Starter #1
The speedometer in my truck is horribly wrong and seems to be temperature sensitive. I have a 1994 Ram 1500 4x4 with the 46rh (a518) automatic. For the first fifteen minutes of driving my speedo is about 30-45mph inaccurate. I know because it read 35 when the state trooper clocked me at 72. As the truck warms up it gets more accurate though never within 10 mph of right. i'm only running 33's so i don't expect tires to throw me off more than about 5mph and definetely not a variable difference. My question is would the speed sensor be more likely than a problem with the gauge cluster? Or is it possibly a short in the wiring somewhere?
 

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I have speedometer issues with mine too and I don't have it all figured out yet. I'm trying to diagnose the problem so I spend money on only one part, rather than throw a bunch of parts at it until the problem goes away. But here's something you might check. If you have cruise control, does it work and hold a steady speed even when the speedometer is bouncing around? I'm pretty sure that the sensor at the back of the transmission is the only one that operates the cruise. So if the cruise works as it should, I think that has to point somewhere else.

In the meantime, here's another idea for you. If you have a smart phone, download a GPS speedometer and use that. It will keep you from getting a ticket.
 

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Discussion Starter #3
ha ha i don't have a smart phone i got a boulder cause i ran over my last one and this is the only one that will survive. That is a good idea though cause i been gauging my speed by how loud my tires are. I'm pretty sure its the speed sensor because it sends a reference signal to the ECM to control torqueconverter lockup as well and i've noticed it coming out of lock up when the speedo reads between about forty and fifty (regardless of actual speed) but clears up when it warms up and is close to right. the torque converter is new so i'm sure its not a problem. The cruise works fine I think, it seems to hold the truck at a constant speed, but I'm driving up and down hills to work so its hard to tell. My next paycheck is going to buy a speed sensor i'll let you know how it works
 

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Hey don't laugh. I did the GPS smart phone speedometer for a while. I don't use it much now, as I got a feel for what rpms the tach reads at various speeds. The odd time the speedo does work, according to the GPS, the speedo is about 2-3 mph slow. I'd still like to fix mine though. I'll watch to see if the speed sensor fixes yours.
 

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Sounds like the speedo sensor/sending unit. Exactly where it is on your truck I'm sure but check the top of the rear differential.
 

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Discussion Starter #6
the sensor's in the transfer case on the 4wd and tranny extension housing on 94-95 2wd models(don't quote me on the 2wd) and in the diff on later 2wd models. in the diff is actually a combo abs and speed sensor. first thing i replaced on my fiance's 98 2wd
 

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Discussion Starter #7
ok not the speed sensor, changed it...wiring diagram shows nothing in between the sensor and the gauge so I'm gonna look for a short. otherwise i'm stumped
 

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What speed sensor did you change?? The one on the driver side of the tranny? There's an RWAL combo sensor on top of the rear diff...I smell that this one is the culprit if your torque converter comes out of lock-up.

Gawd I hate sensors.......

I looked a bit and it seems like a dealer only item
 

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Are you sure the '94s have a differential sensor? I don't find any electric wires going to any sensor on the differential on mine. OD and converter lock up work like they should.
 

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Hayne's says so, and I looked at my 94 and it's there. It's on top of the diff, and the retainer holding it also holds the brake line going on top of the diff. There is a little cover on it (a square box). Under that box is the sensor.
 

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I can't say for certainty witht he 4 wheel drives but for a 2 wheel drive unit they were definately a speedo gear in the tranny. I've changed mine when I changed my tires.
Joe
 

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Hayne's says so, and I looked at my 94 and it's there. It's on top of the diff, and the retainer holding it also holds the brake line going on top of the diff. There is a little cover on it (a square box). Under that box is the sensor.
I couldn't find anything about a differential sensor in my Haynes manual. I've been under there twice before looking for it and didn't see any wires coming from anything on the differential. At your insistence, I crawled under there again to take yet a third look. I'll be danged, this time I find it. Unplug it and go for a drive. The only change I see is the brake light on the dash comes on now. The cruise control still works fine, the transmission including OD works fine, and the speedo works 15% of the time, just like it always does. I think that has to point somewhere else. In my case, I think the only place left is the speedo head itself.
 

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I couldn't find anything about a differential sensor in my Haynes manual. I've been under there twice before looking for it and didn't see any wires coming from anything on the differential. At your insistence, I crawled under there again to take yet a third look. I'll be danged, this time I find it. Unplug it and go for a drive. The only change I see is the brake light on the dash comes on now. The cruise control still works fine, the transmission including OD works fine, and the speedo works 15% of the time, just like it always does. I think that has to point somewhere else. In my case, I think the only place left is the speedo head itself.


Hayne's page 9-24 item 17.

After I looked at it again, it's probably not the cause of your problems. I read fast and saw there is a speed sensor in that rwal, but just for the abs.

Only 2 things can make a speedo haywire (excluding the speedo itself):

Bad VSS (speed sensor on the driver side of the tranny). This is most likely not the case, because your tranny would shift weird. Unless you bought an aftermarket VSS, who would give the right input to the tranny but not the speedo.

Oversized tires. Then again It would not work only 15% of the time. It would be off by a few MPH, no more.

I'll try my buddy the wrecker to see if he ever saw that problem, I'll keep you posted...
 

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Discussion Starter #16
ok been playin a goin over the wirin diagrams. the VSS in the transfer case (4wd) goes directly to my speedometer. I believe this is the case on 94 and 95 models. The sensor on the diff is there if you have rear abs. on these year models it controls soley the abs and acts as a dual abs and speed sensor on later models. if i unplug the sensor in my transfer case the speedo does nothing. but the sensor itself is not bad. i replaced it with no change and back probed the wires and thus far have found no problems. i even changed the sensor in the diff for good measure as that gave me similar problems in my fiance's '98 2wd but to no avail. i'm beginning to wonder if i may have two isolated problems- a bad gauge and a faulty torque converter? I wish it was a stick shift---dreams hopelessly--- NV4500---
 

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It's highly unlikely to be the speedometer itself but not impossible. I would reccomend you find someone with a odbII computer or DRB and see if your speed the computer show matches the actual speed, if your speedometer reads crazy but the actual matched the PCM value then maybe it would be the speedometer. Also have you changed tire/wheel size? Then your computer may just need upating, if your not sure check the tire size against the door panel.
 

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The 1994 Rams are not OBDII. Tires are stock. The odd time the speedo works, it's accurate - well, about 3 mph slow at 60 as compared to a GPS speedometer on my Droid X. I've checked everything else as I can see it. I don't know how it can be anything other than the speedo head itself. I just need to find another one and try it, assuming it's not also bad.
 

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Discussion Starter #19
yeah like thirdmouse said- obd1 and i'm runnin 33's so i expect about five miles off- not forty and definetley not a variable error. its 40mph off for the first ten minutes of running then finally gets closer to right but never within fifteen miles of right and never reads over 65 (i had a buddy clock me at 92 when i noticed the speedo never went past this mark). The torque converter problems seem to have cleared up, maybe just break-in woes or old ass truck idiosyncracies? The TC's barely three and half months old.
 

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Discussion Starter #20
fixed the problem. tachometer went out too so i figured i'd try the gauge cluster- got one at a junkyard and i'm back on the road and accurate... only now my odometer says 127,000 instead of 216,000 guess that may be a good thing but its exempt anyway and i don't plan on selling it. it'll either fall apart under me or go to my children in the future
 
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