2nd Gen Non-Engine/Transmission Speedo problem

[cumminscanuck]

The speedo on my 2000 Ram behaves erratically as soon as I start the engine. It flickers all over the dial whether the truck is moving or not. I have a Scangauge II permanently connected to my OBD port which also shows erratic speed readouts.



I have changed the speed sensor in the back axle and the output shaft sensor in the transmission but the problem persists. Can anyone suggest where I should look next

 

[DavidC]

wiring,plugs etc.

Dave

 

[Road Dog]

Start by unplugging the sensor on the rear diff. With no signal the speedo should be zero. If it's not then there's a wiring or ecm problem.

 

[cumminscanuck]

Start by unplugging the sensor on the rear diff. With no signal the speedo should be zero. If it's not then there's a wiring or ecm problem.

Thank's, I'll try that. Is it likely that this is an ECM and not a PCM problem? The transmission is an auto.

 

[DavidC]

PCM, Powertrain control module, ECM Engine control module. If electronic your looking for the PCM.

Dave

 

[DavidC]

Found this in the 99 service manual
VEHICLE SPEED INPUT
The Vehicle Speed Sensor (VSS) is no longer used
for any Dodge Truck.
Vehicle speed and distance covered are measured by
the Rear Wheel Speed Sensor. The sensor is mounted
to the rear axle. A signal is sent from this sensor to the
Controller Antilock Brake (CAB) computer. A signal is
then sent from the CAB to the Powertrain Control
Module (PCM) to determine vehicle speed and distance

Dave

 

[cumminscanuck]

Found this in the 99 service manual
VEHICLE SPEED INPUT
The Vehicle Speed Sensor (VSS) is no longer used
for any Dodge Truck.
Vehicle speed and distance covered are measured by
the Rear Wheel Speed Sensor. The sensor is mounted
to the rear axle. A signal is sent from this sensor to the
Controller Antilock Brake (CAB) computer. A signal is
then sent from the CAB to the Powertrain Control
Module (PCM) to determine vehicle speed and distance

Dave

Damn, another module (CAB) to check out!!!

 

[cumminscanuck]

OK, I unplugged the new rear sensor, started the truck and the speedo did not move and the brake and ABS lights came on. I reconnected the sensor and the speedo still did not move; a definite improvement. I backed out of the garage and set off on a test run. After 1/2 a mile the speedo started to flicker again but not as excessively as before.



I sat outside the post office with the engine running while the wife checked the mail. The speedo did not move until she got into the cab (NO I can't unplug the wife:-laf), I then started to bounce up and down and, sure enough, the speedo went mad Obviously there is a bad connection somewhere in the circuit and vibration is causing it to make and break. Now I have to find it The new sensor that I fitted had a pigtail that lengthened the cable thus enabling me to zip tie the wiring to the brake line which I think explains why the the erratic behavior was reduced. Previously the wiring was too short to tie up and had been hanging loose in a loop. A good theory me thinks to indicate that the break is near the rear sensor Tomorrow I will start the search at the rear axle sensor and work forward. Oo.

 

[DavidC]

99 shows twisted pair red/violet white/violet from sensor to abs module. Don't know if is the same for 2k but if it is you can use ohmmeter from the abs plug (unplugged) and unplug sensor and short pins together to check for a complete circuit. Good luck

Dave

 

[DavidC]

As an added note I remember seeing posts many years ago of the wires chafing at a point from the axle to the frame, you might look real close in that area as the rear axle movement would cause that.

Dave

 

[DavidC]

99 speed sensor to ABS

 

[DavidC]

99 ABS to PCM, to give you an idea of the circuit

 

[cumminscanuck]

Thanks for the diagrams David, they're a great help as they are the same as my truck.

I am not very confident in my ability to understand and diagnose electrical problems so some guidance on what I have done so far would be appreciated.

I have exposed the twisted RD/VT and WT/VT wires from the sensor (disconnected) as far as the junction to the main loom at the frame. There is no obvious evidence of damage to that point. I have disconnected the plug at the ABS controller and jumped the terminals that correspond to the RD/VT and WT/VT wires from the sensor. I checked for continuity at the sensor plug and found none. I then reversed the process, jumping across the sensor plug pins and checking for continuity at the ABS controller end. Still no continuity. I then checked for continuity from each of the two controller pins to battery ground. I found continuity on the WT/VT to ground at the ABS controller end but NOT at the sensor end. There was no continuity to ground on the RD/VT wire. I interpret this to indicate a short to ground on the WT/VT wire with a break in same.

Is this a sensible process and are my interpretations of these symptoms reasonable?

If this is a reasonable assumption should I resolve the issue by simply running a replacement wire alongside and in place of the faulty one? I would splice in by soldering just short of each plug as I do not have the capability of detaching from or connecting to the plugs. If I do this, is it critical that I use the same gauge wire or can I use heavier gauge?

 

[DavidC]

If you had both ends unplugged (by your description you did) and you put a jumper between the two wires on one end (which sounds like you did) you should read continuity between those wires at the not jumped end. It should be the same as touching both meter leads together only the leads are a lot longer now using the vehicle wiring. If you test each wire to ground you would place a jumper on the individual wire to ground and read from the opposite end to ground (frame) and should read continuity. If any of these read open (no continuity) you have a broken wire that may not be visible and broken under the insulation. If you do not have the wires jumped together and test across the wires it should read open. If it reads continuity they are shorted together somewhere. You need to have the meter on the highest setting. As for the wire it needs to be a twisted pair (cuts down on electrical noise) but I don't think going bigger will change anything. Just don't go smaller. The description of your problem could very well be the conductors are broken under the insulation and making momentary contact at times which could cause the speedo fluctuations. That type of problem happens quite frequently with multi-meter leads which then need to be replaced from the flexing. If your wires are bad I would bet it is in a place that flexes a lot like the frame to the differential but finding it could be a problem. If they were individual wires (not twisted pair) I have often pulled on 2' sections and watched the insulation stretch at the break and simply spliced it back together. I hope I explained this well enough, kind of hard to relay what I mean with a keyboard. Let me know if I have confused you.

Dave

 

[cumminscanuck]

Once again my thanks David for the explanation and for quickness of you response. With the benefit of your enlightenment, I am now certain that I have a broken wire with one side of the break touching ground. This could explain another problem that has coincidentally arisen; my batteries discharge in about 2 - 3 days if the truck is not run. I have disconnected the grounds and they hold a charge OK and when I reconnect the first ground I get a significant spark at first contact which, I believe, indicates a short.

Thanks also for explaining the reason for a twisted pair. The diagram shows the twist to C128, would this be the connection at the frame or is it likely further forward? It is difficult to determine because the loom runs between the frame and the fuel tank and is not easily accessed. The OEM wire appears to 18 gauge which I plan to replace with 16 gauge (that's what I have readily available). I think I should replace both wires to preempt any future failure of the other wire. I am now trying to determine if I need to twist them for their whole length?

 

[DavidC]

yes you need to replace the set (twisted pair). Do not try one single. If one broke the other is right behind it. I believe the harness plugs are behind the drivers front wheel. I had to get to them once and if I remember right it wasn't the best place. I did not have to remove the fender well but I believe there are several plugs in that area. I do not know if the wire stays twisted the entire length but it would not hurt to be so.

Dave

 

[cumminscanuck]

yes you need to replace the set (twisted pair). Do not try one single. If one broke the other is right behind it. I believe the harness plugs are behind the drivers front wheel. I had to get to them once and if I remember right it wasn't the best place. I did not have to remove the fender well but I believe there are several plugs in that area. I do not know if the wire stays twisted the entire length but it would not hurt to be so.

Dave

Great, now I know what I am going to be doing tomorrow Keep your fingers crossed and many thanks again for your help

 

[DavidC]

Good luck, hope it fixes your problem as it sounds like it will with the test results you have. You will be the first to know.

Dave

 

[cumminscanuck]

Final update.

Rewiring the twisted pair did NOT#@$%! solve the problem so I started on the ground connections around the truck. I removed, checked, recharged and cleaned the batteries and terminals. I fitted new ground cables from the batteries to the PCM, frame, engine and body. No fix!! The truck has an exhaust brake fitted and a BD "Autoloc" which controls TCC lock up when the brake is on. The Autoloc reads speed data from the PCM to determine when to unlock the TCC so it was a suspect. I disconnected it from the PCM and "TA DA", the problem went away. Oo. I did get a little help from a neighbor who seemed to have a little knowledge about these trucks, which boosted my confidence greatly. The neighbors name is Piers something!!!