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intermittent erratic speedometer

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Old 05-18-2018, 08:28 PM
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intermittent erratic speedometer

1999 Ram 3500 duallie, automatic, quad cab, 2wd, 5.9 Cummins.

Speedometer jumps around more times than not.

New transmission output shaft speed sensor.

New differential speed sensor(the old one actually checked ok with an ohm meter)

Rear axle sensor wiring checks at 4.9V and good ground on either side of the pigtail repectively.

Transmission output shaft speed sensor reads 2.63V on both sides/pins of the pigtail.

Any help greatly appreciated.
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Old 05-21-2018, 10:48 AM
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Is the erratic behavior that it is dropping? I assume that's the case. but best to ask and be sure...

Did you do a continuity test on the sensor wiring and wiggle/shake the wiring to test for an intermittent break (or connection, as the case may be)?

Can you recreate this with the rear axle on jackstands?
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Old 05-22-2018, 12:08 AM
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Originally Posted by texasprd View Post
Is the erratic behavior that it is dropping? I assume that's the case. but best to ask and be sure...

Did you do a continuity test on the sensor wiring and wiggle/shake the wiring to test for an intermittent break (or connection, as the case may be)?

Can you recreate this with the rear axle on jackstands?
Yes, I have checked and wiggled the wiring. I do not know where the other end of the wiring goes to, to do an end to end continuity test. But I do get a 0.L reading on my ohmmeter when I try a continuity test from either side of the pigtail to the frame. ( also, as mentioned I get a continuous 2.63v on both sides of the transmission output shaft speed sensor pigtail). I believe one side should be 4.9V<5V, and the other side should be ground.

The erratic behaviour has the speedometer needle bouncing below(dropping) and above the given speed of the vehicle. In cruise control mode, the cruise control drops in and out causing continuous(uncontrolled non stop) erratic acceleration.

I am not able to get the truck up on stands.
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Old 05-22-2018, 09:48 AM
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I copied the following from my AllDataDIY subscription - bear in mind this is for a '96, so YMMV with your '99. maybe it will help in finding the other end points for end-to-end continuity tests:


Circuit Operation
Circuit K7 supplies 5 Volts from the Powertrain Control Module (PCM) to the vehicle speed sensor. The K7 circuit connects to cavity B31 of the PCM connector.

Circuit G7 from the vehicle speed sensor provides an input signal to the PCM. The G7 circuit connects to cavity B27 of the PCM.

The PCM provides a ground for the vehicle speed sensor signal (circuit G7) through circuit K4. Circuit K4 connects to cavity A4 of the PCM.

Helpful Information
Circuit K4 also provides ground for the signals from the following:

•Heated oxygen sensors
•Crankshaft position sensor
•Camshaft position sensor
•Engine coolant temperature sensor
•Intake air temperature sensor
•Throttle position sensor
•Manifold absolute pressure sensor
•Vehicle speed sensor


So, I think you are correct that one side of the pigtail should show 5V and the other side ground - assuming that the above also holds true for your '99. For my '96, the connector diagram shows OR(orange) as the 5V supply, BK/LB (Black/light black -???) as the ground, and WT/OR (White/Orange) as the signal.

Are you getting any indication of a trouble code?
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Old 05-22-2018, 11:01 AM
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No fault codes showing.

Anyone have a PCM pinout diagram for a 1999 ram 3500 2wd quad cab cummins?

Dealership won't give me a printout. Say it's proprietary.%^&*
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Old 05-23-2018, 09:13 AM
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Proprietary - NOT! You could get a single-vehicle subscription to AllDataDiy like I have, and the PCM pinouts are on there if you look in the right place. Also, factory manuals are available from Genos Garage.

My previous post mentions "cavities" on the PCM - I suspect those mean connector letters and pin numbers. They may be different for your '99, though.
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Old 05-25-2018, 11:23 AM
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Any updates?
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Old 05-25-2018, 12:35 PM
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Originally Posted by texasprd View Post
Any updates?

I disconnected the batteries and thoroughly cleaned them with baking soda and water to neutralize any external contamination, and cleaned the posts and cables. Reconnected the batteries after 24 hours (which I believe reboots the computer system). The speedometer seems to be working perfect now (for two days anyway.). I have not rechecked the output speed sensor pigtail voltage or ground yet, but I will do this soon.
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Old 05-29-2018, 11:31 AM
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Interesting!

I had an issue two nights ago - torque converter suddenly unlocked, and when it relocked it unlocked again after just a few seconds and continued doing that for the remaining 8 miles home. It was fine last night and this morning, but I pulled codes for excess voltage on TPS and IAT sensor... I will check and see if those share a common 5V feed.

We are both getting weirdness with sensor voltages. Keep us posted.
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Old 05-29-2018, 02:53 PM
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Same issue with my torque converter unlocking and locking. Been going on for quite awhile, but never for more than a few times and then it stops.
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Old 08-31-2018, 12:12 PM
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Speedometer is back to erratic again with torque converter locking and unlocking. Any help greatly appreciated.
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