Dead SPEEDO and ODO

Driving along, everything is fine... then the speedo drops to zero and no more miles. Happened on pavement. Not after a bump or anything, just under acceleration.

Had been a few days of very muddy CO backroads. Truck caked with gunk and muck.

When I monitor the truck with an OBD scanner (TorquePro) it does not see speed while driving either. I have error code P0500 as well.

For a year or so I have not had my front speed sensors connected. So I assume just the two at the back axle were sending a signal. I have however reconnected the front two sensors and replaced the rear two sensors, but this has made zero change is speedometer function (still dead).

Related or possibly unrelated... my gas gauge has become hypersensitive and lazy at seemingly the same time. At a stop light I can easily loose a 1/4 tank while braking and gain it back accelerating. Also, the gauge can take 10 or so minutes to recognize being full after a fill-up. Regardless of how many times the ignition is switched on or off.

The transmission (AUTO) seems to be shifting just fine so it does seem at some level the truck does recognize it's actually moving forward and at speed. Perhaps transmission shift points are not related to the speed sensors at the wheels?

Anyways... any suggestions where to start looking? Blown fuse I have missed? Bad ground?

Thanks for any tips or suggestions.

 

The speedometer gets the vehicle speed from the ABS module and then transmits it to the PCM.
Do you have an ABS light on?

 

That is good info. My ABS system has been in rough shape for several years. The ABS module is still in place under the hood, plugged in... but disconnected from the brake system.

However, it has been this way and speedo was working. I do recall unplugging the module for a moment years back and the speedo dies so I plugged it back in and everything has been working, until this happened.

 

Most likely one of the rear speed sensors has failed or one or more of the wires going to them are broken/corroded.
I'd get the ABS codes read and go from there.

 

I assume ABS codes are different from what a traditional OBD scanner tool can grab?

 

Yes most generic scanners will only read engine (PCM) codes.

 

Do you happen to know the minimum speed sensor signals the ABS module needs to send a speed signal to the speedometer?

Since I have had the front to sensors disconnected for some time without issue, I am assuming the two rear sensors were doing the job. Or perhaps the ABS just needs a signal from one sensor? Or perhaps a particular sensor... left rear for example might send speed signal to the speedo.

 

I believe it uses the right rear for vehicle speed but I'm not 100% on that, I do know it uses one of the rear sensors for vehicle speed.

 

It would make sense it was a rear sensor instead of just a 2 out of the 4 type situation. Otherwise one would think reconnecting the front sensors would get my speedometer working again.

Thanks for all of the help

 

Right rear wheel ABS Sensor on an Automatic Transmission.

Sensor in the transmission for a Manual Transmission.

 

Awesome... thanks for the confirmation.

 

Jimmy H (and whomever else)

Seems I have learned from this thread I have an issue with my right rear ABS sensor sending a signal to my ABS ECM and then that signal translation being sent to my speedometer. Since I already took the "easy" step of replacing the right rear sensor with zero improvement, it seems I may have a wiring issue.

What would the downside be of me using one of the front sensor signals in place of the right rear ABS signal? Just swapping the wires or pins at the ABD ECM plug. Reason being it's easier to troubleshoot the shorter wiring distance from front wheels to the ABS module and there is little reason to believe the front wiring is not still good.

My brake system is fully disconnected from the ABS module and all I really just need the ABS ECM to translate a wheel sensor signal into something the speedometer can understand and display. So confusing the ECM into which wheel is moving would be a non-issue.

 

I don't see why you couldn't but I would still want to look at ABS codes and data before assuming anything.
You may check the IA1 and IB1 connectors for corrosion seeing as the speed signals go through them.

 

The truck has spent it's life in the SW and has very little corrosion or rust throughout the vehicle but there is a lot of dust. If these junctions were bad, would the OBD also loose speed signal?

Thanks for the wiring diagram PDF by the way. Those are always helpful.
______________________________________________________________________________________

Understanding original speed input was only from the right rear speed sensor, I have tried sending a speed signal from different sensors to this input (PIN 44, 45) on the Skid Control ECU.

• Swapped input 15 to 44 and 16 to 45 (Left rear sensor) . No change.
• Swapped input 40 to 44 and 39 to 45 (Right front sensor). No change.
• Swapped input 10 to 44 and 9 to 45 (Left front sensor). No change.

Its possible that all speed sensors are no longer working or wire breaks between them and the ECU... but that seems unlikely to me.

It's possible the ECU actually receives signals from multiple or all speed sensors, converts, then sends signal to speedometer. But that doesn't really explain why my speedometer has worked for months with the front two speed sensors disconnected.

During this process I did discover the rear two speed sensors were covered in oil once removed. I have learned this indicates bad axles seals and the oil may corrupt or disrupt the speed sensors ability to work properly.

I will be replacing my axle seals and perhaps this will / should correct the right rear sensor (apparently the primary speed signal sensor) which could fix the speedometer.


But this still doesn't explain to me why signals from front sensors would not be a valid substitution.

 

Without knowing what ABS code(s) are setting and being able to look at the ABS data it's nothing but a guessing game which IMO is a waste of time and effort. We don't even know for sure that the ABS module is online and communicating without trying to scan it.

 

I understand. I can look into that. My guess is that with me running the truck with missing sensors for so long I would get a lot of unrelated data back. By this I mean the SPEEDO has been working for a long time with the ABS ECU full of errors due to missing signals.

Do you know if the OBD speed signal comes through the OBD port before or after the SPEEDO reads it? If the SPEEDO feeds the OBD port then of course my OBD scanner wouldn't see speed either. But if the speed signal is fed directly to the OBD port from the ABS ECU and my OBD reader doesn't see vehicle speed... maybe that proves the ABS ECU is not functioning.

 

This is the path Automatic:

Speed signal >>---- Skid Control ECU>>----------->>Combination Meter >>------------>>PCM

DESCRIPTION
Automatic Transaxle Models: Vehicles, which are equipped with ABS (Anti-lock Brake System), detect the vehicle speed using the skid control ECU and wheel speed sensor. The wheel speed sensor monitors the wheel rotation speed and sends a signal to the skid control ECU. The skid control ECU converts the wheel speed signal into a 4- pulse signal and transmits it to the ECM via the combination meter. The ECM determines the vehicle speed based on the frequency of the pulse signal.

This is the path Manual:

Speed signal >>---------->>Combination Meter >>------------>>PCM


Manual Transaxle Models: Vehicles, which are equipped with manual transaxle, detect the vehicle speed using the No. 1 vehicle speed sensor. The No. 1 vehicle speed sensor transmits a 4-pulse signal for every revolution of the rotor shaft, which is rotated by the transaxle or transfer output shaft via the driven gear. The 4-pulse signal is converted into a more precise rectangular waveform by the waveform shaping circuit inside the combination meter. The signal is then transmitted to the ECM. The ECM determines the vehicle speed based on the frequency of the pulse signal.

 

Nothing feeds vehicle speed to the OBD2 port directly, as I said the ABS module is responsible for providing vehicle speed to the instrument cluster on automatic transmission trucks. The cluster then sends it out on the SPD terminal to all the other modules that require it.
Think of the modules on the truck like a computer network, think of each module on the network as a separate computer. When you plug a scan tool in your basically plugging into the network, generic OBD scan tools can only communicate with the PCM so if the PCM isn't getting the speed signal from the cluster (which gets it from the ABS module) your gona see 0 mph in generic OBD data.
You need a scanner that can access the ABS module to see whats going on there.

 

Do you happen to know what the ECM converts the pulse signal to so the speedometer can display correctly?

Does the Skid ECU monitor all four speed sensors and then averages them to the speedometer output?

I really just need a box that converts the speed sensor signal to a speedometer compatible signal. For my truck the rest of the Skid ECM functions are not needed. I wonder if the Skid ECM is performing actual signal calculations or just converting the 4 pulse signal into a signal the speedometer can read.

 

Thanks for the clarification. If the speedometer isn't getting a signal it seems like it must be something downstream from that. Wiring, junction, bad ECM or faulty speed sensors.