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Discussion Starter #1 (Edited)
Speedo went out while off-roading a couple weeks ago. I wheel often in my truck, so not a unique scenario. Code is P0500. Tried the following:

1. Checked all fuses
2. Checked visible wire harnesses and connections. The connector on the driver's side ABS sensor is kinda busted up but still engages (see #5).
3. I measured the resistance across both VSS sensors on the trans. They both measured ~620 ohms and both change when metal comes near the end.
4. I measured the resistance across both ABS sensors in the rear. They both measured ~1100 ohms. I did not remove these.
5. I checked the resistance at the connector where both sensors go in above the diff. Both sensors read the same as the measurement at the sensor, therefore the busted connector in #2 is not an issue (it will be replaced regardless).
6. I bought a VSS sensor and replaced each sensor on the trans with it. Neither one made a difference.
7. I disconnected my Yellow Box Speedo Recalibrator, just in case that was bad. No change.

I don't want to replace the ABS sensors, they were $350 at AutoZone! :oops: Plus, since they both measure the same resistance I'm skeptical they're bad.

Some other info - I cut the wires for the front ABS sensors (suspension droop too much) and removed the ABS fuses years ago. The speedo has always worked fine like that, no check engine lights.

I'm thinking an issue somewhere in the wire loom? Any suggestions?

The FSM I have calls out some checks, but apparently somewhere along the line they changed the sensor type. I have two two-wire sensors. The FSM shows one three-wire sensor. So that's not much help.

Any help would be greatly appreciated!
 

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Discussion Starter #2 (Edited)
Some more troubleshooting:

1. Checked the voltage per the FSM (see attached pic). I get zero. But I don't even know if those are the right pin-outs for the 2006.
2. Jacked up one tire in the rear and put the multimeter clamps on the ABS sensor. I get a randomly fluctuating resistance of about ±0.5MΩ. It goes negative and positive. There isn't much change at a very slow speed, but spin the wheel about 90° per second and you get negative readings. I don't know how you get a negative resistance.
3. I get the same resistance for both sensors at the connector under the seat where the loom enters the cab as I do at the sensor. So at least the external looms are intact.

I think I'm going to order a couple ABS sensors and a new connector and replace them both. Hopefully that fixes things.

Any other suggestions, please chime in. Thanks!
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Discussion Starter #3
Pulled the rear ABS sensors. Lots of crud on them. Got excited that was the culprit. Cleaned them off and no dice.

I have new rear sensors and the rear sensor harness coming. If that doesn't work I'm at a loss what to do next.

20200514_154956.jpg 20200514_155215.jpg 20200514_154116.jpg 20200514_154210.jpg
 

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Discussion Starter #4
Replaced both transmission speed sensors and both rear ABS sensors. Also replaced the rear ABS harness with the damaged connectors. No luck.
Tore the instrument cluster apart and don't see any obvious signs of damage or burnt components. Running out of things to try now. Guess my next task is to tear the dash out and start tracing wires. I had a mouse problem in the engine bay last year, I suppose I could have a chewed wire back there. Thinking of ordering a new instrument cluster ($100) and seeing if that helps.

Anyone got any other ideas?
20200517_093618.jpg
 

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Discussion Starter #5
More troubleshooting. Found this check in the manual. Unfortunately, I don't have a oscilliscope so used a multimeter. I get 11.5V with the truck running. I don't see any fluctuation, but it may be too fast for my multimeter to pick up on it. Strangely, when I have the Yellow Box connected, it shows 14.5V.

I also get 0V with the connectors disconnected and checking that pin. Odd - you'd think there would be some voltage coming out.

I also realized the double cab wiring schematic is different than the access cab. I don't have the access cab wiring schematic, so that's not very helpful. But it appears at least some of the pin outs are the same.

Ordering a new instrument cluster. Guess I'll keep throwing money at it. :rolleyes:

Capture.JPG
 

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Discussion Starter #6
Tried a new (used) instrument cluster. No change.

So, I think that narrows it down to something in the wiring or the ECU. Crap.

Next stop is going to be Toyota. Let's see what they say for $150.
 

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Discussion Starter #7
SUCCESS!

My ego wouldn't let me take it to Toyota so did some more troubleshooting today. Figured I'd post the details here for someone who has the same issue in the future.

First found in the wiring schematic that the speed sensors feed into the ECU. (V3 Speed Sensor and O/D Speed Sensor). Wires are as follow:
V3 Speed Sensor
W-R pin 26
Y-R pin 34
O/D Speed Sensor
G pin 35
R pin 27

20200530_104605.jpg

Measured the resistance on both sensors at the trans and at the ECU.
V3=625Ω (626Ω at sensor)
O/D=618Ω (618Ω at sensor)

So wiring was good from sensors to ECU.

Checked continuity from ABS ECU to instrument cluster (BR-B wire). Bypassed the Yellow Box calibrator for this. Had continuity.

20200530_111903.jpg

Finally, installed the new front ABS sensors. Cleared the ABS code, started the truck and the speedo works now.

So, what was it? Not sure. Maybe a loose connection on the ECU or the ABS connector. I had removed the front ABS sensors years ago and the ABS fuses (the ABS sensors were getting bound up at full droop, and ABS sucks in dirt/mud/snow). Speedo has always worked fine. Maybe a strange gremlin where the ABS computer needed to "reset" at a fully functioning state in order to start sending the speed signal to the instrument cluster again?

Just for kicks and giggles, I unplugged both front ABS sensors. ABS light came on but speedo is working fine. So, who knows. Just glad it's working.
 

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Discussion Starter #8
Another interesting tidbit - got code P0717 while driving - transmission input sensor. Forgot to plug it back in after doing resistance checks. So, the speedo doesn't need at least that sensor to function.

Replugged it back in, cleared the code, and all is well.
 
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