'2WD-4WD Mode Change Failure' alarm/alert

Had an issue today. After starting the truck, I received a "2wd-4wd Mode Change Failure", even though I did not make any changes with the selector. That is,... last time I drove I was in 2WD and was still in 2WD when I got the alert.

Clearing the alarm, it just returned moments later.

Drove about a mile, to get the groceries I left for in the first place. Still I could not get it to clear.

I played with moving the selector. Although it would actually move into 4HI, the alert remained, and the 4HI indicator light did NOT come on.

Switching back to 2WD (and it did actually drop out of 4HI), the alert remained.

When I put it into 4LO, the indicator lights for 4LO, TRAC OFF lit up, and it did drop into 4 LO, but I still could not clear the alert.

I'm not sure what I did, but eventually, I was able to clear it.

Did my shopping (may 15-20 minutes), and the process started again. Going thru all the same steps above, and even driving to an empty parking lot trying various things to clear the alert.

As I backed into my parking space at the motel, it finally cleared again.

It is pretty cold where I am at the moment, as I am traveling north for work. About 23*F, wind-chill about -8*, but the wind-chill shouldn't be a factor, especially considering I've driven in colder weather at faster speeds.

Thoughts??

 

I've driven/parked/idled in colder temps. Maybe a wiring thing? Salt, loose wires, corosion etc

I had my Blindspot monitor sensors randomly light up, flicker and different brightnesses and on a single side sometimes. Checked the plug. Probably got road salt in it at some point. Cleaned it out with rubbing alcohol. Problem solved

 

I need a factory service manual, I guess. Not a Hayne's or Chilton's.

 

So, where is this switch I need too locate, and what does it look like?

 

Any modifications? Aftermarket lockers or re-geared diffs?

If not then look at the link below

this post explains it the best https://www.tacomaworld.com/threads/2wd-4wd-mode-change-failure.597135/#post-21303343

apparently I summoned him? Ignore this post and look at the one below

 

3rd Gen Repair Manual, there are quite a few copies floating around. Here is one it looks like 3rd Gen Tacoma Service Manual (huge PDF...) | Tacoma World

Your description of the problem is very odd compared to the typical failures we have seen online. In my experience, when the we see the 2wd-4wd mode change failure, you can only clear it out by restarting the truck (at a minimum).

Regardless, to diagnose, you are going to have to use Techstream or climb under the truck with an ohmmeter... (and as you are not in florida, that second option sounds very BAD right now).
This is a quick overview of how to check the two actuators with an ohmmeter. https://www.tacomaworld.com/threads/4wd-actuator-troubleshooting-transfer-case-and-add.578262/

 

No mods, no regearing, or other.

 

Thanks.

 

Are you 100% sure it was shifting 2hi to 4hi and vice versa?
(I am just so used to the truck locking out further mode changes once it throws that message, so its very odd to me that it might continue to change modes after the message is shown)

A good code reader may well be worthwhile here, even if its not techstream

I THINK a front diff temp sensor failure may cause the noted situation (and there may be other possibilities).
Ref code P17C7 and P17C8
But if that is the cause, then this is a failure I have never heard of... and maybe why it doesnt match the typical failure symptoms exactly.

 

Thanks for following up.

Yes. I am absolutely sure it still actually changed modes, even though I had the alert/alarm.

In 4HI, I had the jumpy/grabby action with sharp turns, although I had the alert and no green 4wd symbol.

In 4LO, the same, but throttle response was typical for 4LO operation. I had the 4LO symbol, the trac off symbol, as well as the one that has the squiggly tire tracks. Still did not have the green 4WD symbol though.

I checked while driving yo/from the grocery, and stopped in a vacant lot next to the motel where I could "play" without obstructing traffic and trying to get it to switch.

I took pics as it was happening last night, to help me remember the sequence for the post, which I can/ will upload after I get some food in my belly.

I will say that starting her up & driving to the motel tonight, from the job (20ish miles), there have been no issues. No alerts. Everything still changes into and out of the various modes.

 

There are (per the repair manuals) failure modes that cause the message but don't lock out mode swap. But I have never seen them.

Looking at the manual, the ones I see are diff temperature related. If you are not missing any important functionality (as far as you can tell, I wouldn't try to investigate until summer.

If you do want to investigate, the diff temp sensor is where I would start (if I didn't have techstream). But it's just an educated guess. Let me know if you really intend to investigate further under truck

 

Thanks. I likely will look into it. I have a four day weekend coming up and the weather won't be as brutal.

I need to get a service manual to show me where everything is.

Funny, tonight was only a couple of degrees warmer than last night.

I wonder if there could have been some ice or what not jamming something up? Maybe taking on a longer trip warmedd stuff up enough to clear the ice?

Winter is just starting here in Duluth, MN & Superior, WI, so if it's temp related... it's just a matter of time before the temp plummets well below last night' and this morning's.

I'll definitely keep an eye on it as the season continues...

Pics in order of taken.... alarm.


Alarm, with 4LO activated...


Alarm cleared and 4HI...

 

At what temp is the low temp alert supposed to be activated?

 

ECU recording range is given as
Front Differential Oil Temperature
Min.: -56°C (-69°F)
Min.: 199°C (390°F)

The manual lists the resistance expected as 120kOhm to 240kOhm (but doesnt indicate if that is over the full temp range, or expected shop temp range)