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Discussion Starter #1
Hello,

I have a 1996 Tacoma 3.4 auto 4x4. I have searched the internet and a few forums for days looking for an answer as to why my O/D light is flashing on and off. as the light flashes you can feel the truck kicking in and out of over drive(at cruising speeds). fluid is full. if I click the o/d off it drives fine no kicking in and out of course. also trans shifts great even into overdrive. only issues is the light blinking and it kicking in and out of o/d. no I have not hooked to a scanner yet either. figured id ask here before paying a diagnostic fee. thanks in advance.
 

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Mambeau / Admin
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You have the A340F electronically controlled transmission. The O/D indicator light means "O/D OFF" (i.e., O/D is disengaged). If it's flashing on and off that might mean something anywhere in the various control circuits (electrical / hydraulic) is failing to shift into and maintain the expected state.

On the ECT autos there's a throttle signal from the throttle position sensor, used to regulate the line pressure. There's also a "throttle valve" in the tranny's upper valve body, but I'm not sure what it does. There's also a throttle cable connection to your tranny. If there's something wrong with any of these three throttle-related mechanisms that might explain it.

There's also a 3/4 shift valve somewhere within the tranny. In theory that could cause a shifting problem from 3rd to O/D (4th) and vice versa.

The O/D stuff is controlled from the (engine's) ECM. Shifting is dependent on your vehicle speed sensor, so there's an outside chance your speedo / VSS circuit is the problem.

It could also be a wiring or actuator problem. According to the Toyota documentation your tranny shouldn't be able to upshift into O/D if the O/D is disengaged. If the O/D is disengaged the O/D OFF indicator lamp should be illuminated.

Is the O/D OFF indicator lighting up and staying lit when you switch the O/D off?

Do you have cruise control? The ECT stuff is cross-wired into the cruise control stuff somehow.

Because I can't find anything in the factory documentation that indicates the O/D OFF light should ever flash on and off, I'm thinking your O/D isn't getting engaged.
 

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Discussion Starter #3
thanks for the info. the light will blink on and off every 2-3 seconds. about every 3 blinks you can fill the truck drop out of overdrive. so to answer your other question it does shift into overdrive. also if I click the button to disengage o/d it does exactly what it is supposed too. truck runs and drives and shifts like it should with no malfunction.

i read in another forum on a older Toyota a guy was having the same problem and all he needed was a good ground from body to engine.(I have not checked that yet as I just seen that today).

i also have not tried to engage the ect button. I will also do that this evening. I do know my shifter is all sloppy as in loose feeling. could there be a grounded wire somewhere from the button in the shifter?

thanks for any feed back.

Eric
 

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Mambeau / Admin
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Yes - there's always the chance there's a wiring / ground fault with the shifter button / switch itself.
 

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Mambeau / Admin
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Here (attached) is the 1996 V6 ECT wiring diagram.
 

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Mambeau / Admin
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There are transmission-specific codes that can be detected using an OBDII scanner. However, I don't know whether any of them relate to the ECT add-ons.

On the other hand ... If you can borrow an OBD scanner and check for codes it might give a clue to what's going on. Remember - some of the things that might confuse the ECT aren't in the transmission (e.g., the vehicle speed reading and the throttle position sensor).
 

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Discussion Starter #8
Ok so the truck does not have cruise control. I couldn’t recall bc I just got it last week. I added the ground wire from pass side head to firewall as it was missing.

test drove and still blinking and kicking in and out. I cut truck off and started back and engaged the ect button and it still does it.

I may try the scanner tomorrow as the shop is done closed. How would I begin to know ifone of the many things you listed is bad other then throwing parts at the truck if no codes are present
 

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Mambeau / Admin
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I don't think the ground wire you replaced had anything to do with the ECT circuit.

Check the wiring diagram (toward the end; the ground points listing). There are 3 ground points associated with the ECT system.

Beyond that ... It's a 24-year-old vehicle, and there are multiple things that could be wrong within the transmission itself (slippage; hydraulic problems; valve(s)).

Now that you've mentioned it's a recent acquisition ... Has this behavior been happening ever since you got the truck last week, or did it start suddenly?

Does the truck have non-OEM sized (especially larger) tires? Anything that would throw off the speed sensor reading might conceivably be screwing up the ECM's ability to know when to shift.

Because the light that shouldn't be flashing is flashing on and off my first guess is that the O/D isn't engaging cleanly or the ECM is getting faked off about what state the O/D is in. This could result from an ECT / electrical fault, or it could be some sort of hydraulic problem or solenoid failure inside the transmission.

My advice would be:

  • Locate and check all 3 ground points associated with the ECT system
  • Check your speedometer behavior and check the vehicle speed sensor
  • Put an OBDII scanner on it and see if it registers any codes.
Unless you find a fault or get codes pointing to something peripheral (speed sensor; throttle position) I think I'd assume the problem is most likely mechanical / hydraulic inside the tranny itself.
 
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