offset442

I'm posting this to let those who have suffered the same fate as me, a second chance to repair there ecu.

1988 Toyota truck 3vze 3.0 v6 MIF

Symptoms,

1.ECU DTC:21 (up stream A/F sensor only on 88 model)
code sets shortly after engine warm up and stays set even if you have replaced the o2 sensor with a proper denso selection

2. engine running perfectly after warm-up despite code

3. all connections between sensor and ecu in good shape

4.voltage measured between Battery positive and termainl "HT" on ecu less than 10 volts uasly 6 volts or less maybe zero depending failure

5.current flow measured between B+ and sensor heater less than 1 amp (should measure approximately 2 amps for a 24 watt heater or 1 amp for a 12 watt heater)

possible Cause(s),

1. after market O2 sensor heater has a heater with too low of resistance uasly 1.2 Ohms is a goog caindate for burning the ECU fusible resistor
on my model The prober O2 sensor had a heater measuring about 6.3 OHM's

2. O2 sensor heater wires (the black pair) melted or somehow shorted together, Burning the ECU fuseable resistor

Solution:

1. get an o2 sensor that has a 6 ohm heater!!!!!!!!!! and or FIX any and all shorts in the heater circuit

2. remove the ecu from the truck and take the from and back covers off of it, you should immediately see a dark brown or black burn spot on the board.

3. ontop of the burn spot you will see a grey resistor that looks cooked next to the pin lebeled "HT" on the board in silk screen, masure the resistance of the cooked resistor and it will measure several MEGA Ohms, you should at this point notice another simliar resistor that will masure around 0.3 ohms and have a label that reads 0.15k this means 0.15 ohms type K

4. VERY CAREFULLY!!!!! unsolder the burned resistor taking CARE NOT TO RIP UP THE DAMAGED COPPER FOIL PADS!!!!!!!! and replace it with any resistor that measures 0.3 ohms on your meter and is of equal size or bigger, (appox 2 watt rating) I got my replacement out of an old computer monitor if your paranoid and/or afraid of having to replace a transistor in the future you can order a replacement fuse-able resistor from allied electronics, or scavenge one out of another ecu of any make/model that has been declared dead

5. you can do this if you want test the npn switching transistor to see if its burned, (google: VOM transistor testing) this transistor is used to turn the heater on and off when the computer decides its needed, most likely it is not harmed if the fuse-able resistor did its job

Tip: if you don't know how to solder on and repair circuit boards DON"T DO IT WITHOUT PRACTICE, SO find a geek that can, one is available in most any computer repair shop, at 1/4 the cost of a new ECU and most will just say 20 bux to solder a single resistor.

MY check engine light is GONE!!!!!! YAY!!!!!

Morals, get the the right DENSO O2 SENSOR, OR AT LEAST LEARN TO USE A DMM OR VOM

Peace!!!!!!!!

Ocean I. House

offset442

Just a quick addition this information also applies to other makes and models, and engines, you can probably apply this repair on almost any non-potted discrete style constructed ECU.

The more modern stuff is using SMD micro electronics sealed in epoxy or a weird thermal gel and is almost impossible for even the most experienced geek to repair

Ocean

jbtvt

Nice writeup, thanks.