# Should both lights flash together on 88 ECM?



## yellowrosefarm (Dec 11, 2011)

So, I'm having trouble with my "new" 88 HB Z24 and I pulled the seat to read the codes on the ECM. On my 86.5 the red light flashes, then the green. On the 88, both red and green flash together. 1 slow flash, 2 slow flashes, then 3 fast flashes, 4 fast flashes and 5 fast flashes and it repeats. I can't make heads or tails out of it. Any ideas?


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## SPEEDO (Jun 9, 2003)

there is a sticky on how to read the codes etc (above)


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## TheRepairMan (Jun 30, 2009)

SPEEDO said:


> there is a sticky on how to read the codes etc (above)



That "sticky" has become a jumbled up mess since there's no way to edit earlier posts. Plus, the most important link in post #1 no longer works, so... I'm going to make yet another attempt to clarify the code reading mystery. My five highlited steps to follow below, and the ECU's five diagnostic modes, are just a coincidence and have nothing in common with each other. Just think "mode 3" and follow the five steps below.
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Upon first clockwise turning the ECU diagnostic POT switch, the ECU's LEDs will flash FIVE "diagnostic modes" in those first 5 sets or pairs, of red and green simultaneous flashes. Diagnostic *codes* are *displayed in "mode 3" only*. Selected from the third set of dual flashes. Read on.

Note:
I suggest you read and understand this whole process before you start going through the steps, then proceed. I'll break this down into five easy steps to print off to have with you during the process of accessing the modes. *Mode 3, self diagnostics*, is the one you want to select to read all stored diagnostic codes. The other 4 modes serve other functions, as are explained in the Nissan manual, and in the sticky at the top of the HB truck board. You MUST select a diagnostic mode by moving the ECU switch. The screw type rotates right and left about 3/4th of a turn. Don't force this switch beyond its stops, as these POT switches inside the ECU have been known to break! 


*1st step*... Make sure the vehicle's battery has been, and is, in a good state of charge. Gain clear access to easily see the ECU, its LEDs, and the mode switch. Get a small straight screw driver handy that fits gently into the ECU's diagnostic mode, potentiometer type, switch slot. 

*Step 2*, turn the ignition key to the "run" position. The ECU diagnostic mode switch, beside the LEDs, should be all the way to the left to begin.

Okay, now start entering mode 3 for diagnostic code reading. Here's how.

*Step 3*, rotate the ECU diagnostic mode switch clockwise, all the way to the right and wait for the dual flashes to count, 1, 2, 3, 4, 5. Just count them all at first so you will know how much time you have to react between the 5 "mode" flashes. These will continue to repeat to give you time to select the correct mode.

*Step 4*, Immediately after you see 3 dual red/green simultaneous flashes, quickly, but gently, rotate the ECU switch counter-clockwise, all the way back to the left. That puts you in mode 3, the diagnostic mode to read stored codes.

*Step 5*, take careful note of the exact number of individual red and green flashes. Early codes consist of two digits, and the ECU will start codes over until all are displayed. Once noted, just turn off the ign. key.



Now, return to the forum with your findings to determine the code explanations. 
(For code definitions see one of my last posts in the "diagnostic code sticky" at top of the HB truck forum.) 
Move forward with repairs as needed. 

(As always, here on this forum, any editing or amendments to the information in this post will have to be done in later posts as editing isn't an option after a period of time. Maybe the admin here will change that someday if we make enough noise.)

-Roger

Addendum: The code reading info above only applies to OBD I, and OBD 1.5, late 1986, early 1987, through 1994, 1995 US model trucks ...as far as I can determine. Other exceptions may apply.


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## yellowrosefarm (Dec 11, 2011)

On this 88 ECM and the 86.5 ECM I have there is no screw to turn for different modes. There is only an on/off switch. The factory service manual I have for the 86.5 makes no mention of different modes. The number on my 88 ECM is MECS-G340


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## TheRepairMan (Jun 30, 2009)

yellowrosefarm said:


> On this 88 ECM and the 86.5 ECM I have there is no screw to turn for different modes. There is only an on/off switch. The factory service manual I have for the 86.5 makes no mention of different modes. The number on my 88 ECM is MECS-G340


Should work the same way. Just follow the steps above and insert your switch to on when to the right, or clockwise is stated, and off when left, or counter clockwise is needed. Your manual "should" have the instructions spelled out also. Keep us posted.

-R


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## yellowrosefarm (Dec 11, 2011)

I did actually try that with the on'off switch but nothing I do changes both lights flashing together. Every place I have looked says the red should flash, then the green. Count the number of red flashes, then count the number of green. These flash together and always the same. 1 long, 2 longs, 3 short, 4 short, 5 short, repeat. That's why I'm thinking trouble inside the ECM.


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## TheRepairMan (Jun 30, 2009)

yellowrosefarm said:


> I did actually try that with the on'off switch but nothing I do changes both lights flashing together. Every place I have looked says the red should flash, then the green. Count the number of red flashes, then count the number of green. These flash together and always the same. 1 long, 2 longs, 3 short, 4 short, 5 short, repeat. That's why I'm thinking trouble inside the ECM.


Yup, sounds broke! The switch should be easily replaceable by any car stereo or electronics repair shop... if you can find someone willing to do it. If a new switch doesn't fix it you have to find an ECU or live with it. 
-


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## yellowrosefarm (Dec 11, 2011)

Finally got it work. I just wasn't waiting long enough. Switch on, wait for 3 flashes of red and green together, switch off, wait, wait, wait,wait,wait, then code flashes begin.


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