Has anybody attempted to repair their ECU?

[FastWoman]

May the electromotive force be with you!

[Zed Head]

Curious as to what your plan is. Maybe some small adjustable pots in place of resistors, for tuning purposes?

I have a 5K taper potentiometer in my car and it is a little touchy, but I've also noticed that the performance seems to change occasionally for no obvious reason. Maybe with tanks of fuel, but hard to tell. I drive my car almost every day and can go a tank or two with out touching the knob, but occasionally I have to add a little fuel to cover a lean spot, or reduce it to change the smell of the exhaust. Who knows, it might also be ECU temperature itself causing some drift, but something is going on.

[240260280z]

Curious as to what your plan is. Maybe some small adjustable pots in place of resistors, for tuning purposes?

I have a 5K taper potentiometer in my car and it is a little touchy, but I've also noticed that the performance seems to change occasionally for no obvious reason. Maybe with tanks of fuel, but hard to tell. I drive my car almost every day and can go a tank or two with out touching the knob, but occasionally I have to add a little fuel to cover a lean spot, or reduce it to change the smell of the exhaust. Who knows, it might also be ECU temperature itself causing some drift, but something is going on.

I think it is the size of the pot and vibration. The nominal sensor resistance at operating temperature is 325 ohms. A 1% change in your pot (due to under-hood temperature changes and/or vibration) would be 50 ohms so it will change the nominal by ~ 17%.... this is not a linear system however you can eliminate mechanical instability by using resistors and a smaller values pot.

Now can you imagine what building a rocket takes given the vibrations and thermal changes!

Edited June 13, 2014 by Blue

[Zed Head]

Actually, I am running now with the pot either completely open (knob stopped out) or a slight resistance. The tension on the knob is high, it's vibration resistant. And the pot is behind the hood release handle in the cabin, so ranges from 60 - 80 F probably. The changes are from outside the pot, I believe, either fuel or 1970's components.

Always ready for a new experiment though. Interested to see what happens.

[sscanf]

My plan is to first of all figure out if the board I bought get works at all. I'll give it a quick look over then plug it in and see what happens. Then I was going to try to figure out what (if anything) has drifted on the board by checking components based on the schematics/markings. If I have to pull a leg on a resistor to check it, I'll probably just replace it if I have it on hand (I have a pretty good stock). I may just wind up shotgunning it a section at a time if I have the components on hand. The transistors may be harder to find. If the board works, the transistors are probably ok anyway.

I guess the question I want to answer is: Why does the series pot on the temp sensor make things better? Has the board changed or has something else changed? The only something else I think has changed is the gas (though I suppose the air has changed a bit - more CO2 ;-)... So my goal is to see if replacing drifted components on the board eliminate the need for the pot.

[240260280z]

It would be interesting to see if capacitors have changed much from their stated values in the last 40 years.

For understanding why the tweaking resistor works you should dig into the 75-83 FSM's fuel sections as well as reading forums and websites on "AFM" and "LJetronic"

[240260280z]

I have seen books on pulse shaping circuits years ago. You may wish to look into this area as well.

[Captain Obvious]

The transistors are easy. They're your garden variety 2N and 2S stuff. Problem will be that the whole box was "tuned" on an ATE fixture at the factory with those piggyback resistors. Who knows what parameters of those transistors have been characterized in that way. Gain, leakage voltage drops... Who knows!

And there's the three IC's. Custom parts with zero documentation made out of unobtanium. :-)

Good luck. We're all counting on you.

PS - Haha!!! Hope you don't mind if I steal that one!

May the electromotive force be with you!

[sscanf]

OK. Well, don't hold your breath, I will take a shot at it. Sad story about those custom IC's, we'll see. But, I have a little news: the "untested" ECU arrived and at first glance it looked untampered with so I plugged it in and it was actually initially promising in that it tried to start. I think I got some action from the cold start valve but that was it, nothing after that.

So last night I look at the boards and found this:

I have not tested it yet but its probably open. Looks like a diode but its hard to tell and the board markings are obscured by the crud. Hopefully tonight I will get it off and scrape off the crud and figure out what it is and replace it from some bench stock. Maybe do some sanity checks on the components in that area of the board along with those big T03 transistors.

So, I have some repair to do before I can attempt a repair to the drift. I have not looked all the way through the files from the "Is the 280Z FI a Good System" thread. Does a parts list exist in there or am I left to experimentation?

Maybe this unit suffered an infant mortality event and is otherwise nearly new. Wishful thinking, I know. I think it would be very interesting to observe the differences between an NOS ECU and a drifted one. I do have a scope and will use it if provoked.

[240260280z]

Sadly there is no parts list.

If you sir, felt the pioneering spirit, you could make a spreadsheet with component annotation and value, that would be a good start to reverse understanding the circuitry and useful for others making the trek.

[Captain Obvious]

So last night I look at the boards and found this:

Woof. That's not good... That used to be diode D190 and based on where it's located in the circuit schematic, I'm guessing it's a Schottky.

Supposed to look like this:

It's the main power feed into the ECU and the input side is hot any time the fuel injection is operating. I'm thinking reverse polarity protection for the whole shebang because it's in right at the supply input and feeds just about everything.

So... Yours passed way too much current and went up in smoke. Problem is though... What was it that was demanding so much current that the diode gave up? The diode probably isn't the underlying problem, it's probably just the result of the underlying problem.

[sscanf]

I figured that the blown diode was the result of a short somewhere and did some probing in the area for shorts. The first thing I checked were the big resistors and transistors (with resistance and diode tests). All seems ok. I took a chance and threw a 1n005 in that spot but I got no joy (and the new diode did not explode). I'll go after it over the weekend with that schematic in hand. I guess I'll be needing a Schottky.

One interesting observation though: It ran for about 2 seconds when I first plugged it in. Just like before. It seems like the motor will run for 2 seconds with NO ECM because that is pretty much what I had when I first plugged this in the other day. Either from residual fuel from the last time it was plugged in or somehow the cold start valve is independently energized during start.