key in start position and no CEL - bad ecu maybe

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kendem
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Car: 1991 nissan 240sx

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So it is internal in the ecu?


kendem
Posts: 62
Joined: Wed Sep 05, 2007 7:58 am
Car: 1991 nissan 240sx

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MY CAR STARTED after i jumped pin 4 to ground. The check engine light came on and stayed on . I shut off the engine and restarted 5 times. Does this mean the issue is resolved?

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VStar650CL
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Hmm, I wonder if there isn't some crud on that old ECM circuit board. I can't imagine any other way you could "jumpstart" a transistor, but apparently that's what you did. I think I'd crack open the box and hit all the electronics with some Deoxit. Great to hear you got it running, though, and yes, as long as it keeps starting, I'd say you can call it resolved. Cleaning the board should help assure that it stays that way. :)

kendem
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Car: 1991 nissan 240sx

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I gotta hand it to you, you really know your stuff. Are/were you a mechanic? I never heard of deoxit. I'm worried that if i open the ecu it may not work after. But I'll take your advice and do it, you have been dead on correct about everything else.

Thanks again for all the help!

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VStar650CL
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I'm a Nissan Master Tech with electrical specialty, I just don't advertise it. The Black Belt wraps his ghi with the White one outside the dojo, I've always felt like humility matters.

The tops on those old ECU's pretty much all just unscrew and pop off, so it's generally no big deal to expose the PCB. Deoxit is the absolute bomb for de-crudding old or corroded electronics. It's expensive compared to CRC and some other contact cleaners, but it's worth the extra when you really need clean to mean clean. It's my go-to for flooded terminal contacts that nothing else will clean up (and believe me, I handle lots of flood cars). You can ask any insurance adjuster that our dealership works with, we routinely save stuff from the junkyard that nobody thinks can be saved. Deoxit and copious di-grease play a large part in that.

kendem
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Car: 1991 nissan 240sx

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Impressive resume. I was going to pick up some crc today at orileys because I wanted to finish this today. I would have to order the deoxit from amazon. Since my car was not flooded do you think crc would be sufficent? I suppose you can use this cleaner on terminal connections also? My F8 to E9 connector was pretty dirty and was a b**** to seperate.
I'd like to replace the connector on my distributor that's in the worst shape of all, but don't think I would find one. BTW where are you located? I'm here in SOCAL.

kendem
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Car: 1991 nissan 240sx

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This morning I turned to key to start and the engine light came on and car started. Great, nothing changed since last night.

I then disconnected the negative battery terminal, waited a minute then reconnected it then put the key in on position and NO check engine light. Car wouldn't start.

I checked pin 4 (key still in on position) and it had 12v so then I grounded pin 4 and heard the essc relay click and the check engine check light came back on and was able to start the car.

So do you believe this is an internal issue with the ecu?

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VStar650CL
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Yah, could be a solder or crud issue, but It might also be a poor connection at pin 4. I was thinking about that, and if the transistor isn't biased internally with a pullup resistor, then it can only be biased through the coil connection. If that's crappy and the transistor is a BJT type, it might very well need some sort of "jumpstart" because of insufficient bias voltage. Even if you don't crack open the ECM, I think the first thing I'd try is Deoxit on both sides of the ECM connector.

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VStar650CL
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kendem wrote:
Sat May 21, 2022 9:03 am
Impressive resume. I was going to pick up some crc today at orileys because I wanted to finish this today. I would have to order the deoxit from amazon. Since my car was not flooded do you think crc would be sufficent? I suppose you can use this cleaner on terminal connections also? My F8 to E9 connector was pretty dirty and was a b**** to seperate.
I'd like to replace the connector on my distributor that's in the worst shape of all, but don't think I would find one. BTW where are you located? I'm here in SOCAL.
I'm in Arkansas, that would be quite a haul for you. The CRC stuff is way better than nothing, just not as good. Most of the wires through F8/E9 are pretty critical, so yes, definitely clean those up too. Helpful hint with crappy old connector shells, douse the whole connector with WD40 or PB Blaster before trying to separate it. When they're sticky it's usually because of mud and crud underneath the release pawl, and a little lube will usually make them come apart much easier.

kendem
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Car: 1991 nissan 240sx

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Will do. Thanks again!

kendem
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Car: 1991 nissan 240sx

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Well yesterday i decided to take a drive and made it a quarter mile before the car died, the same fuseable link blew. Had to get towed again. I did not clean the ecu with deoxit yet but was going to as i ordered it from amazon.

My wife's been bugging me to get myself a new car for years now. Maybe she's right, I'm 65 with a bad lower back. Been fixing these old cars for over 3 decades, have an 89 civic also. I.just can't let go of these old clunkers.

I bought my wife a new 2018 hyundal elantra gt 4 years ago and i made a promise to myself the dealer will do ALL the maintenance even if the wiper blades need replacing.

So after yesterday, convinced the 240 is headed to the salvage yard, this morning i'm already thinking of what else can i d to fix it, maybe there is a resistor burnt-out somewhere, maybe in the ecu? Or maybe i psyco therapy

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VStar650CL
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I believe that fusible link only feeds the ECM and some sensors. Tracing shorts isn't necessarily for the faint of heart, but if you want to try to track it down I'm here for you.

kendem
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Joined: Wed Sep 05, 2007 7:58 am
Car: 1991 nissan 240sx

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I was thinking of connecting a 25a circuit breaker in place of the fuseable link. Then while the engine is running see if i can't cause the amp to trip by moving some of the wires on the circuit. Looks like the injectors, coolant switch, cps, tps, maf could cause the circuit to ground out.

Looking at the wiring diagram i don't understand how pin 4 gets grounded. Like i said before I'm not too good at wiring diagrams

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VStar650CL
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This is as simple as I can make it: There's a transistor inside the ECM which is connected to pin 4 on one side and ground on the other. The transistor isn't shown on the wiring diagram, but when the ECM activates the base (trigger) of the transistor, the transistor acts like a switch turning on and conducts whatever is connected to pin 4 to ground. In this case, that's the coil of the ECCS Relay. Since the other side of the relay coil has 12V from the battery, whenever pin 4 supplies it ground, the relay coil conducts and the relay switches on. The reason it's arranged that way is so the ECM can "wake itself up" to do vacuum testing on the fuel tank when the car is parked, even though the ignition is off. But it's also supposed to wake itself up immediately whenever it sees the key turn on. That's what it wasn't doing before you jumpstarted it.

I did a writeup for Nico about how to chase short circuits which you may find useful. In this case you might end up doing segmented fusing like in Part 3 to get to the bottom of it, since it's obviously intermittent and the short could be a dying component and not just wiring.
post6818947.html
post6819005.html
post6819207.html

kendem
Posts: 62
Joined: Wed Sep 05, 2007 7:58 am
Car: 1991 nissan 240sx

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Incredible write up. I'm studying article one and may need to reread 4 or 5 more times. Just wish i had more time to spend on this, however i am retiring in December.


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