How to test if my ECU is bad in my 89 240SX??

[Prelude Guy]

Hey all, I used to post about a month ago or so about my 89 240 bogging. I haven't been able to test the car out since because i just finished swapping a JDM H22 VTEC motor into my Prelude.

Anyway, I want to start troubleshooting the 240 now. I was kind of concerned that I have not thrown ANY codes since troubleshooting the car. I have disconnected the O2 sensor. I checked the ECU and it did not have any CEL codes stored. I have unplugged the MAF and TPS sensor too. No codes.

I should throw a code for one of these parts, shouldn't I??? How else can I test the ECU??

My car runs and starts up. It idles a little rough...ranging from 700-1300rpms. I am the one who used to post about having a major bogging issue if I pushed the gas pedal down more than 15% of the way. If I push down less than 15% of full throttle, the car drives fine. However, trying to get on the highway is where the problem occurs. It happens in any gear too.

I've changed the O2 sensor, plugs, wires, dist. cap, rotor, fuel pump, and ground wire. My battery light is almost fully lit at idle but will slowly dim out as I rev the motor. However, as I get raise the rpms higher and higher, the car will start to cut in and out a little and the battery light will start blinking as it starts cutting out.

The car starts up as soon as I crank the starter. I am amazed that it does this, even after I let it sit for about a month. However, it doesn't run right. VERY strange.

The car is running better since I bought it, however, I am trying to get it to run as it should so I can drive it during the Winter.

Let me know what you think.

[F3600]

Hi !

My 240sx CA18DET is doing the same thing on me for about one month ... its look like a a vacuum leak but there none... today i took my alternator to get it tested and it was not charging at low rpm.. im pretty sure that the alternator is big chance of being the problem cuz everytime an alternator is going bad there some funky problem affecting all the car ...

When i reinstall my rebuilt, im gonna reply you to tell ya if the problem disapeared

good luck

[Prelude Guy]

Yes, that would be AWESOME!!

Please let me know what you discover.

Andy

[F3600]

Well it was my alternator ! I took it out and got it tested and rebuilted. I installed it tonight , with two big ground cable. One from the head to the body and one from the intake manifold to the battery holderplate , where the ground cable bolt to the chassis.

Now my car is running better than before... (it was barely driveable before that)

look your voltage at the batterie should be at 13 14 volt at idle... if your not sure just took it out and get it tested at a electrical motor shop.. !

good luck buddy !

Ps for the ecu : turn your car key at on

check the two led back the ecu, if there are lit, it should be okay. If there no code, its because something that is not checked by the ecu is wrong.. (like the alternator). I know that sometime the ecu doesnt report part that are disconnected . My o2 sensor is not connected and im getting code 55.

Anyway good luck ! keep reply some news . I want to know if you got it solved.. ! ( took me one month for me .. too long.. )

[Prelude Guy]

AWESOME, MAN!!! You just gave me some hope.

I am going to have the alternator tested tomorrow or Wed.

I'll post back with the results.

I've had the car for over 2 months. It would be SOOOO awesome if I can get this car running correctly. I bought it thinking that the engine was shot. It would be a FANTASTIC surprise to get this car runnung as it should without having to replace the motor. I would like to drive it during the Winter as a daily driver.

I'll post back.

Andy

Well it was my alternator ! I took it out and got it tested and rebuilted. I installed it tonight , with two big ground cable. One from the head to the body and one from the intake manifold to the battery holderplate , where the ground cable bolt to the chassis.

Now my car is running better than before... (it was barely driveable before that)

look your voltage at the batterie should be at 13 14 volt at idle... if your not sure just took it out and get it tested at a electrical motor shop.. !

good luck buddy !

Ps for the ecu : turn your car key at on

check the two led back the ecu, if there are lit, it should be okay. If there no code, its because something that is not checked by the ecu is wrong.. (like the alternator). I know that sometime the ecu doesnt report part that are disconnected . My o2 sensor is not connected and im getting code 55.

Anyway good luck ! keep reply some news . I want to know if you got it solved.. ! ( took me one month for me .. too long.. )

[Prelude Guy]

F3600,

How hard is it to uninstall and reinstall the alternator??

Do you have any simple instructions of how to do this?

How long should i expect to complete this project??

Thanks,

Andy

[ricebike]

hi, there's an online zipped file of the 240sx called "90FSM" floating around...

it is in pdf format; i'm scanning the pages now...agh, can't find a page where it gives a step-by-step procedure to it... guess it was too simple for a write-up

basically have to get underneath the car too & may haveta remove the PS & AC belts (unless u are just gonna replace the alt & not the belt) etc...

oor just goto your local autozone/ advance autoparts store for a free on-car battery & alternator test since it's still driveable

[DAEDALUS]

Why not just check the voltage on the battery? Car off, check voltage, should be above 12.0V. Start car, voltage should be close to 13V, maybe even ~14V. Turn on all electricals...lights, radio, AC/blower high, etc and measure. Should still be close to 13-14V, maybe .1 or .2 lower than the last reading. Also, on a good battery voltage should stay above 10V after cranking for 15 seconds (remove fuel pump fuse).

[tiger]

hey. I was thinking that maybe my alternator is bad.... I just got a new battery and it's just seeming to kill it so fast. My car is running like ***... unstable idle.. the last code it threw was for the MAF and then I got a new MAF and installed it and it is still running crappy... could the alternator be messing with the voltage to the MAFs causing it to run crappy? The car isn't driveable... I had to hook jumper cables to it and when I tried to start it it ran ok for a sec and soon as i took my foot off the gas pedal it straight died. Someone help please. Thanks