BenFenner

I'm pretty sure I'm dealing with a fried ECU here on my brother's new 1998 Civic (automatic). Long story. I'm used to dealing with Nissans. I've been diagnosing electrical gremlins all weekend and so far have managed to find oodles of information by searching this forum. This is the first thread I found necessary to make.

If anyone has a 1996 - 1998 Honda Civic ECU can you test for continuity between pins A11/A24 and pins D11/D12. Pins A11 and A24 are power (to ECU and injectors), and pins D11 and D12 are sensor grounds. I don't think they should be connected, but on my ECU they are. Can anyone confirm that they shouldn't be connected internally?

BenFenner

If you have a 1996 - 1998 Civic and don't want to pull out the ECU to help me, just check to see if the right-side wires (yellow with black stripe) on the injectors have continuity with ground. If yours don't, then my ECU is fried.
Thanks!

BenFenner

With the ECU plugged in and key off I'm seeing continuity like this.

Positive battery pole -> 15 Amp fuse in engine bay -> Pin 1 on main relay







Once I turn the key to the run position I get continuity like this.

Positive battery pole -> 15 Amp fuse in engine bay -> Pin 1 on main relay -> Pin 3 on main relay -> ECU Pins A11/A24, all four injectors, and ground (through the ECU it seems).
Which immediately blows the 15 Amp fuse in the engine bay.





With the key off, and the ECU connected, I've got continuity between the injector "power" wires and ground (through the ECU to the sensor grounds I believe).
Not cool.
If I disconnect the ECU I don't have continuity between the injector "power" wires and ground any longer.

Former User

What specific problems are you having?

Former User

Does 15A hood ECU fuse 44 still blow if you unplug the fuel injectors and the IACV?

Does 15A hood ECU fuse 44 still blow if you unplug ECU connector A?

BenFenner

Thanks for the reply RonJ.

I've packed everything up for the night. I've spent 16 hours this weekend diagnosing the craziest electrical problems on this lemon.
I've unplugged the ECU connector "A" and the injector harness is no longer grounded, so I'm going to take a guess and say I don't think the fuse will blow if I try that. However, I will try that tomorrow after work to be sure.

I will also try unplugging the injectors and the IACV and see how the fuse does then. I'm guessing it'll still blow but I will check to make sure. Now I just need to research and find where the IACV is on this engine. =/

Former User

If the fuse still blows with ECU connector A unplugged, then the short is not coming from the ECU.

IACV location:

Is the Civic a CX, DX, LX, EX, or HX? Does it have a manual or auto transmission?

BenFenner

By the way, I've looked the main relay over and there are no hairline cracks in the solder joints, but I went ahead and re-soldered them all anyway. I've done a couple little tests on it to see if it's working properly but I guess it's time I do the full test procedure. I'll let you know how that goes too.

Former User

First do the component unplug tests I mentioned as the results will help you focus your troubleshooting effort.

BenFenner

Yep, I'm following you there. It would be nice to know the ECU is still good.


Okay I know you don't want to hear this, but I have no clue if it is a CX, DX, LX, EX, or HX. It has an automatic transmission. And I'll just take a picture of the engine bay and post it up here tomorrow and I'm sure you'll know in a second what model it is. =D
I think it has a 1.6L SOHC engine, but really that's just a shot in the dark. The spark plug wires come from the side of the engine and the valve cover doesn't have the tell-tale double hump like on DOHC engines.

Will do. I appreciate all the help. I'm quite competent with electronic and mechanical systems; I'm just a total Honda noob. I have the FSM (PDF) and the wiring diagrams have kept me from needing much help until now. I won't be able to work on the car again until tomorrow afternoon so expect updates then.

Former User

The IACV for automatic transmission Civics is attached to the throttle body. You'll recognize the IACV by it having two coolant hoses running to it.

BenFenner

Excellent. Thank you.

BenFenner

Monday update.

No, if I unplug ECU connector A and turn the key to run the fuse doesn't blow.

Yes. With the ECU fully connected and the injectors/IACV disconnected the fuse still blows.

While I have a little light left tonight I'm going to try leaving the ECU connector plug A disconnected and ground the main relay pins 8 and 2 to see if I get voltage to the fuel pump (the fuel pump has been disconnected this whole time.)

Any other suggestions?
I'm leaning heavily towards a bad ECU (the car drove as recently as last week).

Former User

The short is almost certainly in the ECU. I'd remove the ECU and carefully inspect it for signs of an electrical short. Do you have access to a compatible known good ECU for testing purposes?

BenFenner

Grounding pin #2 of the main relay and putting the key to RUN, then grounding pin #8 gets me 12 volts* at the fuel pump and a nice click from the main relay. I have to agree, the ECU seems toast.

I've opened the ECU and can see no signs of faulty capacitors, resistors or the like, however this is where my expertise sort of falls down. I'm no PCB expert. =/

I do not have a compatible ECU to use for testing, but obviously getting one is my next step. How much do they sell for used around here? I know our Sentra ECUs go for around $50 -$75 but we don't have the large Hondata market pulling prices for OEM ECUs up, but nor do we have the volume numbers pulling prices down.

I might also have to head to the local yard to try to source an ECU although I'd prefer to buy a known good one from this forum. (Searching the classifieds is a bit daunting for me right now though as I don't know the terminology or even what kind of model I have.)



I believe the recent car history goes like this:

Previous owner had trouble with the SRS Unit failing which also caused the ECU to fail. They finally found out the issue and just left all of the burnt out fused for the SRS system either missing or in the fuse box (this is how we bought the car). Then they got a replacement ECU and stuck it in there and sold it running.

Before taking the car home, in an effort to get the headlights working I took one blown fuse (find out later it is the SRS Unit fuse # 23) and replace it hoping for the best. Eventually this allows the SRS Unit to fry the current ECU (again?) and leave my brother stranded. Get the car towed to my place and I find tons of fuses missing. In an effort to fix things (head lights, turn signals, fuel pump not working) I toss in all the fuses the car needs and immediately blow 3 or 4 of them relating to the SRS system and the fuel pump. After a lot of diagnosis I believe I narrowed the problem down to a failed SRS Unit so I left it disconnected and started putting fuses back one by one and checking for problems by turning the key to ACC and RUN and back. Once I get to and put in fuse # 13 for the fuel pump I noticed the problem. Once that fuse is in, the fuse # 44 under the hood blows and that got me on my hunt through the main relay and such for problems, which left me where I am now with what seems to be a bad ECU.

I'm just hoping that if I do get another ECU in there it will work, and I'll just have to leave the SRS system inoperable, or source a known good one of those.

Sorry for the novel, just trying to give some background and hoping someone can confirm the theory sounds at least plausible. I'd hate to burn up another ECU.



*Actually 13.65 volts. See post #23 below.

Former User

Nice job troubleshooting complicated electrical problems. Again, the ECU is almost surely the source of the short. You may want to post WTB ads for both the ECU and SRS unit in the Marketplace.

BenFenner

Thanks for all the help and the words of encouragement Ron.
I'll just take the VIN and find out what model I have.

Just to be sure, this is the correct place to post a wanted thread?
http://www.hondamarketplace.com/forumdisplay.php?f=68

Former User

Yep. That's where you should create a Want To Buy ad for your ECU (and SRS unit). Be sure to post the model numbers. A clear picture of the units may also be helpful.

By the way, the fact that both computers got fried suggests that the car had a large voltage surge or was in a flood. I say this because the ECU and SRS unit are separate components. I don't see how frying one would fry the other.

BenFenner

By the looks of the car, it could have been a soda flood, and also maybe it was in a sand storm. Or all the tar from smoking. This thing has seen better days. However inside the SRS unit and ECU it looked super clean. =(

Thanks for the tips on taking pics of what I need. I'll do that.

They are completely separate units, but they are connected by fuse #13 which feeds Pin #7 on the SRS unit and Pin #5 on the main relay which eventually goes through Pin #2 of the main relay to the ECU (always grounded) and Pin #7 of the main relay to the fuel pump. So they are connected in a round about way. Maybe a huge power surge. Maybe that's why the head lights aren't working. =P

Now that I think about it, I'll have to check the fuel pump to see if it has the proper resistance just in case.

Former User

I'd buy into this theory if the fuse 13 circuit had the short, but it doesn't. Instead it's the fuse 44 circuit with the short. So I struggle to see the fuse 13 connection, though I guess anything is possible.

BenFenner

Okay I take that back, there is a diode in the main relay that should isolate the ECU from the SRS Unit...

Oh, and Fuse 13 certainly had a short at some point. It was DOA when the car was towed to my place.

*sigh*

Electronic gremlins. =(

Former User

Good luck with the fixes Ben.

BenFenner

I think the battery, battery voltage, or maybe alternator (bad regulator?) is the source of the dead ECU and SRS unit.
Should I make a new thread about this, or continue on here?
Battery voltage with engine off is actually 13.65 volts, which I'm finding out is WAY too high. Not high enough to kill components on it's own, but maybe it is a sign of an over-active alternator? Or just combined with normal charging it is too much voltage?

BenFenner

I guess I'll just have to replace the battery and/or take the alternator to the local parts store to have it checked out. That way I'll be sure to avoid any surges those items may have caused in the future.

Former User

Is your battery voltage reading with or without the engine running?