1G How to tell if ECU is bad??

[kc92awd]

1992 AWD 5-speed. Won't start. Getting no spark. Getting no signal to injectors. Pump is working and fuel goes through rail.

I pulled the ECU and opened it up. I can't see anything too bad with it. There is some residue that looks like dried battery acid in one area. That could be a capacitor leak I guess. Is there a way to have the ecu tested? What exactly do leaky caps or burnt ECU's look like? Would it be obvious?

 

[oldman]

If you can see acid on the board, it's probably cooked. I think when you turn the key on, the check engine light should come on for couple of seconds. If not, the ecu is not turnning on....something like that. steve will probably come on soon and give you more details. pm him, he can probably fix it for you as well.

 

[kc92awd]

who is steve and what is his user name so I can pm him? he fixes ecu's?

 

[oldman]

umm..."steve"

 

[steve]

There are pictures of various ECUs with capacitor leakage on my home page.

Most places that fix ECUs will also check yours out for the cost of postage.

Steve

 

[kc92awd]

i was looking at those and most of them look burnt black. mine just has a small patch of clearish/white dried residue that resembles dried battery acid. But there is no black "burnt" look.

the car only has 63K miles and I can't smell a burning smell when the car is cranking. The ecu also doesn't smell burnt. The car won't start at all though.

 

[way2slow4g63]

another way is to smell it if it smells fishy it no good

 

[steve]

There are lots of other things that can cause the car not to start. The ECU is a common cause because of the history of the caps leaking. The electrolyte leakage usually turns the metal parts of the ECU it comes in contact with black from corrosion. A white spot sounds more like something got on the waxy conformal coating and dried.

The fuel pump isn't driven by the ECU until the engine starts, it's powered by the starter circuit to begin with. If the ECU is powering up and running it's software it will turn the CEL light on for a few seconds when you first turn the key on. It can boot and still be bad but things like a bad CAS, Engine Coolant Sensor or blown fuses can also keep the engine from starting.

Steve

 

[kc92awd]

thanks for all the help. Would either the CAS or coolant temp sensor going bad prevent injectors and spark from working?

 

[Ragin_TSI]

To obtain an output of diagnostic codes:
1 verify battery voltage isabove 11 volts
2 the throttle is fully closed
3 the transaxle is in neutral
4connect an analog VM between terminal numbers 12 and 1 of the data link connector under the driver side dash, heres a diagram
1 _
_ _
_ _
|_
|_
_ _
_ 12

Observe the sweeps of the needle for the code
5 turn ignition on but don't crank
read code

the ecu is bad if: code is short 0 volt then a long 1 volt reading

Buy a Haynes Repair manual for all the codes Manual number 68030

Hope this helps

 

[Ragin_TSI]

If your caps are bad you can buy them from http://www.machv.com/ecucaprepkit.html

 

[kc92awd]

To obtain an output of diagnostic codes:
1 verify battery voltage isabove 11 volts
2 the throttle is fully closed
3 the transaxle is in neutral
4connect an analog VM between terminal numbers 12 and 1 of the data link connector under the driver side dash, heres a diagram
1 _
_ _
_ _
|_
|_
_ _
_ 12

Observe the sweeps of the needle for the code
5 turn ignition on but don't crank
read code

the ecu is bad if: code is short 0 volt then a long 1 volt reading

Buy a Haynes Repair manual for all the codes Manual number 68030

Hope this helps

I couldn't get the volt meter to do anything. What setting should it be on? DC 15?
I also plugged in datalogger with MMCD. Just reads ECU communication error.

 

[kc92awd]

There are lots of other things that can cause the car not to start. The ECU is a common cause because of the history of the caps leaking. The electrolyte leakage usually turns the metal parts of the ECU it comes in contact with black from corrosion. A white spot sounds more like something got on the waxy conformal coating and dried.

The fuel pump isn't driven by the ECU until the engine starts, it's powered by the starter circuit to begin with. If the ECU is powering up and running it's software it will turn the CEL light on for a few seconds when you first turn the key on. It can boot and still be bad but things like a bad CAS, Engine Coolant Sensor or blown fuses can also keep the engine from starting.

Steve

Steve,
Thank you for all the help. I really appreciate it. I do not get a CEL light at all. I get some other lights, but no CEL.

So, either the ecu is bad, or it is not getting power? Where do I go from here? How can I test what the problem is?

 

[steve]

First you check the MPI fuse on the positive battery terminal and the room fuse #19 in the fusebox by your left foot.
Then you need access to the ECU to check that it receives power from the MPI relay when the ignition is turned on.

Steve

 

[kc92awd]

The fuses are both fine. I have access to the ecu. Can you please explain how to check and see if it receives power from the MPI relay when the ignition is turned on?

I have been searching like crazy and I found someone who hooked a fused wire up to the power wires on the ecu. What fuse should I use (amps)?

 

[kc92awd]

Steve,
I found some of your old posts. Here is what I've got:

ECU pin 103 always has power.
MPI pin 10 always has power.
When I ground MPI pin 8, ECU pins 102 and 107 have power.
When car is on, ECU pin 110 has power.

Does this confirm bad ECU because it won't turn on even though it is getting power from 102 and 107 when MPI pin 8 is grounded?

Or, I suppose it could be a problem with MPI pin 8 grounding when it should. What does the wire to pin 8 do? Where does it go?

Thanks,
Phillip

 

[kc92awd]

IT STARTED!!! IT STARTED!! IT STARTED!!!

Okay, after buying this car 3 months ago, I finally got it to start. I kept the MPI pin 8 grounded and then I plugged in the ECU and it freaking started!

So I still have a problem. Why is the pin 8 not grounding when it should? I had to keep it grounded with an extra wire to start the car.

 

[steve]

Something has damaged the circuit in the ECU that watches for pin 110 going high from the igniton switch and pulls pin 63 low to switch on the MPI relay and power the ECU up. I've seen a few ECUs where the output transistor for pin 63 on IC 110 has been blown from either the coil in the relay going bad, back EMF from the coil breaking it down, or someone shorting it to +12.

Since the car starts and runs with ECU pin 63 (MPI Relay pin 8) pulled low it sounds like ECU could be repaired.

Steve

 

[kc92awd]

#1) Is there anything wrong with running it for a little bit with MPI pin 8 grounded? I installed a clutch while it wasn't running and I'd like to make sure everything else is working.

#2) Can I send the ECU in to you for repair? How long does that take?

#3) You mentioned either "a coil in the relay going bad, back EMF from the coil breaking it down, or someone shorting it to +12." If I get the ECU fixed, will it just go bad again from one of these issues?

 

[steve]

#1) Is there anything wrong with running it for a little bit with MPI pin 8 grounded? I installed a clutch while it wasn't running and I'd like to make sure everything else is working.

It's hard on your battery since this will leave the ECU up and running as well as power the MAF, ISC, CAS, Solenoids, and injectors all the time.

#2) Can I send the ECU in to you for repair? How long does that take?

Send me an email.

#3) You mentioned either "a coil in the relay going bad, back EMF from the coil breaking it down, or someone shorting it to +12." If I get the ECU fixed, will it just go bad again from one of these issues?

If that's the problem, I'm not sure exactly what is causing it, just speculating on possible causes. On those that I've fixed where the transistor failed, no one has reported a repeat failure yet. Until we know for sure what is causing the problem I can't say if it's internal or external to the ECU or what the chances of another failure are.

Steve

 

[kc92awd]

I guess I don't understand what's going on between the ECU and MPI relay. If I ground MPI pin 8, the car starts. Why not just have a switch from the factory that grounds it when the car is is the on or start positions?

the ECU doesn't get power until that wire is grounded, but the ecu has to tell it to ground? I'm lost

 

[steve]

It can be slightly confusing. The ECU has two power sources. Pin 103 is the backup power that maintains the contents of the RAM in the CPU and provides power to the circuit that switches the MPI relay. One half of the MPI relay is used to provide main power to the ECU and the other parts I mentioned. That comes in on pins 102 and the one below it.

I long since learned not to answer why questions about things I wasn't involved in.
The scheme they used works well even if it seems overly complex.

Steve