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Resolved Dead 420A ECU fix?

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fisher83

Proven Member
89
1
Aug 23, 2013
Tacoma, Washington
I have noticed a lot of threads that have sensor problems that ultimately end up with bad ECU's. Is there any measures you can take to help prevent this? Would adding a ground wire or two off of the case of the ECU help any? Maybe even extra grounds from the engine to the chassis and alternator to chassis and so on and so fourth?

My ECU in my 420a went out today and after checking the CAS, CPS and other sensors that are tied together, I ultimately came to replacing the ECU. The same ECU that had been running my car for the last two years with no issues at all.

So is there anything that can be done to prevent another ECU from biting the dust? I've added more grounds, changed out the small braided factory grounds with 4 gauge wire and ground off the paint to get a better connection used lithium grease on the connectors.

What more can be done? :confused:
 
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A lot of the time with those ECU's there is a bad solder point on the board. There is really nothing you can do to prevent this from happening other than opening the ECU, pulling all the jello like stuff from the board and resolder the bad solder connection on the ECU. It is a very time consuming because the ECU is filled (potted) with a silicone/jello like stuff that has to be slowly scraped away carefully not to damage the board.

Forgot to ask, did you check to see if the ECU was actually working and whether or not it had any trouble codes stored. When you turn the key on does the check engine light come on? You can do what is called a key dance in which you turn the key on-off-on-off-on in a span no longer than 5 seconds. The check engine light will flash the code.

If there are no codes at all, or the light doesn't illuminate than I would suspect the ECU.
 
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I switched to a ms3 full stand alone system, and have a 1998 ecu sitting in a box if your looking to buy one.
 
The CEL comes on briefly then goes off. I tested the voltage on the CAS and CPS and got a low voltage reading even after cranking it over to see if that would change the reading. Everything I read pointed to the ECU being bad.

The voltage signal was supposed to be between like 7.8v to 9.5v on one of the pins and it was only coming in at 4.5v. I read that the voltage could vary depending on where the motor was in reference to the ECU so. I'm not too concerned with getting another stock ECU because I have a m.s.II on the way from symtech, but I was just wanting to prevent something from happening to it.

You would think excess resistance would be the cause for the boards going bad, that's why I was wondering if grounding was part of the issue or if there was another culprit. I know factory wiring, especially grounds, are pretty inadequate. I just wanted to use the stock ECU to break my motor in until I get a little more familiar with the megasquirt. :thumb:
 
I have no spark or fuel. I checked the ASD/fuel pump relay and they are both good. I have a brand new CPS, CAS, coil, O2 sensor. It has fuel, I replaced the fuel pump, filter, checked the timing and compression.

It's a fresh rebuild with only 10 minutes of run time before it died. I also checked the voltage to CPS and it read 5.6 volts on pin #1, 2.5 volts on pin #2 and I have continuity between pin # 3. The voltage is about 2.5 volts below the suggested voltage according to Chiltons. The cam angle sensor read low as well and also had continuity.

I get the check engine light for the 3 second cycle when you turn on the key, but no codes. I did the key dance and got code 55, which is end of codes. My question is it the PCM or is there anything else I'm missing? :confused:
 
Why did you do the key dance? You have OBDII. Go rent a scanner from any auto parts store and find out what the code is. If the scanner can't connect, then the ECU is to blame. I would also pull it out and check it for damage.

You numbers on the CPS seem off. Check it according to these parameters:

http://www.dsmtuners.com/forums/152302942-post5.html

You can also take a read through this thread for more ideas:

http://www.dsmtuners.com/forums/articles-miscellaneous/217951-how-diagnose-no-start.html
 
I read through those threads. I checked the voltage in my chiltons and according to it they were low. I dont have a check engine light staying on either. The threads said that usually if the pcm is bad then the check engine light wont light up when you turn the key but mine does. So thats where im stuck at right now. And as far as the key dance, I figured what the heck, might as well do it so I can say I did everything. I have power to the asd/fuel pump relays.
 
Did you put a scanner on the car? If not, do that. Just because the check engine light isn't on, doesn't mean it doesn't have codes stored.

I haven't read through the Chiltons, but the numbers in that thread came from the factory service manual and I would do the test according them over the Chiltons.

If the numbers still seem bad, that is probably your problem. It seems unlikely that a new part would fail but it depends on where it came from these days. You can also check the old ones if you still have them and perhaps save yourself some money.
 
I took the sensors off of the motor that I pulled out of the car to double check. The motor I pulled out ran fine so I know they were good enough to make it run atleast. Ill put a scanner on it in a few days. I know it will throw a code for the after cat O2 sensor, it always does since I put the frankenstien throttle body on. We will see what else comes up. Ill keep everyone posted.
 
So after muuuuccchhh effort I found the culprit. Underneath the PCM and fuse where the junctions are was a frayed ground wire.

Where the wire harnesses go into the junctions is a plastic cover over them, the plastic was rubbing on the ground wire for the bottom harness. I soldered the wires back together and added an additional ground wire and grounded it to a different place on the car body. I took off the stock ground for the PCM and ground off all the paint and also for the new ground. The bottom harness is where all the ignition/fuel controls are and it only has the one ground wire. The top harness had two grounds.

So hopefully by adding the additional ground wire as well as upgrading the rest of the stock ground wires to 8 gauge wire I will be able to prolong the life of my PCM.

Thanks for all the help and comments. :D:hellyeah::thumb:
 
I found the culprit, it was a bad ground wire for the bottom harness. It was barely intact enough that the PCM could send voltage and it would go to ground but not enough to discharge all of it.

I replaced the ground and added another ground wire off of the terminal (b10 I believe). It did cause my IAC to short out and throw a code, verified by my neighbors snap-on scan tool, and ohmed it out as well. So I will replace that and hopefully have it idling back to normal.

So in the future for all of you out there with the same problem of no spark/no fuel check your grounds if you hear the ASD click and everything else has been checked as far as continuity for the CAS and CPS .
 
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