I have a 1990 Dodge Colt with an (allegedly) defective ECM unit. Two
reputable mechanics told me they cannot work on the intermittent
'check engine' light until the computer is replaced.
The car runs fine at 220K miles and I really wouldn't trade it for the
world. It gets just under 40 MPG around town and around 43 MPG on a
trip, so it's quite a practical vehicle. The car runs like a champ,
except when wet...which has just started to be a problem recently.
Suspect the wires and plugs are in need of changing.
Other than that, the check engine light comes on intermittently.
Sometimes it will go for a month without the check engine light on
though. So, I'm not really sure if this is a cause or a symptom:>:
My mechanic told me he cannot communicate with the computer when he
hooks up his test hardware to the vehicle. As a result, he can't read
error codes or check readings that might give a hint as to the
problem. Based on the inability to communicate with the ECM, both
mechanics said to replace the computer before they can work on the
check engine light issue.
Does anyone have schematics for these units or basic troubleshooting
info? I have training in electronics, but without a schematic or
servicing info, I'm probably helpless with regard to repairing the
I should also say that the idle speed is very low after resetting the
computer, and sometimes takes a week to adjust itself after an ECM
reset. But, after awhile, it does go back to 700 rpm idle and does not
You won't be finding a schematic for the ECU, anytime soon. They are very
rare and just about never get out of the OEM's research labs.
In 1990 the ECU was somewhat simple minded. The diagnostic error code
output was nothing more than a series of pulses, on one pin of the
The intention was that any mechanic with an analogue voltmeter could read
the diagnostic codes. After 1995 the diag codes became more complex and
usually required a diagnostic code reader (code scanner)(not on your car,
but cars manufactured after 1995!!)
The ECM learns your driving habits and the characteristics of the car in
about a week, then retains this info until your reset the ECM memory.
That's why your car runs poorly for the first week. I'll bet that, based on
what you have said here, there is nothing wrong with the ECM. The check
engine light is indeed pointing to another fault somewhere in the car.
I don't have the specific pin out for your ECM, but I'll GUESS that the diag
connector is located next to the fuse block on the left kickpanel (on left
hand drive vehicles) under a black protective rubber cover. All you'll need
to check for error codes is a volt meter (12 volts) and the correct pin
location. Check the service manual (see your local library for the manual).
The manual will have the error codes, their description, contact points, and
procedure for reading the codes.
Some ECM's of this vintage had a problem with the capacitors leaking, and
eating up traces on the PCB. While your into the repairs, you may want to
at least inspect the caps for leakage, swelling, or other damage. Typical
failure, results in the vehicle being totally dead and not running (cranks
OK, just won't run). Replacement caps are typically just a few dollars, and
easy to replace.
For further reading, you may want to try this site. It's for Eclipse and
Talon vehicles, but it will have information on Mitsu ECM cap repairs.
<TRABEM> wrote in message
WOW, awesome info on that website! And, I thank you very much for the
info you gave.
I have the service manual (Chilton, for 1990 Colt), and it describes
the ECU inputs and pin out on the connector. It also describes how to
read the error code with a simple DVM or analog voltmeter.
Here's the problem.....
A local dealership told me they couldn't work on the check engine
light problem without being able to communicate with the ECU. They
told me my ECU isn't talking to the tester they have although it runs
the car just fine. I was put under a lot of pressure to buy a new ECU,
so they could start to work on the check engine light problem. The
mechanic himself told me he should be able to connect his tester to
the ECU and interrogate it to get various data from it in order to
diagnose the problem.
The example he gave was the throttle position sensor. He said he
should be able to read the throttle position from the output of the
ECM. IF it appears to be stuck when it's actually moving, then he has
to determine whether the sensor is bad or whether the ECM has an
He also said he had to know the setting of the motor that maintains
the idle speed...which he said the ECM would report if it was asked
for this data by the test instrument.
So, I was told they could not diagnose the check engine problem
without proper communication with the test hardware....which my ECM
was not providing.
After I read the service manual, I learned about manually checking the
stored error codes and I did the procedure myself, bought a new oxygen
sensor and installed it myself. The problem went away for 3 or 4
months, but then the check engine light started coming on
So, I'm back where I started....at least I didn't spend big bux for an
ECM that I probably didn't need.
I had a serious argument with the Service Manager about the ECM
functionality. I told him to read the error codes manually rather than
relying on the ECM's data communication with his test instrument. He
told me they didn't use second rate diagnostic procedures and gave me
the car back after declining to work on it further.
So......I'm a little confused!! Can the dealer be that fubar'd, or is
there indeed a diagnostic data out pin that they use to interrogate
the ECM with??
Again, it's a 1990 Dodge Colt, 1.5 L engine with 216K miles on it.
Thanks for your help.
Pre 1995 ECU's (OBD1) did not "generally" have (or need) input diagnostic
ports. All they needed to do was output an error code. HOWEVER, there are
third party "Data Loggers" that will work with Colts and allow for some
control of solinoids, injectors etc.
I not sure what this mechanic is thinking of,(I wouldn't want to think he's
trying to make an extra buck here). A good mechanic wouldn't depend on
having a special scan tool for an OBD1 car.
Even the Mitsu., shop manual for my son's 1991 Laser describes using an
analogue meter to read the codes. Nothing is mentioned at all about being
able to "Interrogate" the ECM with a special tool (as is needed for my 2003
I would suggest you read the error codes again, and maybe post the codes
here, and see what the problem is. With the mileage you have, I would check
all sensor connectors for mositure or corrosion. Even just unplugging then
replugging them in may clear the problem.
The idle position sensor is nothing more than a rheostat (potentiometer),
and very easy to test with an ohm meter.
Most mechanics should know, that when you unplug the ECM from the battery
you will lose the memory and the car (ECM) will need to relearn your driving
habits. This is common across many different lines of auto's, and usually
takes about a week, depending on your driving habits.
You may well have a different problem here than you had last time.
In almost all cases you can test the electronic sensor items in your colt
with a simple VOM, instructions will be in your Chiltons manual.
It is my opinion that there is nothing wrong with your ECM (Capacitor issue
You may want to change dealerships though, I think I'd be a little pissed
with your current dealer also. They'd look pretty bad if they fixed your
car now, after telling you that you needed a new ECM, when you really
<TRABEM> wrote in message
OK, many thanks Niro!
I don't know for sure that the dealership was using a Mopar test
instrument, or whether it was an aftermarket brand. But I do know I
was treated like I had some kinda disease when I suggested reading the
error codes manually and the treatment was even more hostile after I
showed the Service Manager the procedure in the Service Manual.
He very much wanted me to fix the ECM before proceeding, and as I said
before, I was invited to take my car elsewhere when I suggested using
the low tech method for reading the error codes.
I am a little worried though...about the ECM. After reading about the
electrolytic caps, I'm wondering if I am on the verge of a
catastrophic ECM failure.
I note that the check engine light sometimes goes off and stays
off.....which (I think) means the ECM has reset itself. I also notice
occasional skipping, which I had thought to be worn out ignition wires
since it happens alot in wet weather (but occasionally when it's dry).
A couple times in the last week, the ignition cut out completely for a
quarter second or so while driving. It came right back on and didn't
sputter after that. During the brief time the ignition winked out, I
saw the tach drop out completely, even though the motor was in gear
and still turning over... So, I'm wondering if the ECM has bad
electrolytic caps and a catastrophic failure is about to happen.
I should say that the check engine light did not come on following the
brief drop out of the ignition....perhaps this is significant, perhaps
It sounds like I need to order some capacitors and do a preemptive
capacitor lobotomy on the ECM (just in case).
I did see the pictures and read the web material describing how to
change the caps and what type of caps to order. Fortunately, I am
fluent in soldering and have electronics training...so it's a minor
operation for me.
Thanks for all the suggestion and guidance. It's much appreciated.
When your changing the caps, check the circuit traces under the caps for
damage, such as cracks or breaks in the copper. Some of the caps I've
changed had a glue like material around the base, which could damage the
trace on removal.
Chrysler Canada quoted me $2000.00 cdn, for a new ECM for a 91 Laser, and at
that it had to be special ordered from the center of the universe, I can see
why they wanted you to replace it.
If the ECM loses the pulse train from the TDC and crank sensor, it will not
fire the ignition coil. This would kill the tach, and the engine would
stop. When the signals return the ECM would continue to fire the ignition
coil and the engine would continue to run. This doesn't normally light the
Have you checked the connectors? These signals are low voltage/current, and
it doesn't take much to affect them.
While you may have a problem with the caps, I believe you have a problem
elsewhere. You have something that is borderline, as the ECM clears the
trouble light when the "sensor" falls back into spec. Maybe an O2 sensor
or airflow sensor, best to check the error codes. If you can't get the
error codes, use the shotgun method and test all the sensors in the car,
there really isn't that many. Your Chiltons manual will tell you how to
test them. And don't rule out that O2 sensor you replaced some months ago
until you've tested it.
Remember that you can use larger capacity (microfarad and voltage) caps,
than whats there now. The caps are all used on the power rails of the ECM.
I do recommend that you replace the caps on spec, but if they don't look bad
you could probably get away with keeping them.
<TRABEM> wrote in message
You sound like a pro.......do you work on these professionally??
I have some training in electronics, and find myself wondering WHY
they use multiple electrolytic caps on the 12 volt rail inside the ECU
box. These days, the whole system should be powered by 5 volts or even
3.3 v. Certainly the microprocessor and the eeprom should be 5 or 3.3
volts. The ECM should source all the power needed by the sensors as
well, so the sensors only need to be grounded externally to work
properly. Thus 12 volt electrolytic caps should not be needed at all,
especially within (inside of) the ECM enclosure.
I know the automotive power system leaves a lot to be desired and it's
a hostile environment for all electronics.....after all, an alternator
puts out pulsating DC which is smoothed by the internal capacitance of
the battery and whatever capacitors are on it's output. But, using
electrolytic caps for this smoothing is a bad idea.
It's bad because the input voltage is always pulsing, so ripple
current is high within the caps. This factor alone could be
responsible for the electrolytic caps failing....because the ripple is
relatively high frequency and because it never stops, the capacitors
have to work very hard to smooth out the pulsating DC input.
A much better way is to feed the 12 volts into a regulator, which
drops the voltage down to 3.3 or 5 volts. A switching regulator could
be used, but is probably not necessary because the power drawn by the
ECM is actually quite low. The regulator would do the job of smoothing
out the voltage supplied to the ECM electronics without involving
large (high value) electrolytic caps.
If a simple 7805 voltage regulator was used, it would need .1 or .2 uF
caps on the input and the output. These caps could be disk ceramic,
which are practically indestructible! Since the regulator electronics
does the bulk of the 'filtering', any capacitors on the 5 volt power
rail would never see large ripple currents, and they to would last
nearly forever....and generic standard electrolytic(s) could be used.
My guess is that the high temp electrolytic(s) are only necessary due
to the large ripple current the caps see (as a result of the
relatively crude alternator output).
I'd be willing to bet the temperature inside the ECU is never very
high, and that the high temp electrolytic(s) are only needed because
of the high ripple current they see.
Even a simple high value inductor on the input of the ECU power feed
would reduce the ripple current seen by the caps. Although one could
argue that a regulator requires a complete redesign, a 1 H choke in
series with the 12 V power line could easily be placed inside the ECM
during reconditioning/repair. The choke would reduce the ripple
current seen by any electrolytic(s) placed on the 12v rail inside the
ECM. And, the choke would not wear out! The presence of the choke
would improve the lifetime of the electrolytic(s) by minimizing the
ripple current they see.
By utilizing the magnetic field stored by the inductor, the inductor
could (potentially) totally eliminate the need for electrolytic(s) to
do the filtering. When the input voltage drops, the extra energy
needed to keep the output voltage at 12v could be drawn from the
stored energy in the inductors magnetic field. Using an inductor to
smooth out the voltage fluctuations would be a great improvement in
reliability.....an inductor should last forever (literally).
If I was working on this problem professionally, I'd put an ac ammeter
in series with each electrolytic cap and quantify the average (ripple)
current flowing in each cap. Then, I'd put a large (appropriate
current rated) inductor in series with the input power and measure the
same ripple currents again. A more meaningful test might be to measure
the temperature of each electrolytic before and after the inductor was
added. I'd be willing to bet each capacitor runs at least 30 degrees
cooler with an inductor in series with the input power lead.
I wonder if newer ECU's use more appropriate methods of smoothing the
input power such that reliability is not degraded by the use of large
Drop me a line, either direct or on this newsgroup.
TO repply directly send an e male to:
anonymoPusone aat thedoghousemail doot comP
Remove the upper case P's to get the proper email address to reply.
No, I do not work on ECU's as a profession. I'm a cert electronics tech,
who professionnally works on mid range computers (25 + years experiance)
down to component level. Cars are a hobby (I'm also cheap and don't trust
dealers). I got into the ECU's when my kids cars started having problems,
and the quick fix from the dealers was "replace the module, and give us lots
of money". Even the auto wreckers back then were demanding a high price for
ECU modules (if you could even find one for a Mitsu). Even Honda modules
were difficult to get, (for a different reason, everyone was "upgrading").
One thing the computer industry taught me was how sensors and associated
circuits worked, in disk drives, of 20 years ago. Virtually all the sensors
found in a car today, were in disk drives and tape drives, 20 years ago
(well except for the O2 sensor).
The problem with the caps is not the ripple or Mitsubishis fault. The
capacitors have a design fault. This is an industry wide problem. I had to
replace 13 caps on a 2001 Aptiva (IBM) computer motherboard because they
were swelling up. I've seen this problem on numerous other circuits, (yes
ripple will cause the same thing). My guess is that Mitsu just found a bad
supplier of caps, for 5 odd years, by chance.
Check out this article :
or this article; http://www.badcaps.com /
and another; http://www.badcaps.com/causes /
When I was working on my ECM, I thought the CPU voltage was 5.0 volts.
There is an onboard requlator, in the form of a power transistor. The CPU
doesn't operate on the 12 volt battery feed (well not directly) and the
5volt rail looked pretty clean on my scope.
My problem was (twice, two different ECM's), there was a break in the power
rail directly under the large cap, in the center of the PCB.
Lets not get over reactive here. I've been inside both Honda and Mazda
ECU's. Generally they all work well, and ECU failures are rare , relative
to the number produced, (unless somebody does something stupid on their
car). Most ECU's have well protected input's and outputs, and the
alternator ripple usually isn't a problem.
High temp components are used because the temperature inside of a car, park
in the sun, in say Arizona, will be well over 120 degrees F.
Electronics have been in cars for many decades (the audio system has been
solid state since the sixties) so most of your concerns have been answered.
If ripple was a problem (as it is in car audio) then they would have
designed for it(as they did in the audio systems) in the ECU.
<TRABEM> wrote in message
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