How to Replace the Capacitors in a 1G ECU
This document is
provided as a guide for an experienced repair technician. While this repair is
not the most difficult one to perform, there are many ways in which the ECU can
be damaged. Any experienced electronic hobbyist should be able to do this
repair. We believe this document to be accurate. However,
TechnoMotive is not responsible for any
errors, nor for any use or misuse of this document.
What is an Electrolytic Capacitor?
A capacitor is generally
used in an electronic circuit to filter out unwanted signals. The electrolytic
capacitors in the DSM ECU are used to filter the power supply. An automobile is
an extremely hostile place for electronics. The output of the alternator isn't
all that pretty to begin with. Couple that with the spark plug high voltage
firing, the radiator fans, the interior fan, turn signals going on and off...
the +12VDC main power line gets a lot of crud on it.
Which capacitor to choose? There are a lot of different types
out there - some large, some small, some really accurate, some "cleaner"
for low noise applications, etc. For power supply filtering, electrolytics are
almost always used. Power supplies generally require a large capacitor to
absorb spikes and dips. You can't afford not to have a steady power supply line
inside of a computer - the microcontroller might act erratically or reboot.
Electrolytics are chosen because they provide a lot of filtering for their small
size. They are also very cheap.
Electrolytic capacitors do have a downside, though. They tend
to leak with age when exposed to heat and many power cycles. The electrolyte
that leaks out is very harmful to PC boards. It can actually eat the copper
traces, eventually making a short on the board. When that happens, the ECU will
either stop working altogether or act very erratically. As the capacitor leaks,
it will also lose its filtering properties, allowing possibly harmful spikes
into the ECU.
Signs of Impending Doom
Luckily, there are usually some warning signs that your ECU is on its way
- A rapid clicking or chattering from under the dash. Usually accompanied
with the engine stalling or losing power during the noise. This is the
microcontroller going into reset over and over and over again due to a bad power
supply. Every time it resets, it will turn the fuel pump relay on and off.
This could also be a bad fuel pump relay, but not usually.
- An usual smell that seems to come from the center console. Especially if
it smells like rotten seafood. Consider that it could also be your heater fan
motor, unless accompanied with a power loss or stalling.
- Your car is older than seven years and sees a lot of extreme temperature
If you experience either of the first two problems above, act on them as
soon as possible. Even though your car may still be drivable, the longer you
let the problem go the more likely you will end up with a hole in your ECU's PC
board or with several blown components on the board. Then you will have to dig
a unit up in the junkyard or buy a new one from Mitsubishi for $1100.
- Purchase the Replacement
Capacitors. Unfortunately, the particular capacitors you need can't be
found at Radio Shack. They can be found at a
For your convenience, you
may press the next three buttons in sequence to order these capacitors from
Digi-Key. When you get to the form, fill in the quantity (probably 1) and then
press "Add to Order". Then hit your browser's BACK
button to return to this screen. After the third button, hit "Submit
Order" at the bottom of the Digi-Key screen to place your order. If
you make any mistakes, hit "Delete All Items" and start again.
The cost of the capacitors is about 81¢, but there will be about $10 in
shipping in handling fees.
the way, in general, you can "up" either the voltage rating or the
capacitance of the capacitor in a power supply application if you wish. For
example, instead of 50V capacitors, you could use 63V capacitors. You just have
to make sure that the body-style of the capacitor is "radial" (both
leads coming out of the botttom, as opposed to "axial") and that it
- 47µF @ 50V (Digi-Key# P5275-ND)
- 22µF @ 50V (Digi-Key# P5273-ND)
- 100µF @ 16V (Digi-Key# P5231-ND)
- Gather Supplies. Although the
instructions below are written for using copper solder braid and a solder
sucker, we highly recommend using a professional desoldering station with its
own air supply. A solder sucker and solder braid will do the job, but are much
slower and not as accurate at the real thing. Make sure you have a good
soldering iron with a large tip. You are also going to need some solder.
- Remove the ECU. The first thing you will have to do is take the
ECU out of the center console. We have a HOW-TO
available with pictures for both the 89-94
and 95+ DSMs.
- Remove the PC board from the case. Now would be a good time to put
a grounding strap on your wrist, if you have one. Take the four screws out of
the side of the case. Keep in mind that they might be incredibly tight. At
least one usually is. Consider using a flat-blade screwdriver very carefully on
the tough ones to allow for better torque. There are also four screws in the
corners of the PC board itself. The same rule applies to those screws as you
try to remove them.
- Locate the Capacitors. Figure 1 shows the positions of the
capacitors and their values. You might want to inspect them for any sign of a
leak or maybe a smoke trace if you've had a problem.
- Add Solder to the Capacitors. Why do we want to add solder
when we will just be sucking it away later? When the boards are wave soldered
at the production factory, solder will attach to the capacitor leads on both
the top side and bottom side of the thru-hole copper pad. It is possible that
there is little or no solder in the hole between the top surface and the bottom
surface. By adding solder back in, we assure ourselves there will be enough
solder to conduct heat from the bottom of the board to the top of the board and
melt the solder on the top surface of the pad. That will make sucking the
solder out all that much easier and ensure that we won't be ripping the top off
the pad when we take the capacitor out. Figure 2 shows solder being
added to the 47µF capacitor. It also shows the position of all of the
capacitor leads relative to the huge diode posts, which are unmistakably huge.
Note that all of the capacitor legs are bent outwards while all resistor and
diode leads are bent inwards.
- Bend the Capacitor Leads. Now would be a good time to straighten
out all of the capacitor leads. Figure 3 shows this being done with a
tweezers while heat is applied with a soldering iron. This will make it easier
to both solder suck and remove the capacitors later.
- Suck the Solder. Figure 4 shows the relatively mundane
task of sucking out the solder. Make certain that the solder is nice and hot
and is flowing well before you fire the trigger of the sucker. If you do it
right, the solder will be sucked off the top layer of the board as well.
- Clean the Pads. The hand-held manual solder sucker rarely gets all
of the solder. Most of the time, you have to clean up after it. Figure 5
shows copper solder braid being used to sop up all of the residual solder to
ensure a clean pull of the capacitor.
- Pry out the Capacitors. Using a tweezers or small screwdriver,
carefully pry out the capacitors. If you encounter some resistance and it
doesn't feel "right", you might want to go back and reclean the pads
or go as far as adding back more solder and beginning the process again.
Remember that you don't want to "pop" one of the thru-hole pads, or
you will be doing some repair. Figure 6 shows the 47µF capacitor
- A Look at the Naked Board. Now that the capacitors are off, you
should give them a good sniff. If any of them smell at all like rotten seafood,
they are leaking. Be sure to pay attention to the area of the board underneath
a leaking capacitor. For example, the ECU in these pictures had the faintest
odor of rotten seafood from the 47µF capacitor. In Figure 7,
notice that the bottom hole pad shows some darkness around the lower pad. It is
the beginning of the oxidation of the copper pad. This ECU was caught just in
time. Another thing that should be noticed in Figure 7 is the marked
polarity of the hole pads. Inserting a capacitor "backwards" can lead
to an explosion.
- Install the Capacitors. It is EXTREMELY IMPORTANT that you
install the capacitors with the correct polarity. If you do not, the capacitors
might explode, doing who knows what damage to the board. In Figure 8,
on the capacitor, you can clearly see the negative sign inside of the arrow
pointing to the lead that is negative. Note that the hole on the ECU PC board
which the negative lead is going into is NOT marked with a "plus
sign". Only the positive hole is marked on the ECU. Be sure to put the
negative lead in the opposite hole. After you push the capacitor all the way
down, take the leads on the bottom and bend them outward as they were from the
factory. This will hold them in place for soldering.
- Solder the Capacitors. Downhill from here on out. Solder each of
the capacitors as in Figure 9. Be sure to trim off the leads so there
isn't a short.
- Cleanup. Almost done! Check out the soldering job using a
magnifying glass. Often, the solder will splatter a bit when using the solder
sucker and solder braid. Use a fine tweezers as in Figure 10 to remove
each small piece of solder or metal that represents a possible short. Then
screw the PC board down into the bottom of the ECU case, put the cover on the
ECU case, and you are ready to go!