Re-sealing the remaining potting compound after some clean up is possible. Electronic potting compound is best, but other sealants might be used for a re-seal attempt. As noted in post#9, mineral spirits can work on the leaked compound, but is relatively slow to dissolve the compound, though safe to car paints. Lacquer thinner dissolves the compound faster, but it is harsher on the paint and can damage it. I have used lacquer thinner to clean a leaking P4120534 ECU, which has been in use since the 1980s, without damage, by working rapidly in multiple short duration applications/removals of the thinner:
Before:
View attachment 1716488182
After:
View attachment 1716488181
The paint on the car is old PPG Delstar catalyzed acrylic enamel.
For more information on complete re-potting, refer to the following:
Chrysler ECU Potting Compound Clean Up and Repair
Chrysler introduced electronic ignition in their vehicles in 1971 and used the same basic system with some improvements through 1989 on many vehicles. The ignition system contained an electronic control unit (ECU) which was filled in the rear with a protective potting compound. Over time, this compound can break down and begin leaking from the ECU.
Chrysler used semi-solid potting compounds in the electronic control units (ECUs), electronic voltage regulators, air conditioning high speed cutout controllers, EGR timers, and other similar electronic modules to protect the internal electronics. In some ECUs and voltage regulators, sand was added to the compound as an additional stiffener for mechanical shock and vibration resistance.
This potting compound can begin oozing or leaking from the back of an ECU, electronic voltage regulator, or other device. The potting compound is not melting, but is slowly breaking down and oozing out. Potting compound has been used for decades in electronics to seal for environmental protection against moisture, debris, etc. and movement of internal components from vibration, mechanical shock, etc. The compounds are typically fast curing two part or single component chemicals that form a solid or semi-solid protective barrier. Over time the compound can break down and "run" or otherwise deteriorate. Heat does not necessarily cause it, but can aggravate the breakdown.
1970s Chrysler ECUs and other assemblies are more susceptible to the oozing than later production parts. Later production was changed to a more rubber-like or silicone-like compound that breaks down less. In general, the parts can continue to function with the compound leaking out unless affected by moisture or other factors. Aftermarket components can also suffer from similar breakdown depending on the nature of their compound. The later units do not really leak the potting because it is an improved composition, typically without the sand. It's easier to clean out too. Most aftermarket units from the 1980s and later are similar. Some are a softer compound, but I have encountered some that seemed harder. Some of the early units from a couple of aftermarket brands do also have sand.
ECUs that have leaked usually still work and can work this way for years. However, exposure of the internal components can lead to corrosion and possible component failure. The leaking ECUs can be cleaned up and re-potted with an electrically non-conductive, thermally conductive, non-corrosive and waterproof compound. Complete re-potting is better, but just sealing the remaining potting is possible without completely stripping the ECU, but is not as assured. Sealing the back of the ECU with compound can be further enhanced by the addition of a plate. Whatever might be used to cover the back must still allow grounding of the metal ECU housing.
These two ECUs were removed from cars in a field. Both are original units that Chrysler installed in the cars. One in 1974, the other in 1976. They both still work but will be cleaned up and re-potted.
View attachment 1716488166
Because it is a chemical degradation, the potting breakdown can occur even when the ECU is not in use on a vehicle. The following image shows a new Autotron ECU, which was sealed in the shrink wrapped box, since 1978, until I opened it the other day. Although not bad, its sticky compound was oozing and stuck to the 1978 newspaper that was used to pack it:
View attachment 1716488169
Cleaning and Re-potting
When the early compound used by Chrysler oozes it forms a sticky mess, and it often has entrained sand, which becomes a sticky sandy mess. Removing the potting is tedious and tricky, especially if trying to not damage the internal components and circuit board. It can be dissolved with lacquer thinner, but it's troublesome to remove from painted surfaces without risking the paint. If the unit has already leaked heavily, that's a head start to removal. Various mechanical means can be used to break away some of the potting if possible. Follow this with application of lacquer thinner. The thinner will dissolve the potting in most cases. In some cases, submerge the unit and let it soak to dissolve the potting inside the case and around the components, but with greater risk to the components. The thinner will eventually dissolve all of the potting, creating a runny, gooey mess. Follow with multiple thinner rinses and manual cleaning to remove the weakened compound. Brake cleaner spray can be for spot cleaning.
If the components are of concern, the lacquer thinner soak can damage them, depending on the amount of exposure. The semiconductors typically will not be affected. The wire wound resistor seems unaffected but the varnish or enamel used on those might weaken. The smaller resistors may lose their markings. The mica capacitors can soften if exposed long enough, and the electrolytic capacitors can start to physically break down depending on the type of construction used. The fiberglass circuit board can become saturated but will dry out.
If there is no concern about the components, just submerge the unit in lacquer thinner and let it sit for a couple of days. It may take a few iterations of soaking and digging to get it cleaned out, but if not concerned about the components, the digging is less tedious.
The lacquer thinner will strip the paint on the few aftermarket units I have done, but it does not really bother the Chrysler units I have done. It seems the black paint may soften some, but a blue unit, and early P4120505 orange units were not fazed. It will remove all markings though, including those on the transistor in many cases. The thinner will not bother the connector or heatsink, but it may weaken the exposed part of the heatsink insulating pad.
After removing the compound, if the case needs refinishing, partially clean it with wire brushing and hand brushing of the case only. If heavier stripping is needed, prior to brushing it, mask the connector and transistor/heatsink, plus cover the back, and carefully bead blast it. The heatsink is anodized aluminum and can be buffed clean if the anodizing is still intact and the aluminum is not corroding. If it is corroding, heavier brushing might be used, but care should be exercised around the insulating pad. The transistor case is nickel plated steel and it can be lightly hand brushed, but any remnants of marking will be gone, and due to its shape, full cleaning of corrosion is difficult.
Evaporust, or similar, can be used in areas that can't be reached with brushing or to get into pits. Some light chemical etching of the external part of the case with phosphoric acid is possible. Don't use the phosphoric acid on the heatsink or transistor. The connector pins can be cleaned with a contact cleaner such as CRC or DeOxit brands. Lacquer thinner can be used for some clean up, especially for any potting on the outside.
The following image shows some ECUs from which I have removed the potting and which will be re-potted when any component repair, further case work, and painting is complete.
View attachment 1716488167
The following image shows "de-potted" ECUs which have been re-painted and tested. I don't attempt to make them look original, just somewhat decently so. Replacement "Electronic Ignition” stickers are available. The repainted ECUs are a P4120505 and an aftermarket version of 4111850. The paint is not the exact color, or sheen, but it's close enough for my use:
View attachment 1716488168
When repainting, don't paint the heatsink, transistor, or connector. I have used a light coat of VHT high temperature clear on the transistor cap to preserve the marking.
After stripping, cleaning, repairing or painting the ECUs, they can be re-potted with an electrically non-conductive, thermally conductive, non-corrosive and waterproof two part (A-B) epoxy compound, such as one of the following:
View attachment 1716488180
After re-potting the ECU looks like the one in the following image:
View attachment 1716488173
ECU Testing
I test the ECUs units before, during and after the process with a C-4166-A, or more on the lab bench for testing or repair as needed. Real testing is done by running the ECU on a vehicle. I use my 1974 D200 with a 440 and its original ignition wiring with a four terminal dual ballast resistor for functional tests and stress testing of my ECUs.
View attachment 1716488179
Other Components Using Similar Potting
Chrysler also used similar potting methods on 1970+ voltage regulators, EGR timers, police air conditioning cutouts, and other similar electronic modules.
Similar rework on these parts which suffered from potting breakdown and oozing can be done such as the voltage regulators and police air conditioning cutouts shown in the following image:
View attachment 1716488172
From Chrysler ECU Cleaning and Repair, G. Lewallen, aka Vaanth
