Melting the potting material out of ECUs

Electrical & Ignition

  1. XCELLR8

    XCELLR8 Well-Known Member

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    I'll make this quick. Bought my 1977 Cordoba in 1996 and shortly after alternator quit. Replaced it with one from an auto parts store. Only driven the car off and on ~300 miles in last 25 years and have melted the black potting out of the back of two ECUs. The first was a factory Lean burn replacement and the second was a MP dizzy with a chrome box and accel chrome coil.

    Question is, can the replacement alternator cause this issue or do I have another problem. The black potting is running down the firewall like candle wax and I never see it doing this while tuning the motor or idling.

    Ideas?
     
  2. Bee1971

    Bee1971 Well-Known Member

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    Age
    Heat
    Humidity

    I have a NEW in the box 5 Pin Standard LX101 ECU from the early 80s
    Never been mounted on a car

    The black potting is sticky and melted in a few spots right in the box

    Another USED factory 5 Pin Chrysler ECU from the 70s same thing
    Melted in areas

    It happens or happened on many cars over the years
     
  3. RJRENTON

    RJRENTON FBBO Gold Member FBBO Gold Member

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    IMO....
    The voltage regulator controls the alternator's rotating field voltage (snd current) and ultimately, the alternator's output and overall system voltage. This is accomplished in the regulator with a transistor that is switching the alternators field voltage (and current) on and off, using a current sinking circuit. IF the alternator's rotating field coil or the slip ring assembly (which provides the means to get power to the rotating field) or the brush holders are shorting to ground (alternators case), the voltage regulator will be switching more current (thru a resistance preventing a total short circuit), causing the internal components to run hot, melting the potting compound, as you've noted. The field wiring to the alternator's field brushes (green and blue wire), MAY be grounded, causing the regulator to overheat. Your car MAY have have a Leece-Neville alternator or a Chrysler "square back alternator" but should be conttolled the same way. I'm sure others will have their own suggestions and opinions as to what the problem is. Just my opinion of course.
    BOB RENTON
     
  4. XCELLR8

    XCELLR8 Well-Known Member

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    Bob - So you are thinking that the alternator is bad and grounding out?
     
  5. RJRENTON

    RJRENTON FBBO Gold Member FBBO Gold Member

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    There is something csusing the voltage regulator to ovetheat.....the alternator itself, grounding out and drawing excessive current in the rotating field or the wiring to the rotating field The regulator MUST be securely grounded to the engine block not just the firewall. Person #2 made reference to the ignition control module (ECU) .... they are potted as well and have bern known to overheat and melt the potting compound.
    You could try disconnecting the field wires at the alternator and see if the overheating stops or not. Just a thought
    BOB RENTON
     
  6. mopar 3 B

    mopar 3 B Well-Known Member

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    Well that's one opinion. Now explain away the ones that have never been used.
     
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    • Bee1971

      Bee1971 Well-Known Member

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      X2


      The initial post was about the ECU I THOUGHT

      I am confused

      LOL
       
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      • RJRENTON

        RJRENTON FBBO Gold Member FBBO Gold Member

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        I believe that the OP was talking about the alternator's voltage regulator.....yes??
        BOB RENTON
         
      • RJRENTON

        RJRENTON FBBO Gold Member FBBO Gold Member

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        Perhaps, you can share your thoughts and opinions as to where the potential trouble areas are. As to the unused products testing bad.....possibly a Quality Control issue by the supplier/rebuilder allowing defective units to get to the marketplace......your thoughts.....?
        BOB RENTON
         
      • XCELLR8

        XCELLR8 Well-Known Member

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        Your all correct. I have a issue where the potting is getting hot and running down my firewall. There was no potting on the firewall until I replaced the alternator. The car has melted the potting out of 2 ECUs, so something is making it melt. All wiring and components have not been changed since I bought the car, except the replacement alternator in 1997, Mp dizzy, Accel chrome coil, and Chrome ECU in 2016. After replacing alternator, I have melted one ECU with the factory stuff and one ECU with the aftermarket stuff.
         
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        • PurpleBeeper

          PurpleBeeper Well-Known Member

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          Once I had pinched the blue ignition wire under the corner of my intake manifold and melted ECUs like that for a month before I found the short. Bob's info rings true
           
        • Sixpactogo

          Sixpactogo Well-Known Member

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          Bad grounds and or bad connections are one of the main reasons for electrical issues IMO. ECU's need a good case ground as does the regulator and alternator. People like to blame the orange box failure on a bad design or manufacturer issue when all along it was a bad case ground. We all like to put 5 coats of shiny paint on and it looks great but paint does not make a good ground. You should have less than .5 ohms between the case and Neg on your battery. I'm betting yours is much higher.
           
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          • RJRENTON

            RJRENTON FBBO Gold Member FBBO Gold Member

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            VERY TRUE....I believe the factory installed a flexible ground connection ftom the back of the block/head/intake manifold bolt to the voltage regulator's case mounting bolt. The ECU should also be connected to this ground, as the ECU'S switching transistor completes the coil's primary winding circuit to ground to create the spark. Poor grounds will cause switching transients that may damage the ECU's internal components.
            BOB RENTON
             
            Last edited: Oct 19, 2021
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            • Bee1971

              Bee1971 Well-Known Member

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              And of course none of this matters on the point I was trying to make of the potting material on Factory and Standard Motor Products Made In The USA from the early 80s

              Melting on new old stock brand new in the box never installed on a car

              I have a few melting right in the box
               
            • Bee1971

              Bee1971 Well-Known Member

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              I have not seen a FACTORY 1972 - 1974 E Or B Body with a ground cable to the ECU

              The E Bodies only
              Got a separate mounting bracket that kept the ECU away from the firewall

              The B Bodies did not have this bracket

              So
               
            • RJRENTON

              RJRENTON FBBO Gold Member FBBO Gold Member

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              YES....I have this bracket as well...purchased when I went to Chryslers at Carlisle, several years ago. Never used it on a B body but for a few $$, why not buy it. My GTX had a flexible ground wire installed from the back of the block to the voltage regulator's mounting bolt. The wire was a braded copper wire....orgional or not....??? I've owned the car for 35+ years....perhaps installed by a dealer many years ago...
              BOB RENTON
               
            • mopar 3 B

              mopar 3 B Well-Known Member

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              True but this doesn't explain the melt down of the gooey cooling stuff on cars with batteries disconnected in storage or OEM's right out of the box that have set for years. Really doubt if any of the electricals in those units have been damaged just sitting there.
               
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              • Sixpactogo

                Sixpactogo Well-Known Member

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                I can't say that I have experienced any of the melted goo you guys speak of with unused parts. I would think it shouldn't get liquid even in 130 degree temps. Guess I gotta be thankful that WI never gets that hot. The melting I referred to is from too much current flow.
                 
              • 1968rt

                1968rt Well-Known Member

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                • Nacho-RT74

                  Nacho-RT74 FBBO Gold Member FBBO Gold Member

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