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I find it hard to believe the connectors are the problem. If they are making poor contact, they will get hot fast. The only sure way to diagnose the problem is to put an amp-meter inline with each glow-plug and see how much current it's drawing. They should all draw the same - 8 - 10 amps at 6 volts.
You can check them one by one by using a jumper from a 12 volt battery through an amp-gauge if you want - instead of jerking around with the relay or controller. If testing with 12 volts - amps will be 4-5 on each plug. Don't do it more than a few seconds at a time. In regard to the correct plugs? The ZD9 Berus that were mentioned in other replies are for the 1987 6.9s only - as well as the later 7.3s up to mid-1994. The ZD9s will work in your engine but they draw less amperage and the timer won't work exactly as it should. The ZD9 draws 7.4 amps at 5 volts DC. Your 6.9 system is setup to use the earlier plug that draws 9 amps at 6 volts DC. I've got a 1985 with the 6.9 and correct plugs and it starts fine in cold weather. The early General Motors 4.3 and 5.7 diesels use the same plugs. I also have a 94 with a 7.3 turbo and the ZD9 type plugs and it also starts find - no better or worse than the 6.9. All 6.9s up to 1987 use the following:
Delco 8G, Beru 0 100 271 102 or 835MJ or GV835, Bosch 80024, Autolite 1107, Wellman
GO80 or dual coil GO88, Champion CH80, Ford and Motorcraft ZD1A or E5TZ12A342B,
General Motors 5613680, IH/Navistar - 1804211C2, Lucas/Delphi DS090A, Delphi HDS305 1987 6.9 and 84-94 7.3 diesels use these:
Beru ZD9, Delphi HDS307, Champion CH84, Lucas/Delphi DS092A, WAP 84 If you decide to put in a manual push-button then it doesn't matter much which plug you use - except - make sure you get a dual-coil self-regulating plug or you'll wind up ruining them.
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