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I have finally fixed my cruise control ECU. This is the '650' type that was fitted to only a few years ('92-'94?).
The problem was due to bad solder joints, the bigger problem was spotting them under the orange coloured conformal coating. The bad joints were visible with careful inspection using a magnifying jeweler's type eyeglass (I had 6x). What you are looking for is a dark (usually irregular or partial) ring in the solder around the component pin.
The fix: re-soldering all of the plastic capacitors on the board, Grey 0.1uf caps C2, C4, C6, C7, C9, C10, C13, C16, C? (behind heatsink for 7805 regulator IC), the larger Grey 10nF capacitor C12 (between the two 14 Pin ICs) and finally a green 22nf? capacitor C11. I resoldered all of these at the same time so I can't pinpoint the troublemaker, but several of them had bad joints.
It is necessary to remove the conformal coating before resoldering. Only remove the coating on the solder joints, it is not necessary to remove all of the coating. It is a polyurethane coating and can be softened using a heat gun (~200 centigrade) before peeling it off. Alternatively use a soldering iron and de-soldering pump to remove the coating and old solder. Whatever method you use, re-solder each joint and check that it is good. It is important to check each joint after re-soldering because the conformal coating on the component side of the board fills the through-holes and can vent when heated causing gas bubbles in the solder during re-soldering!
Although the problem component was undoubtedly one of these capacitors, I recommend re-soldering a number of other components as I had previously done (it may even be worth re-soldering the entire board!):
Re-soldering the capacitors was my lucky break - the problem might be different on another ECU, before this I had re-soldered just the large connector, no joy - although some solder joints showed cracks. Next I resoldered the crystal (metal can near the large 40 pinned IC) again cracked solder joints, but no joy. Then the two smaller (14 pin) ICs and R8 - still no joy. I also re-soldered the relay - just for good measure.
As part of my troubleshooting I had hooked up a buffer board with LED indicators so that I could monitor all of the signals on the ECU connector (and some internal ones). From this I could see that all of the switch inputs etc. were OK and the problem must have been internal to the ECU.
My only problem with it now is that I must have a small vacuum leak somewhere because the car constantly accelerates and decelerates around the set point and won't keep a steady speed :-( If anyone has had luck troubleshooting a vacuum leak I would be grateful for a few pointers.
AndyH
'94 9000 CDE
'97 9000 CSE
posted by 217.207.193...
, Mon, 15 Jan 2007 11:25:43, Mon, 15 Jan 2007 13:21:18
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