A car dealer sold a late model used Ford Tempo to a customer, who drove it a couple of days, then sent it back on a wrecker because it would not run. The dealer's shop then kept the car for a couple of weeks trying to fix it. Late model Fords have a multi-function module, that replaced the old fuel pump, power, cooling fan, and compressor relays, in addition to other controls. These are now in one neat little box with a big gob of wires going into it. The dealer finally replaced this box and the vehicle started. The vehicle was given back to the by now unhappy customer who took off. He again drove it a few days, and it stopped again. The car went back to the dealer again. They discovered that the in-tank electric fuel pump was not running (in addition to other things) and replaced it. It went out again for a while, then came back for a while. Then they sent it to us.
Such an intermittent problem is as difficult for us as it is for anyone else, yet the dealer was on the phone every fifteen minutes wanting to known why it wasn't fixed yet (normal procedure). We discovered that the multi-function module they had already once replaced was again burned out (you know the smell). In addition, the single fusible link that feeds the module, and the devices that it controls, was burned out also. The multi-function module is unlikely or impossible to short the fusible link on it's own, so we began testing each and every device that the module controls for a short. Of course the short wasn't present at the time, and no amount of wire wiggling would cause it to reappear. Replacing the module and the fusible link made the car start and run, but with the knowledge that the intermittent problem was still there waiting to surface someday and take out the third module. After about two days of trying unsuccessfully to get the short to reappear, under pressure of the constant hurry ups from the dealer, we decided to try the following:
Each of the devices that the module controls has it's on power wire leading from the module to the device. With the owner's permission, each of these wires was cut at a place where it later could be permanently spliced artfully, and an in-line 10 amp fuse was installed in each one. The idea was two fold. The 60 amp fusible link which fed all the devices, blew too slowly to prevent the module from burning out first the next time the short reappeared. One of the small fuses should blow instead, the next time the short reappeared, protecting the module this time, and also narrowing down the problem to one circuit. No amount of driving on our part could duplicate the problem, so it was released, after explaining to the dealer, who hopefully explained it to their customer, that it would be back at least one more time some day.
Four days later, it did come back on the hook, and the fuse feeding the fuel pump was blown, identifying that circuit, and preventing the module from failing. The wiring diagram showed that wire to go directly from the module to the fuel pump, and no place else. The seats were then removed from the vehicle, the carpet pulled up, and the wiring harness slit open. The fuel pump wire was found to be occasionally punctured by a sharp body bolt barely protruding into the harness from underneath.
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