We all know that the ECU computer controls the main, frame-mounted fuel pump through the fuel pump relay behind the console. But how is the little lift pump in the tank controlled? Is it a direct connection to some switch, like the ignition switch? Is it controlled by the ECU computer through another relay? How?

And, while we are at it, where are the grounds located that service the fuel pumps?

Reason for asking: see my post: "Yet another frigging problem" Basically, the main pump, a Bosch unit, has been noisey since it was new, and on this past Friday seemed to be ready to fail. It is noisey only after it runs for quite a while, like over 35 to 45 minutes. Really noisey after 2 hours or so. It is only about two and a half to three years old. To quiet it down, I have replaced the filter sock on the in tank pump, and then replaced the in tank pump (and the filter sock, again, too). The main pump appears to have had a serious, but apparently temporary, problem last Friday, yet now, is running quite well, and quieter than it has been in a while. Using the "pull the fuse on the in-tank pump and listen for an increase of noise from the main pump" test, I have established that the in-tank seems to be working. At least when I have tested it.

I need to troubleshoot the problem(s) for once and for all. Maybe the in-tank pump (relatively new) is intermittent, maybe the main pump was a defective unit from the beginning, maybe one of them has an intermittently bad ground, etc. Hence, to do a thorough job, I need to know how the little, in-tank pump is controlled, and where the grounds are for both pumps.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)