Inoperative Cooling Fans

[Sentry95]

I have a '95 Buick Century, v6 3.1L. It has 200,000 miles. The cooling fans are not turning on when the engine gets hot. They never come on, even when the A/C is running.

What I've tried so far is I've replaced the Coolant Temp Sensor. I checked the motors for the fans by connecting them directly to the battery. The fan Relays I believe are good too. A mechanic friend checked them. So the fans, the temp sensor and the relays all work, but the fans won't turn on automatically.

The electrical connectors to the fans don't read that power is coming to them when I test it with a light tester. I think the mechanic said that the relays also aren't reading a ground or something, but he did turn the fans on by "jumping" each relay so he said the problem is not in the relays.

I checked the fuses in the fuse box. Their all good except for the 5 amp fuse labeled "INST LPS". I mean the fuse is good, but there's no power to it's socket. No light in the socket with the light tester. I tried using a new fuse, but the socket is just reading dead.

I know I probably need to do a scan on the car to see if any trouble codes are stored, but I haven't been able to do that yet. I don't have that equipment.

I thought that the problem could either be with a Fan controller, which if there is one I haven't bee able to locate it, or with the Engine Control Module itself. Neither of the fans are turning on, yet their both good fans, motors good.

One thing I'm a bit concerned about is when I replaced the Coolant Temp Sensor the old one was only screwed in a few threads and the new one also would not screw in except for a few threads. It feels like forcing it any further might strip it. So I'm not sure if how far that thing is in matters, but it's not leaking.

Thank you for any advice.

[HotZ28]

If you could find someone with a scanner, it would help; but most likely, you have a PCM quad-driver fault. The engine coolant temperature (ECT) provides the PCM with engine temp readings and if the temp reaches a point to where the fans should come on, the PCM quad-driver will ground the fan relay(s) and the fan should run. The fact that yours does not run even with the AC on, confirms this theory.

Sounds like you need a new/rebuilt PCM, however in the short-term, you could mount a toggle switch under the dash to ground the fan relay(s) the same as your mechanic friend did when he tested the relay to make the fans run. One side of the switch will go to ground and the other side to the relay ground; flip the switch and your fans will run with ignition on.

[Sentry95]

I finally got someone to run a diagnostic on it. Turned out is wasn't the PCM. What happened was the ground wire at the other end of the relay, where it goes into the fuse box or wherever it goes (I didn't do the work myself so I'm not exactly sure) came loose. Someone had soldered it before and the solder broke. So we put a new wire in it.
Thanks for responding. I thought I'd explain what the trouble was in case it might help someone else with a similar problem. My haynes book schematic was also useless to trace the wire. The mechanic had to get the factory schematic.