Cooling fan failure

'04, 4 Liter with 128k miles.

Had the "it's fine on the highway but in town it starts to overheat". So after investigating, found that the original fan was only turning at slow speed; my model does not have the relay under the passenger headlight, but rather the 3-wire harness to the cooling fan, so I replaced it. Worked fine for a short while, but we are back to low (VERY low) speed only.

I'm leaning towards having the electrical system checked; at this mileage there could be a chafed wire causing the issue.

Any thoughts?

There are three wires going to the cooling fan, one is ground, one is power for the low speed fan and the other is power for high speed fan. You need to check power to both fan relays with the key on and engine running. Both should have powers. The computer grounds each relay to activate that specific circuit to power up the fan.

Thanks...hopefully it isn't a computer fault. If it is I'll consider a mechanical fan.