Why is 1999 Chevy Malibu engine over-heating has no heat and fans not running Was told that a blown or cracked head gasket would cause fans not to run?

Answer 1

more than likely it might be a bad relay,fuse or bad motors on fans, if all checks ok check coolant sensor. to check fans, disconnect fan wires, run a wire from positive to battery,do the same for negative to neg. on battery. if they run they are ok!! a blown head gasket will add water to oil in engine and at the worst, it will leak on outside of engine!!

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Your problem is probably the PCM ($125), not a blown fuse or bad relay. This assumes (as you did not mention it) that the temp indication on the dash is working.

The cooling system is comprised of two fans and three relays. All three relays have power available at all times. The PCM provides grounds to these relays to control fan operation. If the Temp Sensor senses 223 deg f, the PCM applies a ground to the Cooling Fan Relay #1, energizing it and allowing power to be applied to a series circuit for both the left and right fans - power goes through the left fan to the right fan (via the deenergized contacts of the Mode Control Relay) - the ground is after the right fan. Since both fans are loading this circuit, they will both run at a lower speed. When the coolant temp reaches 230 deg F, the PCM provides a ground for the other two relays in the system - the Cooling Fan Relay 2 and the Mode Control Relay. With all three relays energized, a parallel circuit is set up so that each fan has its own path to power and ground, causing both fans to run at a higher speed.

For this to be anything other than a PCM failure would require that you have multiple problems as with any one failure you would still have some sort of fan operation - either single fan in high speed or both fans always in low speed - but not a loss of both fans. No single failure of a relay, fuse or wiring problem will cause this problem!

You can confirm this (if you have access to and know how to use a multimeter) by removing the K14, K15 and K 12 relays. Let your car run and heat up. Once the temp indication goes a bit above the halfway mark on the gauge, check for a ground at pin 85 of the K12 relay socket (top left as looking from the drivers side of the car). The PCM should be sending a ground to energize this relay - I bet you will find it is not. Let the engine heat up to the 3/4 mark on the gauge and look for a ground at K15 socket pin 86 (top left) and K14 socket pin 85 (top left). The PCM should be providing a ground to all three relays now causing both fans to run at high speed.

The PCM does have other logic for controlling the fans with the A/C on, and with the ignition off, but I think the above explanation should help solve your problem. If your temp gauge is not showing the temperature (or always reading on the low side), then replacing it will likely solve this problem as this sensor input is used by the PCM for fan scheduling.

I've had this problem for a while and only started to troubleshoot it today. Been adding coolant/water on a regular basis, but as most of my driving is at highway speeds, the engine normally stays pretty cool (until I hit Northern VA traffic).

I am heading back home from Washington DC to Atlanta tomorrow and plan to install a jumper (bent paper clip) between K15 pin 87 and 87A, which will energize K14 causing the Right Cooling Fan to run on high at all times (I already tested it and it works fine). This should keep the engine temp in the normal range even if I get stuck in traffic. Just remember that if you need to do this to get you home, you will want to remove the jumper if you are stopping for any period of time as the fan will continue to run even with the car turned off. The same result could be had by jumping across K14 pins 30 to 87, but I would not recommend this as this circuit carries the right fan load (not just the load of a small relay) and will get the jumper very hot - maybe even melting it or causing an electrical fire in the engine compartment - not good!!! Hopefully I will get around to installing a new PCM in the not to distant future.