Dartht33bagger

This has been an issue ever since I did my engine swap 2 years. Before the engine swap, both fans worked correctly. After a hot day stuck in traffic last weekend and a temperature gauge reading higher than I would like, I decided its time to track this down. I spent 30 minutes reading through a number of threads and gave some of the suggestions a shot. What I learned reading those threads:

1. My drivers side fan works correctly and is controlled by the ECM. My passenger side fan does not turn on and is controlled by either the A/C being turned on or the temperature sensor in the passenger side head exceeding 228F or something.
2. Grounding the wire connected to the temperature sensor in the head should turn on the passenger side fan.
3. Turning on the A/C should also turn on the fan on.

What I tested tonight:

1. Pulling the second fan relay off, I confirmed that the orange wire is supplying constant 12V and the brown wire is supplying 12V when the key is on.
2. Jumping the orange wire to the black/pink striped wire turns the passenger side fan on - Confirms the fan is okay.
3. Connecting everything back up and grounding the temperature sensor wire does not turn the fan on.
4. Swapping the driver and passenger side fan relays still resulted in the fan not running. Because the driver side fan does turn on, this is unlikely to be a relay issue.
5. Starting the car and turning on the A/C only turns on the driver side fan. This confuses me as I assumed that only the passenger side fan would turn on or that both fans would turn on.

If number 5 in the testing section hadn't occurred,it would appear that I just need to put a new pigtail connector on the sensor wire. However, unless I am misinterpreting something, number 5 makes it sound like a potentially different issue. Any thoughts?

Dartht33bagger

I spent a good 7 hours working on the car today. First, I confirmed that the relay is working correctly by using some jumper wires and alligator clips to setup a circuit that I could test with a multi meter. That setup also allowed me to connect the control pin of the relay to ground to ensure the relay worked. After confirming that the relay was good, I found where the A/C pressures switch is at (on the A/C line on the passenger side of the car) and grounded that line. Again, the fans turned on. At this point I was able to positivity identify that the issue was somewhere between the coolant temperature sensor plug and the junection to the A/C pressure switch in the wiring harness.

Using the multi meter, I was able to use the ohms setting to confirm that from the relay to the coolant sensor plug, there was an open circuit, and 0.03 ohms between the A/C pressure switch and the relay. Because the A/C pressure switch and the coolant sensor junction is somewhere in the massive wiring loom behind the engine by the distributor, replacing the wire was nearly an impossible task. This led me down a path that sucked: I needed to run a new wire to the coolant sensor and splice it into the relay line. After fighting the wiring harness for an hour, I was able to extract most of the old sensor wire out, cut it off and cap it off by the ignition coil and run the new 20 gauge wire down the wire loom to the sensor location. Soldering it all up and testing it gave me a successful result: The coolant sensor plug now turns the fan on when grounded!

That is where I left off today. Tomorrow I need to go out and do some final cleanup of the wiring and hook everything up. Fingers crossed that everything will work now. I pretty much had to do the worst possible thing to get this fixed so it better work.

3.4 grape

iTrader: (1)

Electrical problems can really be a pain but I find it very challenging and rewarding when I can find the problem and fix it. The "worst possible thing to get this fixed" would have been if you had to take it to a shop and payed someone to fix it. Hope she fires up today.

PaulyC

Good job man, that's how you go about troubleshooting. You didn't need help after all!

Dartht33bagger

Thanks guys!

I got the car back together tonight and it runs/drives at least! Unfortunately, the cooler weather tonight made it so that the single fan was enough to keep the temperature cool enough that the second one would never turn on. I'll either have to wait for a very warm night or adjust my computer to turn the one fan on at a very high temperature (likely won't do this) to test it.

One thing still up in the air is the A/C pressure switch. I was able to jump the two terminals yesterday to get the second fan to turn on. When I get in the car and crank the A/C to 'MAX', it doesn't turn on. Maybe its possible that my A/C pressure switch is also bad?

Dartht33bagger

Last night was warm enough that I was able to take the car out and get the second fan to turn on! It was only for probably 5 seconds at a time but it was something. Hopefully this will be the end of this adventure.

butchc

Hi, Sorry I didn't notice your thread earlier but I just went though figuring out all the logic and wiring on my 88. Please note that the fan operating logic does vary from year to year and engine to engine so generic answers like the this fan or that fan should always run with the A/C are not always correct, that I did find out early on.. Knowing the correct logic sure helps with problem solving. The electrical supplement for the service manuals are all over Ebay for cheap and highly recommended.

Now IF your 91 has the same fan operating logic as my 88 5.0 TPI I can help you out a bit.
Here is how it all works. It is very important to keep in mind that the the drivers side fan is always controlled by the ECM. However the passenger side fan is not. It is directly controlled by 2 mechanical switches one is a coolant temp switch on the passenger side cylinder head and the other is the A/C system pressure switch as you have found.

Operating sequences
IF Fans are called for by engine temps.
The drivers side fan comes on first and is controlled by a temp sender in the thermostat housing which sends a signal to the ECM (I think 210 but dont take that to the bank, LOL ) If the engine temps continues to rise (230 I think), there is another sender in the passenger side head that turns on the passenger side fan, again this is a direct grounding of the relay, no ECM involved.

If fans are called for by the air conditioning needs this is the operating sequence.
When you first turn the AC on with cold engine neither fan should run. When the system pressure reaches around 240 psi, usually less than a minute the pressure switch turns on the passenger side fan by directly grounding the relay. It also turns on the drivers side fan by sending a signal back to the ECM which in turn grounds the relay to operate the drivers side fan, Thus both fans should come on at the same time and go off at the same time if the A/C is on and the engine temp is below 210. They should cycle on and off. If the engine is above 210 but below 230 the Drivers side fan should stay on and the Passenger side fan will cycle with the return pressure switch command. If the engine temp is above 230 both fans will run irregardless of A/C return pressure. due to the cooling needs of the engine

Hope this helps and not further confuse,
Butch

henryd3

iTrader: (128)

Pretty close but the sender in the driver side head is for the temp gauge. The CTS near the thermostat housing is what sends temperature info to the computer
for fan and other engine tuning needs

butchc

Thanks for that, I had a little brain fart. It is corrected now.