Fan Relay Troubleshooting

Its unlikely but I think I will test my fan relay and make sure I didn't get a bad one. Also, PO ripped out the AC somewhat inelegantly. Not sure how the AC is tied into the fan relay.

I appreciate all the help!!
 
Typically on anything with electric fans, when the AC is turned on the fan comes on to help pull air across the condenser.
 
Well guess what? My new relay is bad. I connected pins 3 & 4 to the negative and positive leads of the battery. This should have closed the circuit and I should have continuity across 1 & 2 but clearly, I did not. Got a new one on order which should be here by Friday. Will report back.

BADFF912-4350-47DC-80C4-B29957502EA4.jpeg
 
New fan relay installed. Still have p1491 error. Multiple other issues with starting and runability to deal with. Might have to raise the white flag and call a real mechanic.
 
Figured out the issue but not sure how to solve it. I am getting the 5 volts to the coolant sensor but when I jump the plug for the coolant sensor, only .9 volts makes it back to the C1 pin 16. So I have a voltage drop along the way. Anyone know how to find the cause of voltage drop in a wire???

137E715B-86B5-435E-BCDE-160E98A9A44B.jpeg
 
Pin 1 is ground and is supplied by the PCM, it should have no voltage on it. Pin 2(aka signal) is not going to be a constant 5v. When you disconnect the coolant temp sensor it should display 5v at pin 2. when you jump pins 1 and 2 together you should read 0v at the pcm.

2 wire temperature sensors are as simple as it gets. The PCM supplies 5v through a fixed resistor in the PCM. The temperature sensor is also a resistor but its resistance changes as the temperature changes. As the resistance of the sensor changes, the voltage also changes because the 5v source is fed through the fixed resistor in the PCM(its really just a series circuit with 2 resistors). the PCM monitors the circuit between the fixed resistor and the variable resistor(the temp sensor).

You really need a scan tool to see what the PCM is seeing as far as coolant temp sensor signal voltage. When the sensor is unplugged the PCM should read 5v. When the 2 wires are jumped together the PCM should read 0v.

Im a little confused how you performed your measurement. How did you determine you had .9v at the pcm?
 
You could jump pins 1 and 2 together at the sensor connector then use your ohm meter and go from PCM c1 pins 16 to 4. You should read less than 1 ohm.
 
  • Like
Reactions: Sputter
Thanks for the reply.
Pin 1 is ground and is supplied by the PCM, it should have no voltage on it. Pin 2(aka signal) is not going to be a constant 5v. When you disconnect the coolant temp sensor it should display 5v at pin 2. when you jump pins 1 and 2 together you should read 0v at the pcm.

2 wire temperature sensors are as simple as it gets. The PCM supplies 5v through a fixed resistor in the PCM. The temperature sensor is also a resistor but its resistance changes as the temperature changes. As the resistance of the sensor changes, the voltage also changes because the 5v source is fed through the fixed resistor in the PCM(its really just a series circuit with 2 resistors). the PCM monitors the circuit between the fixed resistor and the variable resistor(the temp sensor).

You really need a scan tool to see what the PCM is seeing as far as coolant temp sensor signal voltage. When the sensor is unplugged the PCM should read 5v. When the 2 wires are jumped together the PCM should read 0v.

Im a little confused how you performed your measurement. How did you determine you had .9v at the pcm?
Thanks for the reply. Obviously my assumptions are incorrect. I thought when you jumped the coolant sensor it would send five volts back to the pcm which would initiate the fan relay.
  1. I verified 5 volts on the coolant sensor pigtail
  2. I thought that jumping the pigtail would send the 5 volts back to the pcm
  3. I unplugged C1 and probed pin 16 looking for 5 volts but only got .9
just now realized that C1 controls the 5 volt supply so my test was worthless. What’s the right way to test what’s going one?
 
I really doubt your issue is with the coolant temp sensor. If it was you wouldnt be getting a fault for the fan relay circuit.

The coolant temp sensor isnt a switch. Its a variable resistor that tells the PCM the coolant temperature. When the PCM sees above a specific coolant temp(I dont know the actual number but my guess would be 210-220*F) It sends 12v to pin 4 of the relay which energizes the relay and connects pins 1 and 2.

The PCM performs a constant diagnostic where it expects to see a specific resistance to ground at C2 pin 17. This resistance would be the resistance of the relay coil(pins 3 and 4). Your PCM is seeing too much resistance(open circuit) which is why P1491 is active at all times.

It appears you have a good ground since you tested this with a test light(good test). You ohmed out the wire between c2 17 and the relay and it checked out ok(ohm test isnt a great test but we will trust it for now). You have tried multiple relays but still get the code... I have seen many techs get burned by expanded terminals. The PCM uses small round pins and the fan relay uses flat pins. It looks like you are using a needle point lead on your meter and it looks small. However, if you arent careful you could be expanding the terminals. Even worse if you previously forced a larger meter lead into the connector. Can you take a really close look at both of these female terminals and make sure they look like the others?
 
New connector on the fan relay and still no joy. That rules out the connector. Also jumped the temp sensor and verified 0 volts to pin 16 on C1. Is there a way I can check the resistance the PCM is getting on c2 17?

CAA24B5E-A6ED-4B45-8464-EB84DAACB575.jpeg


62AC7986-7833-4998-8DC4-174F26D706F6.jpeg
 
Install the relay, disconnect c2 and ohm from pin 17 to battery negative.

Then disconnect the relay and Ohm pins 3 to 4 on the relay.
 
Last edited:
by ohming c2 17 to ground you should theoretically be ohming out the relay coil.

What does your ohm meter read across relay pins 3 and 4?

Screen Shot 2020-12-09 at 4.55.11 PM.png