High Fan And Pin 17


10 Year Member
Jul 19, 2007
Vernon BC
I suspect my high fan isn't switching on. Note that I'm running an AEM-EMS. However, if I ground pin 17 with a cold engine should I expect the fan to come on or would it only come on at or above its set temperature? It doesn't come on (cold) but there is a "click" sound as if a relay is being triggered. If I run power to the high pin on the fan motor connector it does run. Same with power to the low fan so we know the fan motor is good. Hissin50 are you still lurking about?
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Hey Richard!

I think you're right on the money. Pin 17 should energize the HEDF anytime (by grounding it, you're bypassing the ECM control and temperature requisites).

So with Pin 17 grounded and the key-on, check CCRM pins 6 and 7 for 12 volts. If they're not hot, Check Pins 3 and 4 for constant 12 volts. If you have 3 and 4 but not 6 and 7, that points to a relay control issue.

I know you know, but for others that search in the future, we'll mention to check the high-speed fan wiring terminals (they char) and look for a defective circuit breaker on the fan connector.

If you wanna find a work-around, you have options. You could just wire up your own relay in place of the OEM relay.

Good luck!
That sucks. How to fix it will depend upon if you have a spare CCRM, are willing to splice into the OEM wiring harness, etc. I know you already have a direction (and solution) in mind, Richard. :) Let me know if I can be of any assistance.
I had a local shop check this issue. The problem appears to be that the EMS is not sending or the CCRM isn't getting the signal to switch on the high fan. So, we are going to use the low fan signal to trip a relay wired to turn on the high fan. I'm thinking that the fan should be set to come on at at 200*. Any thoughts about the temp setting, higher or lower?
Since you could ground Pin17 and the relay didn't close [energize the fan], is it known that the HEDF relay simply didn't crap out? If that's all it is, just wire in a new stand-alone relay so the car functions as it did before. Or test the EMS wire that should be commanding the HEDF. It should be pretty simple if you can access the wire and the car is up to temp (easy to do after driving it home, etc).

For the work-around method, you'll want to include a leg to the new relay's 86 that goes to the *input* to the LPCS (so the fan comes on with the AC, like it does from the factory). Tapping the input (not the output) will keep the fan powered-on so it doesn't cycle as the compressor cycles (the factory computer has engineered hysteresis to prevent this, but a simple relay circuit won't). Use diodes on both wires going to new-relay's 86 terminal (to prevent backfeed on both circuits).

For the fan on/off temperature selection, it depends upon your t-stat and how much spread there is in the on/off temps with your tune. You'll probably want the fan to come on about 15=20* over the temp that the t-stat is fully open and shut off about 7-10*F over the temp that the stat fully opens. So if the stat opens fully at 180*F, set it on at 195-200*F, off at 185-190*.

You don't want the stat's fully-open temp and the off-temp of the fan to be too close (or the fan ends up running almost non-stop once the car is up to temp). Also note that if you run an unbalanced t-stat, the temp that it opens at might be much higher than its 'rating'. I.e., a lot of "195" degree t-stats don't fully open till 212-215*F. And thus the fan temps would need to be adjusted accordingly (so the fan isn't running while the t-stat isn't even fully open. E.g., what's the point of having the fan on at 215 and off at 205 if the stat doesn't even open all the way till 212*). More info about the t-stat and how much spread you want between fan temps would help tailor the recommendation.

Just my thoughts. Hopefully that makes some sense.
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Today we revisited the fan pigtail. There is visual evidence on the high fan pin of overheating. So that tells us the high fan is or has worked from about this time last year when I replaced the pigtail. What we are not sure of is the fan motor connector. I'm thinking that the fan motor when working may be drawing to much current indicating a fan motor issue along with the possibility that the fan motor connection on the high side may be faulty and intermittent. I'm inclined to believe that the info I got from AEM tech support regarding switching on the high fan when the A/C is turned on may not be correct. That would explain why there isn't voltage to the output pins when the A/C is turned on. I'm also concerned that the AEM is not switching off the the low fan when the high fan is commanded on resulting in power to both fan modes. Maybe I just have to get used to not running the A/C unless I'm at speed on the highway.
That's a lot of variables. The work-around discussed above should work fine and alleviate the issues as long as low speed is being commanded properly at whatever temp you set it at.