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Cooling fan question...

 
Old 03-09-2019, 06:02 PM
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t5258
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Default Cooling fan question...

Sooo here's my question: I have an 85 Corvette with HD cooling. When the temp hits an indicated 232, only the aux (pusher fan in front of the condenser/rad) fan cycles, the main fan under the hood does not. Going thru the repair manual (chart C-12B-coolant fan inop) everything functions as designed. If I jump the ALDL, it kicks on, if I disconnect the ac press switch, it runs....When I bought the car in November, I noticed the relay had been replaced with an aftermkt one. I've since replaced the pigtail & relay with the correct type, verifying that the wires were routed correctly. Is it normal for the aux to run first? The manual's description of the circuit operation is vague to me regarding the sequence...HELP!!!
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Old 03-09-2019, 06:29 PM
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Main fan should come on before the aux fan.
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Old 03-09-2019, 07:31 PM
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aux fan has a coolant temp sensor on the driver's side bank....not associated with main fan.....main fan should come on with a/c.....or when the ecm says so.....
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Old 03-09-2019, 09:52 PM
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doesn't come on with a/c...Could the coolant temp sensor be bad & not trip a code??
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Old 03-09-2019, 10:17 PM
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A scan tool can read the ECM parameters and tell you what coolant temp it is reading.
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Old 03-10-2019, 01:12 AM
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i had the same trouble. I replaced the temp sensor with a autozone brand. Still had the the trouble. Ended up getting a ACdelco temp sensor and that fixed it.
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Old 03-10-2019, 05:47 AM
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Originally Posted by ihatebarkingdogs View Post
Because the main fan runs with the ALDL jumpered, the ECM is capable of controlling the fan, and the circuit is intact. So it's the programming of the ECM (the "chip"), or the coolant temp input to the ECM. I'd check the resistance of the CTS vs the chart in the FSM. Or just change it. A Delco CTS should be less than $20 pretty much anywhere. Otherwise, something is fubar in the chip.
That makes sense...I'm gonna ohm out the sensor to confirm it's gone south...I figured I'd have at least an intermittent code 14 or 15 though....
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Old 03-10-2019, 11:52 AM
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I had a different problem where the CTS not telling the ECM to lean out as car warmed up causing it to stall. It was throwing a 15 code pointing to the CTS. I unplugged the CTS to confirm it was causing the problem.

After unplugging it I found that the fan would run continuous regardless of coolant temp. You may want to experiment by unplugging yours to see if it changes anything. Of course if it is a problem with the fan itself nothing will change for you.
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Old 03-10-2019, 02:11 PM
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Originally Posted by ihatebarkingdogs View Post
Not on 85. 85 aux fan switch is on PASSENGER side, between #6 and #8.

Aux fan switch moves to driver side in 1986. 1985 on passenger side is one year only.
pretty sure I changed that sensor, and it was on the driver's side....i'll bow to your wisdom until I go out and check....
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Old 03-10-2019, 06:56 PM
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Originally Posted by ihatebarkingdogs View Post
The parameters for 14 and 15 are generally only met with an open or shorted sensor circuit. Skewed resistance conditions generally won't set 14 or 15, because the sensor voltage is still within the acceptable range, but "wrong". I have a Lot of miles (750,000+) in GM vehicles with this sensor and I have yet to replace one for "out of range", or for any other reason than mechanical damage by breaking it while working on something else. I don't think it's going to be the sensor.

What PROM is in it?
Beats the daylights out of me, haven't dug into the ECM yet...HOWEVER...following the Flow chart for fan testing-not the flow chart where the fan won't kick on, it led me to the AC pressure switch. When I unplug it the fan will cycle, so the manual says the switch is bad. I find it hard to believe that the ECM will take this switch's info over the CTS. If the system was completely empty (which it isn't) it wouldn't play any factor in the ECM determining whether to cycle the fan or not, right? As far as identifying the PROM, I'm curious as to why that would matter...this has me kinda puzzled...with it working when the ALDL is grounded, doesn't that confirm the ECM is doing it's job?
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Old 03-11-2019, 05:52 AM
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The main fan should come on before the aux fan. The main fan is programmed for 228 degrees and the aux fan is 238 degrees. You need a scanner to determine what the computer is seeing. It is possible that the computer which is being sent temperature information from a separate sensor does not indicate 228 degrees when the aux fan kicks in. The main fan nay be coming on late and the aux early, both are run from separate sensors. It is also possible that somone programed the main fan to come on latter but I doubt it.

A scanner would really help with this issue.
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Old 03-11-2019, 07:49 AM
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Originally Posted by bjankuski View Post
The main fan should come on before the aux fan. The main fan is programmed for 228 degrees and the aux fan is 238 degrees. You need a scanner to determine what the computer is seeing. It is possible that the computer which is being sent temperature information from a separate sensor does not indicate 228 degrees when the aux fan kicks in. The main fan nay be coming on late and the aux early, both are run from separate sensors. It is also possible that somone programed the main fan to come on latter but I doubt it.

A scanner would really help with this issue.
Agreed...I have an OBD2 scanner, but am a little light on one for this vintage...planning on testing the CTS to see if its lazy on the hi temp end...not expecting much though...
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Old 03-12-2019, 08:03 AM
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Problem solved....took a deep financial risk (all of 16.00), replaced the CTS with a Delco, at 228 the fan kicked on, so I guess the original was getting lazy....
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Old 03-12-2019, 09:35 AM
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Originally Posted by t5258 View Post
Problem solved....took a deep financial risk (all of 16.00), replaced the CTS with a Delco, at 228 the fan kicked on, so I guess the original was getting lazy....
I still have to replace my CTS, but I've been lazy because my 91 runs fine with it unplugged (except for fan running constant). Where did you pick yours at and what was the part number?
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Old 03-12-2019, 11:08 AM
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Originally Posted by CorvetteRules View Post
I still have to replace my CTS, but I've been lazy because my 91 runs fine with it unplugged (except for fan running constant). Where did you pick yours at and what was the part number?
O'Reilly's...AC Delco 15-51107...
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Old 03-12-2019, 11:09 AM
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Originally Posted by t5258 View Post
O'Reilly's...AC Delco 15-51107...
May be more $$....We use O'Reilly's where I work, so my cost was wholesale...probably not a huge variation...
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Old 03-12-2019, 02:15 PM
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Originally Posted by t5258 View Post
Problem solved....took a deep financial risk (all of 16.00), replaced the CTS with a Delco, at 228 the fan kicked on, so I guess the original was getting lazy....
That was my guess, I am glad you solved the issue.
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