Idle Affected When Fans Kick On
Thread Starter
Burning Brakes
Joined: Nov 2003
Posts: 1,234
Likes: 5
From: Herriman UT
Idle Affected When Fans Kick On
86 L98 Idle drops when fans kick on, ometimes to the point that engine dies. For some reason, probably in the custom chip, both fans kick on simultaniously. I notice volts drop from 13.5 to 12.9 when fans start. Fans drawing too many amps? Could that affect idle quality?
Melting Slicks
Joined: Aug 2003
Posts: 3,413
Likes: 404
From: Lakeville MI
5 counts are added to the IAC position when fan activation is anticipated.
Also the injector pulse width is varied as a function of voltage.
Both can be an influence to the idle behavior.
If both fans are being activated together by the ecm then there must have been some wiring changes performed on the car to tie the relay ground connections together.
If the fans are being activated together instead due to the aux. fan switch in the cylinder head, then the ecm would not be able to anticipate this and add the 5 counts to the IAC.
You may want to unplug the aux fan switch if it is still present and let the ecm control the fans if it is indeed wired as I imagine.
Also the injector pulse width is varied as a function of voltage.
Both can be an influence to the idle behavior.
If both fans are being activated together by the ecm then there must have been some wiring changes performed on the car to tie the relay ground connections together.
If the fans are being activated together instead due to the aux. fan switch in the cylinder head, then the ecm would not be able to anticipate this and add the 5 counts to the IAC.
You may want to unplug the aux fan switch if it is still present and let the ecm control the fans if it is indeed wired as I imagine.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
Each fan motor draws 14 amps running and about double that momentarily to start, or 56 amps until both motors get up to speed. The alternator supplies this 56 amps, and at idle speed, cannot, this is why the voltage drops momentarily. The ECM attempts to maintain constant idle speed, but cannot instantly react to instant loads (like the alternator supplying power to both fan motors suddenly turned on). I don't know what effect an aftermarket memory chip does to the ECM circuit that attempts to maintain constant idle speed, but I suppose it is possible for it to interfere with that function.
Team Owner
Joined: Mar 2001
Posts: 54,124
Likes: 30
From: Cincinnati, Oh USA
The auxilliary fan switch is on the left head between spark plugs #1 and #3. Previous owner probably put a lower temp thermostatic switch in that turns on at 217 F. The ECM is programmed to turn on the main fan at 228F and the OEM switch turns the aux fan on at 238 F. In my opinion, and I own an 87, you don't need the aux fan on until 238 F. The factory settings for the fans is perfectly ok and safe for the engine. GM says to turn off the engine at 260 F and let it cool down! 2nd to last time I had my heads worked on, we also installed a Stewart water pump (OEM pump leaked) that has more flow than stock and we let the engine idle in summer temps and the main fan came on at 228 F and turned off at 211 F and repeated. 228 F is not harmful to your engine! Make sure your radiator has debris cleaned out of the front of it, C4's collect all kinds of crap from the road and this reduces the cooling capacity of the radiator. I'm lazy and I use a garden hose and blast water backwards through the radiator while holding the fan blade.
Melting Slicks
Joined: Aug 2003
Posts: 3,413
Likes: 404
From: Lakeville MI
In this case, the ecm should be able to anticipate the fan activation.
Since both fans are wired together which increases the load, you may just want to increase the normal 5 IAC counts that are added in anticipation of the fans to something like 10 counts to compensate.
Thats what I would do.
Raising the idle rpm to increase the alternator output in the vicinity of the fan on coolant temperature would be another option. Based on the temperature, you know when the fans will be running, so you can raise the idle rpm for the relevant range of coolant temperature.
Since both fans are wired together which increases the load, you may just want to increase the normal 5 IAC counts that are added in anticipation of the fans to something like 10 counts to compensate.
Thats what I would do.
Raising the idle rpm to increase the alternator output in the vicinity of the fan on coolant temperature would be another option. Based on the temperature, you know when the fans will be running, so you can raise the idle rpm for the relevant range of coolant temperature.
Last edited by tequilaboy; Jul 30, 2007 at 10:47 AM.
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Thread Starter
Burning Brakes
Joined: Nov 2003
Posts: 1,234
Likes: 5
From: Herriman UT
In this case, the ecm should be able to anticipate the fan activation.
Since both fans are wired together which increases the load, you may just want to increase the normal 5 IAC counts that are added in anticipation of the fans to something like 10 counts to compensate.
Thats what I would do.
Since both fans are wired together which increases the load, you may just want to increase the normal 5 IAC counts that are added in anticipation of the fans to something like 10 counts to compensate.
Thats what I would do.