I am pretty sure I have a bad Fan Pressure Switch causing my secondary fan to be on permanently.

On my scan tool, an OTC Enhanced with the pathfinder 3 cartridge it shows Fan Request as one of the data streams, but it says it is off.

On page 6E3-C12-2 of the factory service manual is shows the pressure switch, leading to pin D11 of the ecm, and is labeled Fan Request.

On page 6E-10 it says:
"Fan Request
State of the A/C fan control switch is displayed. It
should read "yes" when fan is requested. Some
engines may display the state of the 2nd fan, if used."

So I believe that with the pressure switch stuck on the fan request indicator should be on.

Do you think there is something wrong with my ECM or a problem with the scan tool?

 

Iirc 89 is normally closed switch if it's open the fan is requested.the secondary fan is also controlled bye a temp switch in the cylinder head between 6&8. The scanner might be looking at the primary fan. With ac on does the status change ?

 

I believe it is stuck open, when it is plugged in the fan always runs. When I unplug it, the fan shuts off.

So I think with it stuck open the fan request should show on in the scan tool, but it doesnt it shows off, even though the fan is running.

No with the a/c on it doesnt change, it also does not change with the primary fan running.

 

Check the switch for continuity.

 

 

Switch has continuity, about 1 ohm on the multimeter.

 

Also we seem to have different pressure switches.




This is from the 88 Pdf, but its identical in my 89 book.

It shows the switch as normally open.

 

Also Rockauto shows the same part number for 85 to 92.

ACDELCO 1550812 Refrigerant Pressure Switch

If this is the right switch and I believe it is, so one of the books may be wrong.

Either way if the switch activates at 190 from your diagram or 233 then on an 80 - 90 degree day, it should always be active.

If I am understanding the diagram correct.



From the 88 Factory service manual.

and It would be even worse with R134, which I have.




Not sure where this is from but the R12 lines up with the service manual, so I am hoping the R134 is accurate.

Sorry this is off topic for this section of the forum.

I am still wondering why my scan tool doesn't show Fan Request as being on.

 

85-88 is no switch 89 up nc, does the status change on the scanner with primary fan operation ?

 

This diagram is incorrect. The A/C pressure switch has to be NO to operate properly. Both the head temperature switch and A/C pressure switch will close to enable the fan relay. Note that either one can run the fan.

RBob.

 

Neither is bad, the fan request flag is for the primary fan. The secondary fan request pressure switch is bad as you are reading about 1 ohm of resistance without the A/C running.

RBob.

 

The fan request does not show on with the primary fan running either, it always shows off.

 

I wouldn't worry about it, the scan tool could be wrong.

RBob.