john d Posted October 16, 2021 Share Posted October 16, 2021 fans will not run on high speed. changed cts. motors good. will run if high speed relays grounded contact 85, only high speed. puller fan will only run high with ac on. Any suggestions Link to comment Share on other sites More sharing options...
Ronnie Posted October 16, 2021 Share Posted October 16, 2021 Welcome to the forum John! Are you allowing the engine to get hot enough for the fans to run on high? I think it has to get up to about 230 for the fans to run on high. Link to comment Share on other sites More sharing options...
john d Posted October 16, 2021 Topic Author Share Posted October 16, 2021 yes sir. Glad to be here as well Link to comment Share on other sites More sharing options...
Padgett Posted October 16, 2021 Share Posted October 16, 2021 (edited) The fan circuit is in section 8A-31 in the 90 FSM. Stock low speed comes on at about 97C coolant or with AC and high at about 107C (readout at ED04). Can override at ES09 and ES10. Edited October 16, 2021 by Padgett 1 Link to comment Share on other sites More sharing options...
john d Posted October 17, 2021 Topic Author Share Posted October 17, 2021 Appreciate response from Padgett. I don't have shop manual if more info could be forwarded would appreciate. also would like guidance as to purchase on FSM Link to comment Share on other sites More sharing options...
Ronnie Posted October 17, 2021 Share Posted October 17, 2021 Below is the wiring diagram that is in the section of the FSM relevant to your cooling fans. I doubt you have a wiring problem but you might have a bad connection. If the fans will run on high by grounding the relay, it would seem the ECM isn't commanding them to come on high when the engine reaches 230*. The question is why. The first thing you should do is as Padgett suggested. Go into the onboard diagnostics (no scanner needed) and look at ED04 to see what temperature the ECM is seeing when the engine is over 230* on the gauge and the fans should be running on high. Then go to see if you can manually turn the fans on using Overrides ES09 and ES10. Instructions on how to do this can be found in the "Diagnostics" menu at the top of the page. 1 Link to comment Share on other sites More sharing options...
Padgett Posted October 17, 2021 Share Posted October 17, 2021 (edited) Note the controls are the two Grounds, A & B applied through the engine control module as a function of the engine coolant sensor and the AC. BTW for years you could just download the service manuals and other things from a website with GM's permission. Then they revoked the permission and we got lawyer demands for removal. Edited October 17, 2021 by Padgett 1 Link to comment Share on other sites More sharing options...
john d Posted October 19, 2021 Topic Author Share Posted October 19, 2021 Thanks to all. Now looking at high pressure ac switch. Seems to be remaining open. When connector is removed fans work with ac control. OHM'd connector, it's good. When changing sensor will I lose all freon? Link to comment Share on other sites More sharing options...
Ronnie Posted October 19, 2021 Share Posted October 19, 2021 I'm not familiar with the high pressure A/C "Switch". is it located on the compressor. There is a High Side "Temperature" Sensor on the high side A/C line as shown in the photo below. There is a Low Temperature Sensor and a Low Pressure Switch on the low pressure line. The temperature sensors are variable resistors. I can't imagine either on of them keeping the fans from running on high but anything is possible. The Low Pressure Switch can be replaced without losing the Freon. If you remove the Low or High Side Temperature Sensors the Freon will escape. The temperature sensors are no longer available new. Link to comment Share on other sites More sharing options...
Padgett Posted October 19, 2021 Share Posted October 19, 2021 (edited) What is BD27 reading with engine off and cool ? BTW when removed some freon will escape but not much if you plug the hole quickly. With R134A running under $4 a can I would not be concerned. Edited October 19, 2021 by Padgett Link to comment Share on other sites More sharing options...
john d Posted October 19, 2021 Topic Author Share Posted October 19, 2021 Ronnie it is the high side temperature sensor in question as you pictured. When connector is removed fans will run on high with ac on. Link to comment Share on other sites More sharing options...
Ronnie Posted October 19, 2021 Share Posted October 19, 2021 Do you have any BCM codes stored in diagnostics? Is the A/C cooling as it should? Link to comment Share on other sites More sharing options...
john d Posted October 19, 2021 Topic Author Share Posted October 19, 2021 will check codes tommorow. ac is running as it should. Can vehicle run correctly with fans on high when ac is on? never going into low speed. No check engine light on now. Link to comment Share on other sites More sharing options...
Padgett Posted October 19, 2021 Share Posted October 19, 2021 Will cool even better on high. Link to comment Share on other sites More sharing options...
Ronnie Posted October 20, 2021 Share Posted October 20, 2021 The high temp sensor does send an input to the BCM which sends a signal to the ECM. It appears to me that you would need to ground the high temp sensor connector to make the fans run on high instead of just unplugging the connector. This diagram goes with the one I posted earlier. Link to comment Share on other sites More sharing options...
Padgett Posted October 20, 2021 Share Posted October 20, 2021 I just reprogrammed mine to work with a 180F Superstat, Low comes in at 184F and out at 182F, High in at 188F and off at 185F. May see 190F at a long hot stoplight but comes back down. Link to comment Share on other sites More sharing options...
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