Radiator Fans
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Joined: Jan 2024
Posts: 9
Radiator Fans
'95 LT1
3 relay setup
I've read a few posts about when the fans should and shouldn't be running and I haven't seen my question get answered in any of them. So I'll give my scenario.
Only the passenger side fan is running currently. Even with A/C on. I saw somewhere to unplug the MAF so I tried that and it didn't trigger the driver side fan.
I replaced the fan motor and still can't get the (driver side) fan to kick on. I've also swapped some relays around as well. No luck.
Any thoughts?
3 relay setup
I've read a few posts about when the fans should and shouldn't be running and I haven't seen my question get answered in any of them. So I'll give my scenario.
Only the passenger side fan is running currently. Even with A/C on. I saw somewhere to unplug the MAF so I tried that and it didn't trigger the driver side fan.
I replaced the fan motor and still can't get the (driver side) fan to kick on. I've also swapped some relays around as well. No luck.
Any thoughts?
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Joined: Nov 1998
Posts: 71,098
From: Hell was full so they sent me to NJ
Re: Radiator Fans
Have the fans ever worked correctly, or is this a new problem? If you are not the original owner, could someone have tried to wire up a manual switch, and messed around with the wires from the PCM.
http://shbox.com/1/fan_sw_diagram1.jpg
When the fans are on, both should be running. On low speed with the A/C on, or when the coolant temp reaches 226°F. They should both switch to high speed when the temp reaches 235°F.
The problem could be on the control side of the relay, the load side of the relay, or a failed fan motor. If the signal (from the PCM) side of the relay is open or shorted, should set a code in the PCM - DTC 77. Have you scanned for codes? Swapping the relays one at a time with a good one from another service in the under hood box is a way to check the relays. But you've anlready tried that. Also check fuses for the circuits.
http://shbox.com/1/fan_schematic_1995.jpg
The load (12v) side of the relays originates at a fusible link in the red plastic “+” box near the battery. Since there is only one source of 12v, and one ground, if one fan is running, the voltage supply or ground is not the problem. Individual fan motors can be tested by unplugging them and supplying them one at a time wit 12v and a ground. Was that how you determined the motor needed to be replaced?
There are several codes that turn on the fans. I've never seen a breakdown of whether it’s one fan or both, but the way the circuits from the PCM are set up, I don’t see how, with the 3-relay circuit, it would be possible for the PCM to turn on only one fan. Note that all 95’s should have the 3-relay system. The “early production”/“late production “ split actually applies to the 94 model year. GM just reused the 94 factory service manual schematic, without updating it to delete the 2-relay schematic.
http://shbox.com/1/fan_sw_diagram1.jpg
When the fans are on, both should be running. On low speed with the A/C on, or when the coolant temp reaches 226°F. They should both switch to high speed when the temp reaches 235°F.
The problem could be on the control side of the relay, the load side of the relay, or a failed fan motor. If the signal (from the PCM) side of the relay is open or shorted, should set a code in the PCM - DTC 77. Have you scanned for codes? Swapping the relays one at a time with a good one from another service in the under hood box is a way to check the relays. But you've anlready tried that. Also check fuses for the circuits.
http://shbox.com/1/fan_schematic_1995.jpg
The load (12v) side of the relays originates at a fusible link in the red plastic “+” box near the battery. Since there is only one source of 12v, and one ground, if one fan is running, the voltage supply or ground is not the problem. Individual fan motors can be tested by unplugging them and supplying them one at a time wit 12v and a ground. Was that how you determined the motor needed to be replaced?
There are several codes that turn on the fans. I've never seen a breakdown of whether it’s one fan or both, but the way the circuits from the PCM are set up, I don’t see how, with the 3-relay circuit, it would be possible for the PCM to turn on only one fan. Note that all 95’s should have the 3-relay system. The “early production”/“late production “ split actually applies to the 94 model year. GM just reused the 94 factory service manual schematic, without updating it to delete the 2-relay schematic.
Thread Starter
Registered User
Joined: Jan 2024
Posts: 9
Re: Radiator Fans
Thanks for the info. What your saying makes sense to me. I found the graphic that basically said if one fan comes on, they should both come on. No matter the scenario. I actually jumped the old fan motor via 12v and it worked 🤦.
I read somewhere about cleaning the fusible links, but I'm not sure if that is worth trying in my situation. Do you think so?
As far as the manual switch goes, we just bought the car in December and the temperature here in South Carolina just got to the point of needing A/C or worrying about the car running hot. So I'm not sure if both of them have been working.
I read somewhere about cleaning the fusible links, but I'm not sure if that is worth trying in my situation. Do you think so?
As far as the manual switch goes, we just bought the car in December and the temperature here in South Carolina just got to the point of needing A/C or worrying about the car running hot. So I'm not sure if both of them have been working.
Administrator
Joined: Nov 1998
Posts: 71,098
From: Hell was full so they sent me to NJ
Re: Radiator Fans
What do you even mean by “cleaning the fusible links”? They're fully insulated except the ring terminal on the bolt. If the terminals in the “+” box are corroded, clean all of them. I owned my Formula for 26+ years and never saw corrosion in that box. The box did still have the cover on it. If yours has the cover broken off, it might cause a problem. Measure the voltage at the fan motors…. if it's 12 volts you are OK. For that matter, if one fan is working the circuit has 12 volts and a good ground.
http://shbox.com/1/fusible.jpg
There is a second ground coming off pin J1 of the #3 fan relay. Make sure that is good. Location of the ground will be shown in the 1995 factory service manual. If you don’t have a manual, free download, courtesy of GaryDoug:
https://www.mediafire.com/?40mfgeoe4ctti
Simply looking at the schematic shows that the relays are sequenced to put the two fan motors in series so they each get 6 volts and run on low speed. The relays put the two fan motors in parallel, so they both get 12 volts for high speed. I can’t see any way the PCM could command a single fan to run.
Did you check for DTC 77? Did you check to see if someone messed with the wires from the PCM to the relays, looking at the locations shown in the Shoebox photos I linked?
http://shbox.com/1/fusible.jpg
There is a second ground coming off pin J1 of the #3 fan relay. Make sure that is good. Location of the ground will be shown in the 1995 factory service manual. If you don’t have a manual, free download, courtesy of GaryDoug:
https://www.mediafire.com/?40mfgeoe4ctti
Simply looking at the schematic shows that the relays are sequenced to put the two fan motors in series so they each get 6 volts and run on low speed. The relays put the two fan motors in parallel, so they both get 12 volts for high speed. I can’t see any way the PCM could command a single fan to run.
Did you check for DTC 77? Did you check to see if someone messed with the wires from the PCM to the relays, looking at the locations shown in the Shoebox photos I linked?
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