Topic: fan temp sensor problems

[robertsiii]

when i did the fomoco fan mod i installed a temp sensor in my intake manifold. when the switch is in the on position the sensor turns the fan on at 210 on my gauge. the solinod engages  to start fan then bounces on off on off  then finally locks in  and runs till 180. when i leave the switch off and let temp get up to 230 and turn switch on it comes on and runs fine.   any ideas?  

[sharpxmen]

when i did the fomoco fan mod i installed a temp sensor in my intake manifold. when the switch is in the on position the sensor turns the fan on at 210 on my gauge. the solinod engages  to start fan then bounces on off on off  then finally locks in  and runs till 180. when i leave the switch off and let temp get up to 230 and turn switch on it comes on and runs fine.   any ideas?  

1. not sure why the temp sensor would be in the intake manifold, you should have it past the thermostat or at the rad in the cooling circuit.
2. what switch? is that switch meant to disable the electric fan circuit? if yes then it works as designed.

or maybe you can explain how it is wired and where is that switch if I misunderstood your question (as i read it i don't see a problem with the way it works).

[chardrc]

1. not sure why the temp sensor would be in the intake manifold, you should have it past the thermostat or at the rad in the cooling circuit.

on a 1990  tbi the manifold has coolant running through it from the thermostat housing and then going to the heater. so it is on the same side of the thermostat as the radiator (iirc). actually i could be wrong... there may have been a slot open buy the thermostat in the block to bypass for the intake manifold hose but my memory isn't crystal clear on the matter.

[sharpxmen]

on a 1990  tbi the manifold has coolant running through it from the thermostat housing and then going to the heater. so it is on the same side of the thermostat as the radiator (iirc). actually i could be wrong... there may have been a slot open buy the thermostat in the block to bypass for the intake manifold hose but my memory isn't crystal clear on the matter.

that's what i thought, all the cars i've seen with coolant circuit in the intake manifold had it before the thermostat - i still think should be on the radiator circuit, i'm not sure if the had the probe in the actual coolant that goes thru the manifold or just connected to the manifold somehow in which case would be totally wrong

[robertsiii]

 the switch im referring to is mounted on my dash. on runs sensor or auto. off is for water crossing . my rtherostat  housing does not have a large enough port for sensor to screw into.  so coolant leaves themostat through small hose and goes to intake. i used a T to tie in temp switch with that hose . wiring setup runs through 50 amp 5 terminal  bosch relay to soleniod which turns on fan. temp sensor engages and disengages at set temp.  ill scan wiring diagram and post. also will post pic of temp sensor on intake.

[chardrc]

 ya the manifold is definitely on a bypass to the thermostat..

[sharpxmen]

the switch im referring to is mounted on my dash. on runs sensor or auto. off is for water crossing . my rtherostat  housing does not have a large enough port for sensor to screw into.  so coolant leaves themostat through small hose and goes to intake. i used a T to tie in temp switch with that hose . wiring setup runs through 50 amp 5 terminal  bosch relay to soleniod which turns on fan. temp sensor engages and disengages at set temp.  ill scan wiring diagram and post. also will post pic of temp sensor on intake.

... i reread your first post - i think i understand what you are saying now - the problem you are referring to is the "chatter" of the relay  when the thermostat engages? if so, i would maybe test the thermostat (thermocouple switch) with a needle type multimeter and see if the closing/opening of the relay is caused by it or it's a relay problem (you would see the needle bouncing as the thermocouple closes and opens if that's the case). If you find that it is faulty replace it. If it closes but the relay "chatters" then replace that one (the relay) and see if that solves your problem.

[st.chevrolet]

Am I missing something here, why a 50A relay to activate a solenoid to turn on the fan? Why not just a 70A or 80A relay alone? Solenoid may be your chatter problem.

[sharpxmen]

Am I missing something here, why a 50A relay to activate a solenoid to turn on the fan? Why not just a 70A or 80A relay alone? Solenoid may be your chatter problem.

even with the 2 cascaded (although i agree with you there should be a bigger relay instead) i don't think it should be bouncing on and off because of that

[st.chevrolet]

even with the 2 cascaded (although i agree with you there should be a bigger relay instead) i don't think it should be bouncing on and off because of that

I agree! I was just saying the solenoid could be the culprit not the relay.

[sharpxmen]

I agree! I was just saying the solenoid could be the culprit not the relay.

yeah, could be - one too many possible points of failure

[robertsiii]

ive got a 70 amp relay ,i think ill just remove the solenoid and run straight relay as mentioned.  i only run low speed on fan so 70 amp will suffice.

[jfrabat]

ive got a 70 amp relay ,i think ill just remove the solenoid and run straight relay as mentioned.  i only run low speed on fan so 70 amp will suffice.

I run a 40A on the low speed and dont have any issues.  With 70A you can even run the high speed side!

[sharpxmen]

I run a 40A on the low speed and dont have any issues.  With 70A you can even run the high speed side!

just needs to make sure the wires match, but other than that he should be fine on the low speed with the 40 amp.

[st.chevrolet]

The Fomoco fans draw 14 amps on low and 31 amps on high, however on start up they can draw almost twice that amount.