Coolant temperature sensor on 1987 with TPI
Thread Starter
1st Gear
Joined: Sep 2018
Posts: 1
Likes: 0
Coolant temperature sensor on 1987 with TPI
With digital gauges. Had the cooling system flushed and 180 degree thermostat installed to replace original 190. Before that, fan turned on at 185 and after thermostat opened, gauge indicated steady state temperature around 198F. Now, thermostat opens around 180, fan turns on at 188 but temperature continues to rise to the 120 range. Decided to change coolant temperature sensor and Haynes indicates sensor location in front of the intake manifold. Unplugged that sensor and it has 4 terminals. What is that sensor? Secondly, from this forum, I've located the single wire sensor that I wanted to replace in the right manifold. Thanks in advance for any assistance.
Advanced
Joined: Mar 2013
Posts: 85
Likes: 4
From: Whetstone Arizona
I just installed a 195 degree cooling fan switch from GMP Parts in my 87. According to the literature that came with it the 86 and newer year models have the cooling fan switch located on the left cylinder head between plugs 1 and 3. The 84 year model has the CFS in the right cylinder head between plugs 6 and 8. The 85 year model only uses the a CFS if it has and optional auxiliary cooling fan and it is also on the passenger side between 6 and 8. Seems like the 85 thru 89's had the same optional aux fan but the switch was in the passenger side head for the 85 and the left head for the rest.
Racer
Joined: Dec 2017
Posts: 250
Likes: 41
From: Sydney NSW
The wiring diagram in this thread should be of help to you as well.
https://www.corvetteforum.com/forums...-problems.html
https://www.corvetteforum.com/forums...-problems.html
Race Director
Joined: Sep 2003
Posts: 16,838
Likes: 341
From: Database Error Indiana
Cruising
Joined: May 2018
Posts: 11
Likes: 0
87 corvette cooling fan junk
wondering if someone can set me straight. I have a 1987 corvette. it has the aux cooling fan in front and main fan behind radiator. I replaced all relays. aux fan comes on at 238-240 degreees like supposed to i believe. the main fan behind the radiator will come on if I jump the a and b ports on the aldl plug. however the main fan behind radiator will not come on at 228 degrees from what I've read it's supposed to. My a/c is in operable currently. I have a fsm for 1987 but in the diagnosis section for cooling fan it says if a/c is inoperable to refer to section I or 1, I can't be sure but I can't find this section. I have checked my coolant temp sensor, I have 5v at the plug when ign is in on position. the resistance for the cts came back at i believe 4500 ohms. 20k setting I got 4.5 is that right? the temp outside was around 45 degrees so on the tables I found the ohm reading seemed low, although I am no expert. If I'm missing something thank you in advance for everyones help
Cruising
Joined: May 2018
Posts: 11
Likes: 0
thanks waddiejohn for responding. so that main fan only comes on if the a/c is activated? it doesn't come on at 228 degrees like I've been reading? the only fan that keeps the car cool is the front aux fan? that comes on at 238ish? I know these cars have a high coolant temp, just want to be sure it's functioning as it should. so if I disconnect the pressure switch for the a/c it should activate the fan because it thinks the a/c pressure is over x amount of psi if I have the a/c on? thank you again for all your help.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Advanced
Joined: Mar 2013
Posts: 85
Likes: 4
From: Whetstone Arizona
The fan in front of the radiator, the one farthest from the firewall is controlled by the thermal switch on the drivers side of the engine and should activate around 227 degrees F.. The fan between the engine and the radiator is tripped by the A/c pressure switch. If you unplug it nothing happens. That switch connects to ground when it's made. If you want to use it you'd have to run wires from one side of the connector plug to a switch of some sort to then another wire to the other side of the plug. There is a relay in the circuit and the wires from the plug actually run to the relay so if you connect the two wires from the pressure switch together with the key on and your fan doesn't run either the fan is bad, the relay is bad or there is a faulty wire or connection in between. Hope this make sense.
Melting Slicks
Joined: Aug 2003
Posts: 3,403
Likes: 398
From: Lakeville MI
For 86-89 cars (with standard tuning) the main fan is activated by the ecm if the coolant temp is greater than 226° F or if the A/C high pressure switch is active. Both conditions can activate the fan independently.
Example log showing the main fan control 226° F On and 220° F Off switching points (from a stock 88):
Warning: the sheer magnitude of the coolant temperature displayed in this log may cause severe and irreversible emotional distress to many members of this board. You have been warned.
https://datazap.me/u/elijah/gas-stat...-1993-2084-938
Note: Fan will be deactivated 15 seconds after the coolant temperature falls below the 220° F Off threshold.
Example log showing the main fan control 226° F On and 220° F Off switching points (from a stock 88):
Warning: the sheer magnitude of the coolant temperature displayed in this log may cause severe and irreversible emotional distress to many members of this board. You have been warned.
https://datazap.me/u/elijah/gas-stat...-1993-2084-938
Note: Fan will be deactivated 15 seconds after the coolant temperature falls below the 220° F Off threshold.
Last edited by tequilaboy; Apr 18, 2021 at 03:35 PM.
Cruising
Joined: May 2018
Posts: 11
Likes: 0
right on, so the fan farthest from the fire wall I did test previously by grounding the plug on the sensor between cylinders 1 and 3 and that came on, seemed to come on around 238 or 240. the fan closest to firewall I tried hooking up a jumper on the high pressure switch like you talked about, I turned the car to on position and messed with my climate control but unfortunately that fan did not come on. however this fan does come on when I jump a and b on the aldl port. so maybe Im not jumping the right plug or there is something wrong with my climate control device or there is an issue with a wire in the circuit. does the car have to be running? thanks again for all the info makes me feel like I've actually accomplished something with this car. soon she'll be back on the road
Drifting
Joined: Jan 2021
Posts: 1,314
Likes: 384
From: Northern California
The engine does have to be running Yes for the main fan to come on at 227/228F if the A/C is off. The ECM controls the main fan in that case, using the ECM temp sensor in the front of the intake manifold near the water pump. That sensor can have sealant on the threads and it does not have to be grounded to the manifold / block.
The dashboard gauge shows the temperature seen from the gauge temp sensor in the head, one wire terminal, and that sensor is grounded through the engine block. It can have some sealer on the threads but should have a portion near the end which is left bare metal for ground contact to the head.
When you ground terms A/B on the ALDL, you enter diagnostic mode which runs the fan by default even with engine off.
The dashboard gauge shows the temperature seen from the gauge temp sensor in the head, one wire terminal, and that sensor is grounded through the engine block. It can have some sealer on the threads but should have a portion near the end which is left bare metal for ground contact to the head.
When you ground terms A/B on the ALDL, you enter diagnostic mode which runs the fan by default even with engine off.
Cruising
Joined: May 2018
Posts: 11
Likes: 0
Ok. My main fan only comes on if I put the car in diagnostic mode.
It does not come on at 228 when the car is running.
my ac doesn't work.
I checked my coolant temp sensor for resistance at multiple degrees and the plug has 5v. Seems to be working.
All relays are good, like I said jumping a and b on the aldl makes the fan work so the relay is good.
could a coolant temp sensor not reach fan activation temp?
It does not come on at 228 when the car is running.
my ac doesn't work.
I checked my coolant temp sensor for resistance at multiple degrees and the plug has 5v. Seems to be working.
All relays are good, like I said jumping a and b on the aldl makes the fan work so the relay is good.
could a coolant temp sensor not reach fan activation temp?
Team Owner
Joined: Sep 2001
Posts: 62,161
Likes: 1,733
From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
For 86-89 cars (with standard tuning) the main fan is activated by the ecm if the coolant temp is greater than 226° F or if the A/C high pressure switch is active. Both conditions can activate the fan independently.
Example log showing the main fan control 226° F On and 220° F Off switching points (from a stock 88):
Warning: the sheer magnitude of the coolant temperature displayed in this log may cause severe and irreversible emotional distress to many members of this board. You have been warned.
https://datazap.me/u/elijah/gas-stat...-1993-2084-938
Note: Fan will be deactivated 15 seconds after the coolant temperature falls below the 220° F Off threshold.
Example log showing the main fan control 226° F On and 220° F Off switching points (from a stock 88):
Warning: the sheer magnitude of the coolant temperature displayed in this log may cause severe and irreversible emotional distress to many members of this board. You have been warned.
https://datazap.me/u/elijah/gas-stat...-1993-2084-938
Note: Fan will be deactivated 15 seconds after the coolant temperature falls below the 220° F Off threshold.
ECM is reading the temperature given to it from the CTS at the front of the engine. A scanner will tell you if the dash reading and the ECM reading match, they should. If it is reaching the temperature that is set in the ECM, which is adjustable in the chip tune, the relay gets grounded by the ECM and the fan comes on. CTS does not do anything but read temps to the computer.
If the ECM is commanding it on, the relay is what to check. If not, then someone could have tweaked the tune or there is a problem in the wiring either from ECM to fan relay or relay to fan.
Last edited by vader86; Apr 19, 2021 at 06:55 AM.