head scratcher cooling issue


Staff member
IVE got a 1996 vette, in the shop that has cooling fans that work fine if tested,individually out of the car, the three fan relays on the radiator shroud have been replaced, the sensor in the pass side heads been replaced and all the fuses and fuze links test good but the fans won,t turn on when every things connected, trouble codes indicate relays but those are newly replaced

if your reading throught this thread theres a great deal of useful related info here

any new ideas gentlemen?
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grumpyvette said:
IVE got a 1996 vette, in the shop that has cooling fans that work fine if tested,individually out of the car, the three fan relays on the radiator shroud have been replaced, the sensor in the pass side heads been replaced and all the fuses and fuze links test good but the fans won,t turn on when every things connected, trouble codes indicate relays but those are newly replaced

any ideas gentlemen?

I would assume that you've looked at the grounding circuit? When I've had electrical problems on past Vette's I've owned, it's always been a grounding problem.

Good luck,
lovehatec4 posted this info

"The electric fans on mine don't come on until 228 for the main fan unless the ac is on. The axillary fan in front of the radiator doesn't come on until it reaches around 238-240. Mine is an 85 and they made changes on later model C4s. Check all grounds and the fuses in the fuse panel (right side dash). Since you already changed the relays, did you also change the pigtails that plug in to them. Each fan has its own relay and connector. These connectors loose voltage and are frequently the problem. You can use a simple jumper wire and test light to diagnose the problem. With the test light, make sure that the large red wire on the fan relay connector has power with key on (not running), if it does great. Then with a jumper wire, jump from the red wire on the relay connector to the green wire. does the fan spin? if so great.If not, track down the electrical short since you already stated the fans are good when you bench tested them. Next, jump from the black wire to the green wire. If you hear a clicking in the relay replace it and the connector at the same time. If you don't hear a clicking noise, then its the sending unit in the cylinder head. Check each relay and connector 1 at a time. Important, verify that the wire colors on the relay connectors are the same colors as the wire colors I described. As I said earlier mine is an 85 and I couldn't want to cause you any problems. I'm just trying to help from memory. I hope this helps.. "

1996 and up corvettes are OBD2, which cannot use the paper clip trick.

the coolant temp sensor is located on the Pass side head between # 6 and 8 plugs.

Fan control varies by year.
As I recall.
'84 has temp switch for fan only ,no ECM control;
'85 has ECM control but temp switch acts as over ride backup direct to fan
'86- '89 as stated above
'90 onward ; both fans on ECM ; no temp switch.
And all have provision to turn the fans on with A/c on.
heres more info I found if your having a similar issue















Measured Value
Engine Coolant Temperature Sensor. 185 Ohms @ 210F, 3400 Ohms @ 68F, 7,500 Ohms @ 39 F.
Engine Oil Temperature Sensor. 185 Ohms @ 210 F, 3400 Ohms @ 68 F, 7,500 Ohms @39 F.
Oil Pressure Sender/Switch. 1 Ohms @ 0 PSI, 43 Ohms @ 30 PSI, 86 Ohms @ 60 PSI.
Fuel Quantity Sender. 0 Ohms @ Empty, 45 Ohms @ 1/2 Full, 90 Ohms @ Full.
MAT (Manifold Absolute Temperature Sensor). 185 Ohms @ 210 F, 3400 Ohms @ 70 F, 15,000 Ohms @ 40 F.
Outside Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
In Car Temp Temperature Sensor. 4400 Ohms @ 60 F, 2200 Ohms @ 85 F.
MAF (Mass Air Flow) Sensor. .4 Volts @ idle, 5 Volts @ Full Throttle.
Oxygen (O2) Sensor. .1 Volt Lean Mixture, .9 Volt Rich Mixture.
TPS (Throttle Position Sensor). .54 Volts Idle, ~ 5 Volts Full Throttle.

Sensor Locations


Engine Coolant Temperature Sensor. Front of engine, below Throttle Body.
Engine Oil Temperature Sensor. Left rear of engine, just above the oil filter.
Oil Pressure Sender/Switch. Top, left hand rear of engine.
Fuel Quantity Sender. Top of fuel tank, beneath filler pipe escutcheon panel.
MAT (Manifold Absolute Temperature Sensor). Underside of manifold air plenum at rear.
Outside Temperature Sensor. Right side of engine, top right corner of radiator.
In Car Temp Temperature Sensor. Coupe: above left seat near interior courtesy light, Convertible: center of cargo compartment lid.
MAF (Mass Air Flow) Sensor. Front of engine ahead of throttle body.
Oxygen (O2) Sensor. Left side of engine, in exhaust pipe.
TPS (Throttle Position Sensor). Right side of throttle body at the front.


this is the most consistently accurate I.R temp gun I've used for testing[/img]

http://www.testequipmentdepot.com/extech/thermometers-and-humidity-meters/infrared-thermometers/high-temperature-infrared-thermometer-58to1832f-50to1-laser-pointer-42545.htm?utm_source=bing&utm_medium=cpc&utm_campaign=NEXT - Bing Shopping - Extech&utm_term=1100200223789&utm_content=All Extech Products
Cooling System Operation and Testing

Electric cooling fans attached to the radiator keep the LT1 from overheating when there is little or no air passing through the radiator core (car going very slow or stopped and engine running). It is normal for the temps on the gauge to go up to the middle or past middle of the gauge before the fans kick on. The middle of the gauge is in the range of 210º - 220º. With factory programming, the PCM will command low speed fans (or primary fan) "ON" at 226º and "OFF" at 221º and high speed fans (or secondary fan) "ON" at 235º and "OFF" at 230º. The fans should come on before it gets to any part of the red zone. (see "dual fan configuration" below about primary and secondary fans)
The f-body LT1 uses a 180° thermostat as stock.

The PCM gets it's temp readings from a sensor that is in the water pump. If the reading the PCM receives is inaccurate, the fans may not come on at the correct time. The PCM also uses this temperature for lookup in fuel calculation tables. If there is a problem that causes the reading to be always low (cold), the PCM will add extra fuel. This can cause hard starting when warm and an overly rich condition when running.

The gauge gets it's information from a sensor that is in the driver's side head. Inaccurate gauge readings can be from this sensor or it's wiring (the wire burned on a header pipe is common). The temp that the PCM sees can be monitored with a scan tool and compared to the gauge reading. They should be close, but don't expect them to be "perfectly" synchronized.

The fans are programmed to come on when the a/c is turned on. A/c Pressure monitoring sensors feed the PCM info and depending on the situation, the PCM may command the fans off for brief periods. Also, when the car reaches sustained higher speeds, the fans may be commanded off so incoming air can flow through the radiator unimpeded and provide the cooling needed.

Fans will also come on when the SES lamp comes on. The PCM does this when certain (most) DTCs are detected to protect the engine from a situation where it may overheat.

There are two versions of the dual fan configuration:

# 1993-1994 - Primary and Secondary fans that operate at only one speed. When initially commanded on, only the primary fan (driver side) comes on. It operates alone at full speed. If the temp threshold is met for addtional cooling, the secondary fan (passenger side) also is commanded on. At this point, both fans are running at full speed.
These fans use a two relay architecture that can be seen in the fuse/relay panel that is under the hood.

# In late 1994 and into 1995, there was a change to low and high speed fans. When initially commanded on, both fans will come on at a low speed. When the high speed temp threshold is met, they both bump up to high speed. A three relay architecture is used for this fan version (seen in the fuse/relay panel). By adding a third relay, low speed can be achieved by running the power to the fans in series. This way, each fan does not get full voltage and runs at a slower speed. High speed happens when the relays switch to provide full voltage to both fans. Low speed is less noisy and should result in greater fan longevity. High speed is not always needed.

2 Relay System PCM Commanded Fan Operation PCM Wire Color Grounded Fan Operation Relay Operated
#1 #2 #3
Primary@226º Drk Grn @A11 Primary (LH) fan full speed X - n/a
Secondary@235º Drk Blu @A10 Secondary (RH) fan full speed X X n/a
3 Relay System Low Speed@226º Drk Grn @A11 Low Speed (both fans) X - -
High Speed@235º Drk Blu @A10 High Speed (both fans) X X X
For both fans to operate in either system, both relay leads must be grounded. Grounding only the Drk Blu wire will result in only the RH fan operating at high speed.

Here are some fairly simple things to check for various complaints:

~Fans are not operational at any time~
these charts are for the 1985-89 vette cooling fans



# Check fan fuses in the underhood fuse/relay panel
# Check fan relays (same location). Aside from getting out any electrical equipment to test the relay, you can swap it with another one (such as the fog lamp relay) and test for function. See if the relay works for the fog lamps and/or the swapped-in relay makes your fans work. Nearly all the relays in the panel are the same, except for maybe the ABS relay.
# You can jumper two pins on the DLC that should cause the fans to come on. 1993-1994 cars with the 12 pin DLC can jumper pins A and B. On a 1993, that is the same way that you would retrieve trouble codes from the ecm. The 1994 won't give you any codes, but the fans will engage. 1995-1997 uses pins 5 and 6 on the 16 pin DLC to initiate what is called "field service enable mode". That will cause the fans to come on and operate most sensors for sanity checking. After placing the jumper on the correct pins, turn the key to ON (don't start). If the fans work after jumpering the DLC, your PCM is capable of operating the fans and all fan wiring/relays should be ok.
# Deeper problems can be solved through testing and using the wiring schematic.

~Fans don't come on except when the a/c or SES is on~

~Temp gauge continues to rise with no automatic fan operation~

# With a scan tool, check to see what temp the PCM is seeing from the sensor in the water pump. Make sure you are aware of the temps the fans come on (stated in the beginning of this article). If the temp it sees is incorrectly low, it won't know to turn the fans on. Another possibility is that the temp is really ok, but the gauge is reading wrong. That is why you need to use the scan tool to see and compare the readings. Info on testing wiring and sensor can be found here.
# If that looks ok, then your PCM may have issues. You could always try resetting the PCM by pulling the PCM BAT fuse for about 30 seconds.

Testing the ECT (Engine Temperature) Sensors and Connections







Pro tip before starting - Label your relays Relay 1, Relay 2, and Relay 3 according to the wiring diagram (your first post) and what your physical relays represent. Even if its just a sticky note. Get it all straight and stick to the same annotation while you troubleshoot.

You'll need a multimeter than can measure DC voltage and continuity:

Remove all three relays so you're only dealing with the sockets

DC Voltage tests:
1. Confirm 12V between the socket for pin 85 and the negative battery terminal on all 3 relay sockets
2. Confirm 12v between the socket for pin 30 and the negative battery terminal on relay sockets 1 and 2

Continuity tests:
1. Confirm continuity with the end of the dark green wire and the socket for pin 86 for relay 1
2. Confirm continuity with the end of the dark blue wire and the socket for pin 86 for relay 2 AND relay 3.
3. Confirm continuity between the socket for pin 87 for relay 1 and side B of the left cooling fan connector
4. Confirm continuity between side A of the left cooling fan connector and side B of the right cooling fanconnector AND the socket for pin 87 for relay #2.
5. Confirm continuity between the socket for pin 87 for relay #3 and Negative Battery Terminal
6. Confirm continuity between side A of the right cooling connector and Negative Battery Terminal.

Do the steps in order and use the negative battery terminal for your connection when I specify to. Verifying at the negative battery terminal will ensure you're circuit is making a good connection to the chassis ground. If it doesn't make it all the way back to the battery, it's a crap ground and testing it my way will reveal the problem

ECT Temperature vs. Resistance Values

ºC ºF Ohms
100 212 177
90 194 241
80 176 332
70 158 467
60 140 667
50 122 973
45 113 1188
40 104 1459
35 95 1802
30 86 2238
25 77 2796
20 68 3520
15 59 4450
10 50 5670
5 41 7280
0 32 9420
-5 23 12300
-10 14 16180
-15 5 21450
-20 -4 28680
-30 -22 52700
-40 -40 100700

Use a Digital Volt Meter (DVM) set to ohms to measure resistance. Note: Use a high impedance meter (at least 10 megohm) when dealing with the PCM. Most modern DVMs will do, but your old analog meter can damage the PCM. It is also a good idea to get a " reference" from the meter you are working with. With the DVM on the ohms scale, touch the two meter leads together and note the ohm reading. It may not always be perfectly zero, but may be within a tenth or two. Now when you take an ohm reading, you will know what the meter will show when there is really no resistance.

* The sensor in the head has only one terminal. This sensor is for the temperature indicator on the dashboard. Place one test lead on the sensor terminal and the other on a known good ground. Compare the reading to the table. If your car is cold from sitting overnight, the reading should be close to ambient temperature.
* The sensor in the water pump has two terminals. This sensor is for the temperature input to the PCM. Place a test lead on each of the sensor terminals to take the reading. (When reading resistance, it does not matter which lead goes to which terminal)

If the sensor seems to be ok, you may also need to test at the harness connector for proper lead conditions. Use your test meter set on the dc voltage scale to do this. You will need the key in the RUN position, but don't have to start the car.

* For the one lead connector at the head, place the red test lead on the connector terminal and the black test lead to a known good ground. With the key ON, you should read battery voltage (+12vdc or close to it). You can also ground the lead and see if the gauge in the car deflects to full hot.
o If you get no voltage, switch the meter to ohms to see if the lead is grounded.
o No voltage or no ground mean that the lead is open.
o If the gauge is at full hot all the tme, the lead is grounded back toward the gauge. It could be possible for the lead to be pinched and grounded toward the gauge and broken and open back toward the sensor (like in the case of the wire getting caught somewhere during some major engine work). Physically tracing the wire from the sensor into the harness should locate the problem.
* The two lead connector at the water pump has a black (ground) lead and a PCM +5vdc power lead (probably yellow). Place the black meter test lead to black connector lead and the red meter test lead to the other connector lead (yellow on my 1995). You should read +5vdc because this is monitoring voltage being supplied from the PCM.
* If you get no reading:
o Test the yellow lead by placing the DVM red lead on it and the DVM black lead to ground. A +5vdc reading will indicate the lead is ok.
+ If you get no voltage, switch the meter to ohms to see if the lead is grounded.
+ No voltage or no ground mean that the lead is open.
o You can test the black connector lead by using the ohms scale on the DVM. Place the DVM black lead to ground. Place the DVM red lead to the black lead of the connector. If the lead is ok, you will get an ohm reading close to zero. If you get no reading or a very high one, the lead is open or partially open.
* OBD-I DTCs 14 and 15 or OBD-II DTCs P0117 and P0118 are typically associated with problems the PCM sees with the sensors or circuits.

Footnote: If you ever have to test the IAT, it operates the same as the two lead coolant sensor. The same temp vs. resistance table above is applicable to the IAT, as well as the +5vdc lead and ground wire at the harness connector.
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I ran across this thread and just wanted to say THANK YOU!
I've been fighting a problem locating a reason the corvette I recently purchased, cooling system just will not function,
I think thats the reason the previous owner sold the car too me rather reasonably!
I have been searching the internet and found this web site link,
I carefully looked at the diagrams and read the links and sub-links and it eventually became rather obvious,
(I had previously installed a sensor that was not compatible with the year corvette I own)
Ive tested about everything I could think of... now I see I was beating a dead horse....
yeah, I bought the correct sensor and miraculously the fans now work! THANK YOU!

Your certainly not the first, nor will you be the last guy, to find a mis-matched or the wrong part number sensor installed on a car.
a shop manual and a few diagnostic tool are the basic way,
you will find out what your dealing with in the vast majority of cases,
never skip READING THROUGH linked info on this website,
you might not need it immediately but youll be amazed at how often ,
you,ll read something here, and later on, think..
I remember reading about that.....













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knowledge is a powerful thing and it can't be taken away, except by CRS/Alshiemers.