Are you working on a C4?? 
low coolent temp
Team Owner
Joined: Mar 2001
Posts: 54,124
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From: Cincinnati, Oh USA
No one here is clairvoyant. Always state what year C4 you are asking help for.
The dash gauge for temp coolant will read LO all the time if the temp sender is open circuit (check the sender with an ohmeter) or if the green wire that goes to the sender is open circuit somewhere up to the cluster connector. You can test if the wire is open by grounding the green wire and turning the ign sw to run. Your dash temp gauge should read 330 F if the green wire is ok to the cluster.
The dash gauge for temp coolant will read LO all the time if the temp sender is open circuit (check the sender with an ohmeter) or if the green wire that goes to the sender is open circuit somewhere up to the cluster connector. You can test if the wire is open by grounding the green wire and turning the ign sw to run. Your dash temp gauge should read 330 F if the green wire is ok to the cluster.
Thread Starter
Burning Brakes
Joined: Oct 2002
Posts: 971
Likes: 0
From: Rockwood TN
Sorry Guys I forgot to update my profile. I bought an 85 last week to go along with my 77. The 85 I'm not going to mod out just going to use it as a cruiser. I replaced the temp sending unit, the fan wasn't coming on and I found a bad fan relay connector but the true cause for the bad connector was a bad fan motor drawing to many amps and burned up the connector so I replaced the connector and relay and the fan works fine now but I still get a low coolent temp.
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Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
On an '85:
-cooling fan temp sw = RH rear of engine, near the oil dip stick
-ECM coolant temp sensor = front of engine below TB
Is there a sensor in the block below #1 & #3 spark plugs, with a Grn wire? If so check the connection to that senaor.
-cooling fan temp sw = RH rear of engine, near the oil dip stick
-ECM coolant temp sensor = front of engine below TB
Is there a sensor in the block below #1 & #3 spark plugs, with a Grn wire? If so check the connection to that senaor.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by 65Z01
On an '85:
Is there a sensor in the block below #1 & #3 spark plugs, with a Grn wire? If so check the connection to that senaor.
Is there a sensor in the block below #1 & #3 spark plugs, with a Grn wire? If so check the connection to that senaor.
RACE ON!!!
Thread Starter
Burning Brakes
Joined: Oct 2002
Posts: 971
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From: Rockwood TN
I'll try it tomorrow. I do have a question though. If thefan switch is on one head and the temp sensing unit is on the other head then what in the world is the sensing unit on the front of the intake do I'm sure it's not there just for looks. No wait sorry guys I forgot the general doesn't always think like we do.
Team Owner
Joined: Jan 2003
Posts: 20,293
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From: Summerland B.C. Canada
I'll try it tomorrow. I do have a question though. If thefan switch is on one head and the temp sensing unit is on the other head then what in the world is the sensing unit on the front of the intake do I'm sure it's not there just for looks. No wait sorry guys I forgot the general doesn't always think like we do.
Thread Starter
Burning Brakes
Joined: Oct 2002
Posts: 971
Likes: 0
From: Rockwood TN
OK a new question I went to NAPA and told them I wanted a temp sending unit and they gave me one looked like the old one but could it be the one that goes in the head are they the same or what. I believe I'll just go and crank the 77 and relish in the fact that it doesn't have but a couple of vacuum hoses
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
The one in the front of the manifold is the Engine Coolant Temp Sensor. ECM uses it to control fuel delivery (in one respect, it replaces the choke) and to turn on the main fan by grounding the relay - usually when it signals 226 degrees. The sender in the right or left head (depends on Year) controls the Display. An open circuit is LO; grounded it shows something around 300. Tell the NAPA guy you want the Gage sender and he can usually fumble around in his part book and find it or you may be able to look up the part number at www.napaautoparts.com
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
On an '85 the ECM only turns on the fan when the A/C line pressure rises too high, the coolant temp switch in the LH side only brings on the fan when coolant temp exceeds the preset.
BTW, I agree that you need to pick up a HELM FSM for your car. My '85 FSM is three times the thickness of the Haynes which covers all C4s, says a lot doesn't it.
BTW, I agree that you need to pick up a HELM FSM for your car. My '85 FSM is three times the thickness of the Haynes which covers all C4s, says a lot doesn't it.
Last edited by 65Z01; May 1, 2007 at 02:24 AM.
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
"On an '85 the ECM only turns on the fan when the A/C line pressure rises too high, the coolant temp switch in the LH side only brings on the fan when coolant temp exceeds the preset."
??? - Did you rewire it or something? The CTS signal is used by the ECM to drive the Main Fan Relay when the a/c is off. Otherwise, with the a/c on, Fan Switch on the High Pressure Line opens at about 220 -230 psi causing the signal voltage at the ECM to rise to whatever the voltage potential is. With that, it grounds or drives the Relay. Section 6E and 8A should outline it for you. Head switch is used for Aux Fan only and if the Cooling System is the way it left the Factory, it will rarely if ever reach the threshold for operation.
??? - Did you rewire it or something? The CTS signal is used by the ECM to drive the Main Fan Relay when the a/c is off. Otherwise, with the a/c on, Fan Switch on the High Pressure Line opens at about 220 -230 psi causing the signal voltage at the ECM to rise to whatever the voltage potential is. With that, it grounds or drives the Relay. Section 6E and 8A should outline it for you. Head switch is used for Aux Fan only and if the Cooling System is the way it left the Factory, it will rarely if ever reach the threshold for operation.
Team Owner
Joined: Dec 2000
Posts: 90,675
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From: SE NY
Cruise-In II Veteran
Thanks for the 6E3-36 reference; I hadn't been able to find the ECM's coolant temp sensor in the Electrical Diagnostic section of the '85 FSM.
So there is an apparent redundancy in the '85 as on pp 126 of the Electrical Diagnostic section is shown the Coolant Fan Temp Switch, which "also" controls the main cooling fan...hmmm, interesting.
Then on pp 124 is shown that the Aux Cooling Fan Temp. Switch (RH rear of engine) controls only the Booster Fan, while only the ECM controls the main fan.
So his main fan control depends on if he has the Booster Fan option.
In any case, on pp 6E3-9 is shown the "Engine temp sensor (gauge, overheat)" in the LH side of the engine...
BTW, thanks for the reference into the FSM text body...
So there is an apparent redundancy in the '85 as on pp 126 of the Electrical Diagnostic section is shown the Coolant Fan Temp Switch, which "also" controls the main cooling fan...hmmm, interesting.
Then on pp 124 is shown that the Aux Cooling Fan Temp. Switch (RH rear of engine) controls only the Booster Fan, while only the ECM controls the main fan.
So his main fan control depends on if he has the Booster Fan option.
In any case, on pp 6E3-9 is shown the "Engine temp sensor (gauge, overheat)" in the LH side of the engine...
BTW, thanks for the reference into the FSM text body...
Last edited by 65Z01; May 1, 2007 at 12:43 PM.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by 65Z01
So there is an apparent redundancy in the '85 as on pp 126 of the Electrical Diagnostic section is shown the Coolant Fan Temp Switch, which "also" controls the main cooling fan...hmmm, interesting.
Then on pp 124 is shown that the Aux Cooling Fan Temp. Switch (RH rear of engine) controls only the Booster Fan, while only the ECM controls the main fan.
So his main fan control depends on if he has the Booster Fan option.
Then on pp 124 is shown that the Aux Cooling Fan Temp. Switch (RH rear of engine) controls only the Booster Fan, while only the ECM controls the main fan.
So his main fan control depends on if he has the Booster Fan option.
RACE ON!!!