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So, my V6 '91 Daytona has had a consistent problem where the radiator fan will not run unless the AC compressor is engaged. I have dealt with the problem for over 2 years, but just got around so seriously diagnosing it. Thought it might be the coolant temp sensor, but it seemed like it worked fine because the car threw a check engine light when I disconnected it. Replaced the sensor anyways and nothing changed. Checked the fuses, none were blown. Swapped the fan relay and the AC relay with each other, and nothing changed with either system. Checked the relays with a multimeter, and both were functional. Then came checking the wiring. I made sure both relays were getting power from the 40A fuse, and both were good. I then checked the relay to ground continuity though the fan motor, which was fine. To make sure that the fan circuit itself was fine, I stuck a jumper wire in the relay socket and as soon as the battery was reconnected, the fan started. At this point, I knew it was a problem with the activation side of the circuit. Checked the continuity between the SBEC (pin 2) and the coolant temp sensor, which was fine. There was also continuity between the SBEC (pin 31) and the fan relay. So now I have no idea what to do from here. The fan is supposed to engage at 210 degrees under 40 mph and 230 degrees over 40 mph, but it will boil the 50/50 coolant mix and the fan still won't run. I have no idea what else could be causing the issue, so any input would be greatly appreciated.
 

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You need to see engine temperature, you may have a faulty thermostat and the engine is not reaching the proper operating temp.
The engine controller will not cycle the cooling fan on until it sees a temp of 217 degrees F on a 3.0L.
(These voltage readings were taken from my Son's 3.0L)
Backprobe the CTS connector with your digital voltmeter and watch the voltage reading, the voltage translates to a temperature.
If you are not reaching the proper operating temp, replace the thermostat (195 degree) and repeat.

Backprobing the CTS.JPG CTS VOLTAGE CHART 2.jpg
 

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1)Since the fan is controlled by pin #31 on the SBEC and works fine with the AC turned on. The fan, relay and SBEC fan control circuitry are likely fine as well.
2) The coolant is boiling but the fan will not turn on. Either the ECU does know the coolant temp or cannot do anything about it. Since the ECU will control the fan on AC, the issue is likely the ECU does not know what is the correct coolant temperature.
3) Best to check CTS as per NAJ as this sensor is the only way the SBEC can determine engine temperature.
4)Also check codes just in case Fault Codes - Mini-Mopar Resources
 

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Discussion Starter #4
I just checked my codes, and I got a 12 and 55. The 12 was probably because I just disconnected my battery yesterday. The thermostat could be an issue, but I replaced it a year or two ago and it seems to function just fine. During every warm-up period, the temp will slowly rise to a bit above 1/3 on the gauge, drop down below it quickly, then rises again and settles on the mark. I guess the next step is to test the sensor. How do you get the multimeter probes in to test it? Stick wires in the back of the plug until it reads?
 

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How do you get the multimeter probes in to test it? Stick wires in the back of the plug until it reads?
Yup commonly called backprobing. My running cars are turbo and if you unplug the CTS the Fan will run.
Does yours? I did not check my 91 FSM to see what is "normal" for the V6
 

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Discussion Starter #6
I don't see any indication of the fan supposed to be running with the sensor unplugged in the FSM. In the troubleshooting table in the cooling section, the symptom "fan always runs" only corresponds with "normal with AC compressor clutch engaged, otherwise see electrical group 8." It says the same thing under the section dedicated to the fan operation conditions. I have unplugged the coolant temp sensor before with the car running, which immediately caused a check engine light. I remember the fan did not turn on and did run when I switched the AC on.
 

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I don't see any indication of the fan supposed to be running with the sensor unplugged in the FSM.
I checked 8D-17 in the ignition section and both turbo and V6 SBEC activate the fan with the CTS disconnected. Since your fan did not run, It is likely a fault in the SBEC CTS signal conditioning circuitry.
I guess we will wait until you are able to backprobe the CTS vs temperature before we can proceed.

here is a diagram of 87 signal conditioning circuitry, if you want more info...LMK
270233
 

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Discussion Starter #8
It's working! The fan now comes on as it should. Apparently my thermostat is either a 190 degree or 195 degree one, so it opens WAY before the fan runs. With the AC on, the needle will settle on the 1/3 mark (about 190 degrees). With it off, the needle will hit just above the 2/3 mark (about 215 degrees), the fan will start, and the needle will slowly drop back to the 1/3 mark. Not sure if replacing the sensor a few months back, spraying loads of electronics cleaner on everything, or reconnecting the big 60 pin SBEC connector a few times fixed the problem, but it's good now. Now I have to readjust my interpretation of what is "too hot" on the gauge. Thanks NAJ and chromguy a bunch for the help.
 

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The gauges in these cars are not 100% accurate, they are only a guide to where coolant temp is.
Basically if the gauge reads extremely high or extremely low there is a problem, anywhere in between is normal.
Not sure about the 3.0L since my Son no longer has his but the 2.2L/2.5L temp gauges even read differently in the summer than in the winter although the coolant temp is the same.
If you want to know exact coolant temp backprobe the CTS connector and read the voltage as I described or use a scanner to read coolant temp.

Backprobing allows you to read live voltages without piercing wires and damaging connector terminals
You insert the backprobe pin (you can also use paper clips) into the rear of the connector until it makes contact with the terminal and connect your digital voltmeter to the pins.

Gauge fluctuation especially in the cold winter months is normal as the thermostat opens and closes.
Chrysler sent out a TSB (Technical Service Bulletin) about gauge fluctuation for the 2.2L/2.5L engines showing that gauge fluctuation was normal.

(5)Cooling System TSB.jpg
 

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So, my V6 '91 Daytona has had a consistent problem where the radiator fan will not run unless the AC compressor is engaged. I have dealt with the problem for over 2 years, but just got around so seriously diagnosing it. Thought it might be the coolant temp sensor, but it seemed like it worked fine because the car threw a check engine light when I disconnected it. Replaced the sensor anyways and nothing changed. Checked the fuses, none were blown. Swapped the fan relay and the AC relay with each other, and nothing changed with either system. Checked the relays with a multimeter, and both were functional. Then came checking the wiring. I made sure both relays were getting power from the 40A fuse, and both were good. I then checked the relay to ground continuity though the fan motor, which was fine. To make sure that the fan circuit itself was fine, I stuck a jumper wire in the relay socket and as soon as the battery was reconnected, the fan started. At this point, I knew it was a problem with the activation side of the circuit. Checked the continuity between the SBEC (pin 2) and the coolant temp sensor, which was fine. There was also continuity between the SBEC (pin 31) and the fan relay. So now I have no idea what to do from here. The fan is supposed to engage at 210 degrees under 40 mph and 230 degrees over 40 mph, but it will boil the 50/50 coolant mix and the fan still won't run. I have no idea what else could be causing the issue, so any input would be greatly appreciated.
! have a 92 v6 Daytona and have all available wiring diagrams from alldata. Unfortunately the fan circuit is only shown in an A/C context. There is not sufficient data to trouble shoot your issue and since you have a 91 the info could be wrong. A second connection to the relay coil goes to the "engine node module" as well as cavity 31 in my 92 diagram. May I suggest installing a thermal fan switch which could activate the relay by connection to the same terminal if it sends voltage when activated. This will allow the fan to operate with regard only to coolant temperature or A/C, losing the speed/ temp variations
 
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