Mishimoto Dual Fans On And Off

bigbros93

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Oct 31, 2011
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Hey gang,

I have 12" dual mishimoto fans in my 93 LX. I've been having some overheating issues and noticed that the electric fans turn on and off and don't stay on consistently. I am currently using a 40 amp relay and noticed that it was a little melted and deformed so I went and bought a new one. After installing the new relay the fan ran perfectly for about 3 minutes and then started shutting on and off. I finally hooked up the fan directly to the battery and I ran it for 5 minutes and it ran perfectly. Is it possible that a bad relay would cause the fan to shut on and off rather than running consistently? Is a larger relay the answer? Please let me know if any of you have had issues similar to this one. Thank you in advance.
 
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a91what

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Yes.

Basically you are overloading the relay circuit.

How do you have this wired? What type of controller are you using?
 

jrichker

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The heat damage to the push on tab connectors are evidence of a high resistance connection. High current loads like an electric fan and high resistance connections generate heat, which in turn creates high resistance which creates more heat. The vicious cycle continues until something either catches fire or the resistance gets so high that the load (in this case, a motor) quits working. Examine the push on tab connectors for discoloration, loose fit and damage to the wire insulation or plastic connector shell. Replace what you find is damaged.

Voltage drop testing of connections and grounds.

Use a Digital Volt Meter (DVM) to measure the voltage drop across a connection or wire. Adding length to the test leads may be required, and does not affect the accuracy of the test. Use 16-18 gauge wire for the test leads if you have to lengthen them.

Voltage drop increases with the increase of current in a circuit and it also increases with heat. Put a maximum current load on a bad wire or connection and it gets hot and drops more voltage across the wire or connection. As it heats up, resistance increases which makes more heat. Round and round you go in a vicious circle until something catches fire or fails.

Voltage drop testing must be done while the usual load is on the circuit. If it is a starter, it has to be tested while cranking the starter. If it is lights, A/C or fan, they must be turned on high while testing. Fail to do this and you will not get accurate results

1.) Most grounds use the negative battery post as their starting point. Keep this in mind when checking grounds.
2.) The voltage will be small if the ground is good: less voltage drop = better connection.
3.) Be sure that the power to the circuit is on, and the circuit is being used in its normal manner. For instance, if it is a light circuit, the lights on that circuit should be powered on.
4.) To measure grounds, place one DVM lead on the battery negative post and the other on the wire or connector that goes to ground.
5.) 5.) Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections (sensors are low voltage devices and small drops can have a large effect on the devices dependent on sensor accuracy)
0.0V Connections
A voltage drop lower that spec is always acceptable.
6.)
See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .

attachment.php?attachmentid=64167&stc=1&d=1286329941.gif
 

bigbros93

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Orlando, FL
Thanks for the reply. I'll admit that at the time, I wasn't to knowledgeable and purchased the first temp controller I found online. I just recently got the car running after 2 years since I installed the fan and discovering this issue. Both fans are running through 1 40 amp relay along with a 25 amp fuse and 30 amp circuit breaker. Thinking I either need a larger relay or simply run each fan off its own relay, fuse, and circuit breaker. Should a larger relay suffice? Thanks again.
 

jrichker

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Thanks for the reply. I'll admit that at the time, I wasn't to knowledgeable and purchased the first temp controller I found online. I just recently got the car running after 2 years since I installed the fan and discovering this issue. Both fans are running through 1 40 amp relay along with a 25 amp fuse and 30 amp circuit breaker. Thinking I either need a larger relay or simply run each fan off its own relay, fuse, and circuit breaker. Should a larger relay suffice? Thanks again.
Go back to the Manufacturers' website and check out the current draw specs for the fan setup that you have. Size the wiring and the circuit protection according to the fans specs. Again, remember that push on tab connectors can and do increase in contact resistance over time in an under hood automotive environment. This can lead to problems as time and mileage increase; high resistance connections in a high current circuit is a bad thing...
 
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a91what

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A little Dialectric grease goes a long way in keeping corrosion to a minimum and circuit resistance down.
 
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bigbros93

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Thanks to everybody that's been able to chime in on my little problem. Since that time, I've replaced the wiring and now have each fan running off it's own relay, circuit breaker, etc. and it's working like a dream. One more question though to see if anybody out there is doing this.

Currently, the fans come on with a temperature sending unit on the water neck so when the temperature gets to 185 (I think) the fans come on. However, say I'm going down the road and stop for gas while the fans are going. I turn off the car, fans stop, get some gas and a slurpee, and when I go turn it on, the water temp is still over the 185 and the fans want to come on right away. Does this put extra strain on the system since they want to turn on as soon as I turn the key? Would anybody recommend some kind of switch to disable the fans at startup then turning them on?

PS - Next Tuesday Mustangs turn 54. For those in the Orlando area, ACE Café in downtown is having a meet with 1 Mustang from every year. Should be a hoot.
 
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Blown88GT

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The 40A automotive relays are NOT rated for motor loads. They will not last. Bosch has a 75A relay that is rated for motor loads.
Bosch sold to Tyco who sold to TE, etc...
71DfM4Zd0DL._SL1500_.jpg


I have some new spares (original Bosch), PM if interested.
I used to use relays & a sensor in the water neck. The best solution is a PWM fan motor controller. Relays are a "hard start", PWM is a "soft start".
 
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bigbros93

Member
Oct 31, 2011
7
1
14
Orlando, FL
The 40A automotive relays are NOT rated for motor loads. They will not last. Bosch has a 75A relay that is rated for motor loads.
Bosch sold to Tyco who sold to TE, etc...
71DfM4Zd0DL._SL1500_.jpg


I have some new spares (original Bosch), PM if interested.
I used to use relays & a sensor in the water neck. The best solution is a PWM fan motor controller. Relays are a "hard start", PWM is a "soft start".
Thanks for your advice
The 40A automotive relays are NOT rated for motor loads. They will not last. Bosch has a 75A relay that is rated for motor loads.
Bosch sold to Tyco who sold to TE, etc...
71DfM4Zd0DL._SL1500_.jpg


I have some new spares (original Bosch), PM if interested.
I used to use relays & a sensor in the water neck. The best solution is a PWM fan motor controller. Relays are a "hard start", PWM is a "soft start".
Thanks for the advice. This is the first that I hear about PWM. Would this simply be in place of the relay and keep the rest the same? Also, do you keep the water neck sensor to turn the fans on and off? I will def read up on this a little more. Thanks again!