Results 1 to 8 of 8

Thread: OEM fan fail. How it works

  1. #1
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    6,313

    My VIN:    03572

    OEM fan fail. How it works

    One of my fan fail customers sent me an extra OEM fan fail unit he had. What he found with his OEM fans were tripping his 40 amp fan breaker very quickly it would kick off then on a few second and keep repeating. He has ordered a clamp on DC current meter to read the real fan currents.

    This is the first OEM fan fail I have had in my hands. I bench tested it and what it does is turn on the fan fail light when total fan current is below 20 amps. It does not compare the fan currents, inside the unit the two fan terminals are connected. The plastic case is very brittle (I guess from heat) so it shattered when I tried to open the unit. It looks like there is a reed relay in a fine wire coil that gets powered with the fan relay voltage input. Then there is four turns of heave gauge wire wrapped around that coil that carries the fan current and counteracts the fine coil.

    I measured the voltage drop when both fans were powered drawing 10 amps each. One fan pin dropped 0.105 volts and the other fan pin dropped 0.080 volts. So that voltage drop is what produces the heating of the unit (1.85 watts) plus the fine coil current of 63 ma at 14 volts (0.88 watts). Of course when your fans draw more than 10 amps the heating increases.

    My electronic fused fan fail drops 0.036 volts and 0.035 volts with the same 10 amp loads. My standby current is 10 ma. at 14 volts.
    Last edited by Bitsyncmaster; 03-16-2017 at 03:47 PM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #2
    Delorean Guru
    Join Date:  May 2011

    Location:  Northern NJ

    Posts:    5,752

    My VIN:    10757

    Club(s):   (DMA)

    Quote Originally Posted by Bitsyncmaster View Post
    One of my fan fail customers sent me an extra OEM fan fail unit he had. What he found with his OEM fans were tripping his 40 amp fan breaker very quickly it would kick off then on a few second and keep repeating. He has ordered a clamp on DC current meter to read the real fan currents.

    This is the first OEM fan fail I have had in my hands. I bench tested it and what it does is turn on the fan fail light when total fan current is below 20 amps. It does not compare the fan currents, inside the unit the two fan terminals are connected. The plastic case is very brittle (I guess from heat) so it shattered when I tried to open the unit. It looks like there is a reed relay in a fine wire coil that gets powered with the fan relay voltage input. Then there is four turns of heave gauge wire wrapped around that coil that carries the fan current and counteracts the fine coil.

    I measured the voltage drop when both fans were powered drawing 10 amps each. One fan pin dropped 0.105 volts and the other fan pin dropped 0.080 volts. So that voltage drop is what produces the heating of the unit (1.85 watts) plus the fine coil current of 63 ma at 14 volts (0.88 watts). Of course when your fans draw more than 10 amps the heating increases.

    My electronic fused fan fail drops 0.036 volts and 0.035 volts with the same 10 amp loads. My standby current is 10 ma. at 14 volts.
    Once you start tripping the breaker and it auto-resets it gets "softer" and will trip at lower and lower levels of current. It may even seem as if the fans are being turned on and off by the otterstadt switch when they are really being shut off by the circuit breaker. Quick way to tell is to put a voltmeter or test probe on the circuit breaker and see if it is popping. Often you can hear it click. The OEM fan fail relay is properly named. It is a relay, it does control the fans and it fails.
    David Teitelbaum

  3. #3
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    6,313

    My VIN:    03572

    Quote Originally Posted by David T View Post
    Once you start tripping the breaker and it auto-resets it gets "softer" and will trip at lower and lower levels of current. It may even seem as if the fans are being turned on and off by the otterstadt switch when they are really being shut off by the circuit breaker. Quick way to tell is to put a voltmeter or test probe on the circuit breaker and see if it is popping. Often you can hear it click. The OEM fan fail relay is properly named. It is a relay, it does control the fans and it fails.
    The OEM fan fail only cuts off the fan circuit if it burns out. The only "relay" is the fan fail light circuit. My guess is the connections of the fan current on the circuit board inside the fan fail went bad when it heated up. Probably the solder melted. The power feed pin on this unit had melted the plastic a little.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  4. #4
    Senior Member
    Join Date:  May 2011

    Posts:    491

    Dave - this was new to you ?

    When I got a blue "realy" in my fingers around 15 years ago I immediately opened it and found what you saw, too.
    the solder joints, the whole PCB can't handle the high fan current.

    I think the current sensing via two coils and a reed relay is pretty clever, but we all know the rest of the story.


    BTW - I used the place of the blue relay for a second fan relay - each fan has its own relay in my car.
    never had a problem since...

  5. #5
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    6,313

    My VIN:    03572

    Yes I think it was a pretty clever design for the time.

    I seem to remember some people asking if they could use the OEM fan fail if they installed low power fans. My testing shows the fan fail light will illuminate if the current is below 20 amps. So I think all the low power setups are below 20 amps so they would light the fan fail light every time the fans run.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  6. #6
    Matt Drive Stainless's Avatar
    Join Date:  Mar 2016

    Location:  Washington D.C.

    Posts:    330

    If you ever design a pwm fan circuit that avoids the hysteresis, I will buy it, Dave.
    New & Improved V3 S/S Alternator Tensioning Bracket - $39.99
    New & Used DeLorean Parts on eBay HERE.
    S/S Upper Alternator Bracket for Saturn/Autozone Alternator - $39.99
    QA1 Coilover Suspension Kit - Front - $679.76, Rear - $619.79, Complete Kit - $1,224.55
    Mirror switch rebuilding service w/ new springs & ball bearings - $49.50
    HEI Performance Ignition Module (Exchange) - $149.99
    Aluminum welding services also!
    http://www.drivestainless.com

  7. #7
    Senior Member Bitsyncmaster's Avatar
    Join Date:  May 2011

    Location:  Leonardtown, MD

    Posts:    6,313

    My VIN:    03572

    Quote Originally Posted by Drive Stainless View Post
    If you ever design a pwm fan circuit that avoids the hysteresis, I will buy it, Dave.
    Technically, my fan fail could eliminate the fan relay and could be designed to PWM the fans but it would need more pins and would need some wiring changes. I don't really see much in temperature gauge overshoot. Maybe your otterstat has to much hysteresis.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  8. #8
    Delorean Guru
    Join Date:  May 2011

    Location:  Northern NJ

    Posts:    5,752

    My VIN:    10757

    Club(s):   (DMA)

    That fan fail relay is just another example of the whole system being under-designed and over-rated. The electrical system has so many failure prone weak points.
    David Teitelbaum

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •