I want to control an auxiliary fan on my 1969 Mercedes which is currently wired to run until the temperature drops below the sensor level. Sometimes this takes quite a long time even after the car has been switched off.

I do want the fan to run, but for only a couple of minutes after I switch the car off, so I installed a Beuler Time Delay relay. However, it fails to function as designed when I use the car's switch to open the trigger connection. When I disengage the trigger manually by unhooking the wire from the relay, the timed circuit stays closed for 2 minutes before opening.

I have been told to put a diode between 85 and 86. Not being an electronics expert, I would like to get your opinion on what to do.

  • Welcome to EE.SE, but even as a complete newcomer don't you think that providing links to the parts you mention might be helpful? There's a schematic button on the editor toolbar complete with relay symbols if you want to add a schematic.
    – Transistor
    Commented Apr 6, 2016 at 18:40
  • 1
    So after you remove power from the trigger pin it stays closed for 2 minutes, correct? This sounds like it is functioning correctly. What are you expecting to happen when you remove power from the trigger pin?
    – rpmerf
    Commented Apr 6, 2016 at 20:05
  • How do you have this wired? How does it know when the ignition is off? Or is this the issue you are experiencing?
    – rpmerf
    Commented Apr 6, 2016 at 20:07
  • @transistor this is a new user and can't seem to message them over here or over there. Any suggestions? Commented Apr 6, 2016 at 22:37
  • @DucatiKiller: Sorry, I don't understand your comment but I see the post has been migrated so the schematic editor won't be available. I see that one detailed answer has been posted already. I'll bow out back to electronics.stackexchange.com. Thanks.
    – Transistor
    Commented Apr 6, 2016 at 22:49

2 Answers 2


There are a few questions in there, so I will try to address them all. The Bueler Time Delay relays are unlike the vast majority of other relays you will find. The difference is that these relays are so-called "smart" relays. They have embedded circuitry that handles the triggering as well as the timing functions of the timed relay.

For most relays (i.e. "dumb relays"), the internal construction consists of a solenoid coil that actuates an armature which connects and disconnects a circuit (based on the type of relay). There is a standardized pinout configuration for automotive relays, and pins 85 and 86 are the "control pins". When you pass a DC current from pin 86 to pin 85, a magnetic field is created within the relay coil causing it to switch. See this video for a more in-depth explanation of how this works.

The reason why you were probably told to use a Flyback Diode on pins 85 and 86 is because that is the proper way of protecting the relay coil circuit of a run of the mill relay. However, these timed relays are designed with a circuit that handles all of this for you. It has a logic component that is capable of timing events, and it is constantly monitoring the "trigger" pin (pin 86). Using a Flyback Diode on this type of relay is unnecessary.

I believe the reason why the relay works as expected while manually unhooking the wire is because these relays trigger on a "pulse". From the information I was able to gather on these relays (see this video), the relay actuates for 2 minutes when the trigger pin is tied to 12V, not when it is disconnected. It's possible that you are causing a pulse on the trigger pin with the wire that you do not notice because it is happening on a millisecond scale.

You can test this by manually triggering the relay with a piece of wire. Does the relay switch immediately as soon as you touch 12V to pin 86, or does it wait until you remove it? If the relay switches immediately, the easiest way to trigger this relay would be to install a momentary switch (or hijacking an existing one) somewhere in the car. Just press the switch before you exit the car and the fan will continue to run for the predetermined time.

If this is undesirable, you can look into creating a single-shot circuit that fires each time you turn off the vehicle.

  • Thanks I have the correct connection, the constant power to the fan is via 30 and the trigger is on 85. As I switch on or start the car the terminals are engaged and the power starts flowing to the fan but when I switch the car off the something goes wrong somewhere as the power to the fan is immediately switched off but if I manually disengage the power to the trigger while the switch is still on, the relay works as designed and holds the power to the fan for 2 min. before killing the circuit.
    – user16255
    Commented Apr 7, 2016 at 16:51
  • I have to run one more test to see if the ground connection is causing some problems. I will hook the ground connection 85 directly to the battery rather than the common ground I am currently using, maybe some circuit using the common ground is causing some back flow when I switch the car off.
    – user16255
    Commented Apr 7, 2016 at 16:51
  • I wish I had the knowledge to answer this question. Very nice Sir. +1
    – Ppoggio
    Commented Apr 9, 2016 at 19:55

When you turn the ignition off, the positive trigger to pin 86 on your relay turns into ground due to other loads being present on that same circuit. If you install a diode anywhere in line just before pin 86 to prevent capacitors of this relay to discharge back into the vehicle, the timer should work properly.

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