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Discussion Starter #1
I installed a Mark VIII Fan using one of the 75 Amp Bosch relays about 3 weeks ago. I am using a temp switch in the intake crossover that creates a ground when the temp is over approx. 192 degrees. I originally had the positive activation terminal of the relay wired directly to the battery. One day I went golfing and after I got out of the car I heard the fan turn on. I did not worry about it because I assumed it would shut off when the engine cooled down. When I finished golfing the battery was dead. At the time I assumed that the temp in the intake crossover had stayed too high and kept the switch on. Because of this experience, I connected the positive activation terminal of the relay to an ignition on only circuit. When I shut the engine off, the fan stops, no matter what the temp. At least that is how it should work. Today when I shut off the car, the fan kept running. I pulled the positive wire off of the activation terminal of the relay and the fan still kept running. I disconnected the battery to get the fan to shut off. After golf, with a cool engine the fan started running again when I reconnected the battery. I got it to stop running by hitting the relay with my knuckles.

Is this a common defect? Are the 75 Amp Bosch relays prone to problems? Is it possible I did something to the relay to damage it? If so, what could have caused the damage? I have another 75 Amp Bosch relay. I ordered a spare when I got the fan.
Should I just install it or should I look for another relay? What other relays would work well?

Thanks, Joel
 

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Sounds like the internal contacts on that relay are melting which causes them to almost weld themselves together. Either they are getting hit with a larger amp draw than they are designed for or they are defective from the factory. Sometimes if they don't make good enough contact they will just arc across contacts causing a meltdown. Also might depend on size wire you've got feeding the relay. Goodluck, Tony

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70 Chevelle SS396/TH400/3.73 "What does not kill me, makes me stronger."
 

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Is your relay close too your fan? What badchevelle said is on the money. If you hit the relay and the fan stops your contacts are bad. Replace the relay.

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65 Malibu SS, Crocus Yellow with black interior. Let the fun and back pain begin.
 

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Joel. man you can't seem to win 'ey? My Bosch relay is doing just fine.

I'm intertaining to ideas in my head.

1.) the cantacts are getting enough current to arc weld themselves together. Odd, and possibly a deffect.

2.) the relay coil is being activated by some stray magnetic field possibly created by the big fan motor. (not likely
)

I don't know what to tell ya man. You can pop off the case and clean the contacts. maybe file them down, but they are SUPPOSED to be coated to prevent such arcing.

-Oh yeah. that "painless wireing" thermostatic switch works AWESOME! comes on at 203 and shuts off at 285. I have a SPDT swtich that lets me turn it on if I wish or let is ground through the thermostat (in intake) swtich if I like.

My supply for the relay comes from the "radio" post in the fuse box and ground is achieved by first coming into the cab to the SPDT swtiches middle post. Then I control, via the rocker, wether it goes to a firewall ground (so the relay is latched when the key is turned back or to ACC). Or it can be sent out to the thermostatic switch and that adds in the criterea of being 200* yaddda yadda

Mine has worked fine for at least 3 weeks now. Call Bosch and complain. -get some free ****!-

-Also my fan is hooked up with very short 10-12 gauge wire AND the realy is on the ground side of the fan, i.e. after the fan. this may help things out.

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71' 3880# with me. Big Block 402, TH400, 3.73 posi,
13.1sec @ 105 MPH (poor 2.1 sixty foot and rookie tuning)
--will be racing at Chevellebration 2001!--
Picture of me roasting the tires and other guy stuff
Video of me staging (smoke of course)
 

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Discussion Starter #6
ANOTHER ONE BITES THE DUST!!!!

I installed my "spare" relay. It worked ok for a few days but yesterday when I got home and shut the car off it kept running. The "spare" appears to have the same problem as the original.

Before I installed the spare, I called the place where I got it. They said that if I send the defective "one" back they will send me a replacement. With the results I am having, I am not sure I want any more of this model of relay.

Can anyone recommend another relay that would work well with the Mark VIII Fan?

What about using two 30 AMP Relays in parallel? I know someone on this site is running that setup. I would have tried that setup originally but I thought a single relay would reduce clutter.

Thanks, Joel
 

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My guess is that the startup current of the fan is what is welding the contacts. Motors generally draw more current as they are starting than their "running" current.
Has anyone ever tried, or considered trying, a ford solenoid instead of the "high" current relay? Those things gotta have the current capability. I'm just not sure how they would do in a continous duty application (i.e. would it get hot after having been energized for an hour).
 

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Joel;

How big is this relay physically. I wonder if it is really a true 75A relay. Is it one at this link;
http://www.bpg-inc.com/Power.htm

If so, it should be able to handle the inrush.

A couple of thoughts here;

1. The relay being mounted on the fan is subjected to constant vibration that may be burning the contacts.

2. Your wiring/connections at the relay are overheating and burning up the relay internally due to the heat passing through the terminals to the contacts.

3. You need a diode connected either across the relay contact or the motor to eliminate the back EMF voltage spike that occurs when the contacts open. Connect a large diode with the band towards the +ve side.

You could try a H bridge controller from this guy. He advertises that he has a few 12V units.
http://members.tripod.com/~divelec/hbridge.html

It says it is 35A continuous and I think the fan is 38A but maybe with a pot you could slow down the fan a bit to get the current draw lower. Or, you could just try it anyways. There may be enough heatsink for a few more amps.

These guys also have the same type of stuff but a lot money. http://www.vantec.com

With a speed control and a little wiring you could use a meter sender to actually control the speed of the fan.

Peter
 

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Joel you could try to run two relays parallel. I'd run more than the 30's though. These fans must be unbelievable to draw that much current. Does anyone know what F**D used to kickstart em'?

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70 Chevelle SS396/TH400/3.73 "What does not kill me, makes me stronger."
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by Gene McGill:

Has anyone ever tried, or considered trying, a ford solenoid instead of the "high" current relay? Those things gotta have the current capability. I'm just not sure how they would do in a continous duty application (i.e. would it get hot after having been energized for an hour).
<HR></BLOCKQUOTE>

Been there, done that. They melt after too long and stop working. They also draw a good bit of current themselves! about 1.8Amps.

Peter has some good ideas. My back EMF has never been a problem.I don't know why your Bosch setup isn't working. These contacts are screw post terminals and built to handle good amounts of current.

-good luck anyhow.


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71' 3880# with me. Big Block 402, TH400, 3.73 posi,
13.1sec @ 105 MPH (poor 2.1 sixty foot and rookie tuning)
--will be racing at Chevellebration 2001!--
Picture of me roasting the tires and other guy stuff
Video of me staging (smoke of course)
 

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Discussion Starter #11
The relays are much larger than a typical 30 AMP Relay and have the screw type terminals for the heavy wires. I got them from BPG and they are part # 0-332-002-156. It appears they are rated for 100,000 cycles. I think mine lasted about 99,900 cycles less than the rating.

BB_Mike, why do you think the Ford Starter Relay failed?. I am not concerned with 2 AMPS to activate the relay. Is there something about the design that makes them unsuitable for continuous duty use? If so, what is it?

Thanks, Joel
 

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You could always look to a motor contactor which you can get in almost any rating. These are the things usually used to start big 3-phase motors. Unfortunately, probably fairly expensive and hard to get with 12VDC coils.

The Ford solenoid is probably only meant for intermittent duty since it is running a starter.

Peter

[This message has been edited by Peter F. (edited 05-22-2001).]
 

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Joel,
This may sound too simple, but why don't you get whatever relay is used on the MKVIII's ??? It has got to work if Ford is using it as oem. Just go to the local Ford dealer and ask for a fan relay for a MKVIII. It may take a little more work to get it wired up, but if it works in a Lincoln, I sure think it would work in a Chevelle.
The other thing to look at is your supply and ground wires. Check the voltage at the supply wire when the fan is running and make certain that you have very close to battery voltage. If not....low voltage can cause high current, and might be burning up the relays.
Hope this helps,

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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100
 

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I just finished installing the mark8 fan in my 67 last week. I ran 2 30amp relays in parallel and have not had any problems yet. Of course I've only put about 40 miles on the car since the install. Before using the dual relay setup I placed a call to the [email protected]#$ dealership and asked for the fan relay for the mark8 (I think I asked for a 97'). The parts man came back on the phone and said they did have one. It only cost--(are you sitting down?) $127.00. I said thanks anyways. I'm all for clean installation and parts that actually work, but give me a break!

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MY 67

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Discussion Starter #15
I called a couple parts stores and then a Ford Dealer. The part stores did not list a cooling fan relay for a Mark VIII and the Ford Dealer said it was not acutally "a relay" but "a group of relays" that I would need to buy and he was not sure it would work. I did not get a price quote but got the impression it would be quite a lot.

I spoke with an Auto Electric Repair Shop today and the guy told me the problem could be from having the main supply wires reversed. I did not think this should matter. Can anyone confire or refute this? I'll try it since it will only take a couple minutes but I doubt it will have any effect.

Thanks, Joel
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by Joel Koontz:
I spoke with an Auto Electric Repair Shop today and the guy told me the problem could be from having the main supply wires reversed.
Thanks, Joel
<HR></BLOCKQUOTE>

I don't get it. You mena he thinks the fan is blowing in the wrong direction? Or does he mena it makes a difference to put the relay before -or- after the fan.

I have a 40Amp screw terminal fuse that screws directly into my positive battery terminals "wire pinching bolt". From the fuse I go to the fan, all of 1.5 feet away. From the fan I go to the relay, all of 4" away. From the relay I go to the frame bolt that holds the main stearing knuckle in place. From there it is jsut extra feel good stuff: Braided 1" wide (but thin) wire to the block. From the block to the battery with a 4" covered piece. -I also have a smaler 18AWG wire from the (-) post to the passenger fender well.

-Hope that helps. My relay has switched at least 100 times, if not more.

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71' 3880# with me. Big Block 402, TH400, 3.73 posi,
13.1sec @ 105 MPH (poor 2.1 sixty foot and rookie tuning)
--will be racing at Chevellebration 2001!--
Picture of me roasting the tires and other guy stuff
Video of me staging (smoke of course)
 

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Discussion Starter #17
BB-Mike, he does not mean the fan is blowing the wrong direction. There are two "high power" screw type terminals on the relays. He said it may make a difference which one is connected to the feed and which is connected to the load. I would not think it should make any difference but I am no expert so I tried reversing the leads. It worked fine today but that is no proof of anything as it worked OK most of the time before.

I have my relay in the positive wire. Would there be some advantage to putting it in the negative wire? If so, what/why?

I also stopped at a Radio Shack today and asked if they carried any High Amp 12V Relays. He said they don't stock any. When I explained the problem I was having he suggested putting a capacitor between the two big terminals on the relay. He said this should keep the contacts from "melting together". Would it be OK to do this? Would it work? If so, why/how? He said I could us a capacitor like one from a points ignition system. I have those lying around. Would it be OK to use one of them? If not, where could I find a suitable capacitor?

Thanks, Joel
 

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Joel;

If the connection mattered it would probably be marked on the relay. I doupt there is any arc shoots or anything like that inside the relay that would make the connection matter.

The capacitor may help. All it would basically do is keep the contacts from arcing as much when the relay opens. The arced, pitted contacts cause more voltage drop, which leads to more heat, which leads to a melted contact.

What guage of wire did you use to hook things up with? When your fan is running feel around the relay terminals and see if they are getting hot. Be careful because a hot terminal could cause a quick burn.

I still wonder about vibration. You said your relay was mounted on the fan. Is there a high speed vibration in the fan that could be causing the contacts to slightly vibrate against each other? This could make them arc continously which would wear them out quickly. Of course, a relay that size should have a decent amount of contact pressure so I don't know if this would be a factor or not.

Peter
 

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Discussion Starter #19
The Pos. & Neg. "supply wires" are 10 guage wire and come directly from the battery. I have 16 or 18 guage "activation wires".
 
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