[Socrates]

Howdy,

I picked up a rather nice quad of 30/40 amp Bosch relays off a early 90's caddy at my local bone yard to supply my Taurus electric fan and headlights. I'm setting up 2 of the 2553347 relays in parallel to feed the fan (also numbered 2295 or 2316 followed by Z7365, the the 2553347).

I've read that most GM relays have a diode in them to prevent the inductive-kickback or back-emf inherent in a fan motor. Anyone know for sure if the relays I have include the internal diodes, and do they run across the trigger or the main power leads, both, or??? Should or can I run another diode for safety?

My Google search here wasn't helpful, though I found this relay is also be called an RY-241 and used for electric fans and for A/C power for 91-93 caddies, buicks, etc.

["QUICK-SILVER"]

The caddys had a seperate computer to control the fans. Switch put power on the trigger side of the relay and the computer grounded the trigger to kick them on. Depending on how you wire things up you may not need diode protection. You can check the ones you've got with a DVOM. Just check for continuity between terminals then reverse probes and see if you get the same reading.

About the taurus fans. From what I understand they're as strong as the mark VIII's BUT they are REAL 2 speed fans. You use two relays, just one is low speed and the other is high speed. So they don't have the big surge problems that the single speeds do. You'll run on low most of the time and have high kick on when needed. IMO there one of the best electric fan set ups there is. Dual cooling without spending a fortune on a controller.

Let us know what you figure out.

[Socrates]

Thanks for the reply, Quick-Silver!

I answered my own question by tearing open a spare relay with the same part number. There is a diode crossing between #85 and #86. I believe I'd need one going across #30 and #87 for my needs with the Taurus fan, right?


As for the Taurus fan, I have both it and the Mark VIII fan sitting side-by-side in my shop. They appear to share the exact same drive motor, just different blades attached and different shrouds, of course, with the Lincoln fan having about 2" longer blades.

Both of my fans are actually true 2 speed fans, both with two "power" wires, one thinner gauge then the other that spins the fan slower then the thicker gauge wire. I have read that only the early Mark VIII fans are 2 speed, and the later just had two wires, whereas my early model has 3 and dual speeds, yes? Also, as I understand it, the 2 fan speeds are not supposed to be energized at the same time, and that only one or the other speed should be running, not power to both low and hi at once, correct?

I am going to try the low speed only first, and then the high if needed. We do have many 100deg days in VA in July, so the high might be best. Both speeds on both fans do move a TON of air, and the high speed isn't that much noisier so, apart from the extra current draw, not a big deal using the high speed only, especially as my fans are being fed from the 140+ amp CS144 alternator through dual relays with only ~4ft of 10ga wire in the circuit...at least, that's my story as of now. We'll see how it pans out in the real world this weekend!

[BILTIT]

My MKVIII fan has 3 wires but only one speed.

["QUICK-SILVER"]

Funny thing about the Bosh relays, everyone says go to the parts store and get a universal one, and when the part can't cough one up, everyone says find a better parts store. The deal is how many different part numbers there is for a 30amp bosh relay. You got with and without diodes, 4 pole & 5 pole, and who nows how many different diode configurations. Each one with it's on specific application.

85 & 86 are your trigger terminals. Depending on what you trigger with will determine what needs kick back protection. Yours have 85 protected because of the caddys ground pole trigger. They didn't need the spark shooting back into a computer.

I don't think one between 30 (battery) and 87 (component) is needed.

Two speed fans that I've dealt with use 2 hot wires and one ground. One hot wire turns the fan on low and the second wire energizes more windings for high. Low carries the brunt of the current while high comes in like a booster not carrying much of a load.

What BILTIT said about the mark VIII is what I've heard the most of. Have heard about the two speeds also. The only diagram I've found was for the newer model one speed and both hot wires went to the same place inside the motor.

Anyhow, with an ohm meter and hot battery it'll be easy enough to figure out.
2 speed sound like a plan to me. Let it run and then kick it when needed. 165 degree temp sensor for low and a 185 to kick high in.
Let us know what you come up with.

[BILTIT]

Here is some very good info on the MKVIII and control circuits.

http://www.geocities.com/smithmonte/...rkVIII_Fan.htm

[Socrates]

I installed the Taurus fan with the relays listed above, no diode, fan wired to high speed only thus far. At idle, I set the fan to kick on at 175, the fan cools the car to 160 very quickly, and the cycle repeats periodically.

Driving around town the fan didn't come on much if at all, unless I was racing around, pushing the car hard on twisty back roads, a WOT blast on the onramp, etc, then the engine would creep up to 185'ish briefly then cool down to 160. The Taurus fan is pretty loud running on the bench, though I can barely hear it under the hood over the 3" exhaust, headers and clattering lifters

To correct myself above, upon closer inspection, it looks like it's a resistor inbetween #85/#86 in the caddy relay, not a diode...

I've had an electric fan in my Scout II for years without a diode to "protect" the relay and it's still going strong, so maybe the diode is overrated or a figment of a lonely electrical engineers imagination?

[george kujanski]

The reason diodes are used across an inductor (any inductor, motor, relay coil, A/C compressor clutch) Is to clamp the voltage spike that occurs from an inductor when the current thru it is abruptly cut off.

That said, it's a good idea to have diodes across the relay coil terminals because when the computer (actually a transistor in the computer) turns off the relay coil, a voltage spike is generated by the relay coil which, if the voltage is high enuf, will destroy or damage the transistor. The band end of the diode goes towards the terminal which is connected to system voltage (+12).

AS far as a DC motor is concerned, a diode can be used there also, for the same reason, but the diode rating has to be the same as the motor running current. When the relay contacts open the motor current continues thru the diode, but decreases exponentially, but the peak current is equal to the motor running current.

Not having a diode across the relay contacts is not that bad, however, because the contacts of a relay rated for 30A DC, for example, will handle the voltage spike from the device connected to it. Also, the only things connected to the relay contacts are the motor and the battery, both of which will not be harmed. The innards of a computer, however are low-voltage devices.

George

[Socrates]

Thanks George! That all makes sense. I'll runs the fan without a diode and carry a couple spare relays in the trunk too box just in case.

["QUICK-SILVER"]

A well positioned diode from power wire to ground, near the component being turned off and on, will let the spike go straight to ground, greatly extending contact life in the relay.
Commonly refered to as a free wheeling diode but may actually be something else.

Same as the internal diode in the relay protects the triggering device.

[stellar]

QS You've got my curiosity up. Is it a zener diode ? Or avalanche diode? Something else? Is it necessary or just a protection devise to extend relay contact point life?

["QUICK-SILVER"]

Originally Posted by stellar

QS You've got my curiosity up. Is it a zener diode ? Or avalanche diode? Something else? Is it necessary or just a protection devise to extend relay contact point life?

So don't feel like the Lone Ranger on this one.
First best guess: Like the ones on AC (Air Condition)clutch plug-ins but bigger. Got to be for switch potection only on flyback. The larger ones on radiator cooling fans (wired the same as the AC plug-in) is usually incased in plastic with the typical +l<- diode markings on the side.
Use to see a lot of them under the hoods of cars when 'more modern' computer type controls started being used. Got to remember I've been out of commision nearly 14 years now. So don't know whats happened recently.
I do know in searching electric car stuff, neice had a broke electric scooter, some of the freewheeling diodes are rated at 1200volts and 450amps.....

Anyhow any added info would be appreciated. Know what they do just don't know where to get them other than old cars.

BTW been trying to pm you but takes so long to type I keep getting cut off. Going to give it another shot.
Clay

[Sun Tuned]

Quenching diodes.

[george kujanski]

Not zener or avalanche diodes. Avalanche diodes are similar to zeners and the 'zener' term is used for them also, but that's another story.

The "freewheeling' or 'clamping" diodes are just diodes that are used across inductors to 'clamp" (limit) the voltage spike. As I mentioned B4, when used across relay contacts that switch large inductors or motors, the diodes need to have a high current rating, similar to that of the motor or inductor.

There are other clamping devices used for this purpose nowadays, one particularly called a varistor, which limits voltage also, typically used in 110volt AC powered appliances. They're also used in surge suppressors, the type used for computer power strips.

[stellar]

Thanks guys, good info. I feel like we stole the thread from the philosopher, but where else better to learn than under one of the worlds greatest teachers, Socrates. Don't drink the hemlock.