Create an 80 to 120-Amp Relay System (TJ?s Only, Not for YJ?s or Rubicon?s on JU )

[Daless2]

Create an 80 to 120-Amp Relay System (TJ?s Only, Not for YJ?s or Rubicon?s on the JU Forum)

Disclaimer for JU Forums Only:

I have no clue if the Laws of Electrons and Current Flow apply equally to Jeep YJ?s and Rubicon?s as they do on the Jeep TJ?s. I am confidant and will stand behind this design for TJ?s requiring the ability to switch a high current draw device, such as the Ford Taurus Electric Cooling Fan.

I DO NOT know or have any experience in knowing if it will work on a YJ or a Rubicon. If you need an answer to this I would suggest you check the other forums. Perhaps there is a post and discussion already going on. Else I am sure you could start a new thread there.

Anyone know for sure??????



That said, here it is.

A few days ago someone put a post up (was it for a TJ or a YJ I don?t really remember) asking for a source for a High Current Relay required when installing a Ford Taurus Fan installed in a Jeep.

If that request was for a TJ, then I have a rather cost effective solution for you.


Take Three Standard 40 Amp Bosch Type Automotive Relays and wire them together in a parallel circuit.

By doing this, the current flow through all three relays is reduced to approximately one third of the total current flow through the circuit.

How much do these relays cost? About $0.89 each!

Here?s the circuit. It?s pretty self-explanatory.





Wiring it up

A big fat wire (battery cable type) comes from the battery and goes to a junction point, where it splits off into three 10-gauge leads, which go to Pin 30 on each relay. (Please keep these 10 gauge wires shorter then 5 feet in length. This shouldn?t be a problem for you. Just group the three relays together back to back to back.)

Three additional 10 gauge wires (again less then 5-feet long) connect up to each relay via Pin 87 and connect up at a junction to another big fat battery type cable, which will go to the Taurus Fan.

When the three relays are activated simultaneously current will flow through each relay via Pins 30 to Pins 87.

To activate the three relays build a small, parallel wiring harness that goes from your in-cab switch or the fan thermostat to relay coil Pin 85. Then connect Pin 86 on each relay to Ground.

When the switch or thermostat sends +12 volts to the relay via Pin 85 all three relay will simultaneous activate the relay coils.

You can use 18-gauge, even 22 gauge wire for the relay coils connections as this is a very low current (amp) draw.

I have tested this relay circuit in my shop on a TJ. It works great. Things started coming apart when I forced 159 amps continuous draw.

As stated before, the Ford Taurus Fan requires the ability to handle 80 amps of current draw at start-up. Given each of these relays will handle 40 amps this circuit can easily handle the draw. In fact, any two of these three relays can and does handle it.

I tested just two 40-amp relays wired in Parallel. The system burns up at 103 amps. Way more then you need however I would encourage you to use the three relays wired parallel to have excesses current flow capacity

I hope some of you may find this useful.

Have a great night folks.

Frank

[Tumbleweed]

Dang Frank, I was hoping this might work for a Taurus fan in some other type vehicle but I see it won't!
Nice write up as usual. These electric relays, especially multiple ones can drive me nuts. Gonna try to save this write up for future use.

[John]

Frank!!!! I'm willing to experiment with this on a YJ... risky I know, could put the whole time/space continuum in a tail spin and all that.
The real question then is how will my Chevy motor like being surrounded by Ford parts???????

[papromike]

I seems odd to me that the fan would draw that much current.


You could use the 3 bosch relays, however you need to remember that relays have duty cycles, and will thermal if continually used


I would suggest, if you need that much current To use a starter solenoid.

I use a Ford Starter solenoid for my onboard air compressor, it is rated at 150amps continuous, I run it off a 80 amp circuit breaker, it is much less wiring involved

[TObject]

Relays are the electrical components I am least familiar with. But since they are electromechanical devices, I will speculate that including three relays in parallel to spread the load is not a good idea. And here is why:

Let's say that one of the three relays is just a little quicker than the other two. It engages first, and all the current rushes through. You are entering the land of gamble here, will the other two relays engage rapidly enough, before damage to the first relay occurs, or not?

papromike's solenoid idea sounds good. Solenoid is just a big ass relay, right?

Among semiconductor components the correct one to use would be a thyristor. But you probably don't need that, as the latency of the fan engagement is not very important.

[mrblaine]

Only use a starter solenoid if you can absolutely verify that it is rated for continuous duty. I find it odd that the most temporarily used of any relays in a vehicle is rated for continuous duty. Typically the current to close the contacts is very high and will either melt the energizing coil or melt the trigger wire.

I inadvertantly use a starter solenoid in an ambulance once and nearly burned it down. Fortunately the compartment I ran the wiring through was all aluminum and no combustibles other than the insulation on the wire were present. The boss found it none too funny that his crew was standing outside the ambulance on the side of the road with smoke pouring out of the patient compartment.

Sergey, typically electrical damage is time based. The longer you are in an overload situation, the more damage occurs. The inverse is also true and with the millisecond engagement rate of the Bosch relays, I don't think you can hurt them.

Ever see a wheatstone bridge in a shunt for an ammeter rated for 500 amps DC? The center is about the size and thickness of a small fingernail clipping and it handles 500 amps no problem.

Different scenario I realize, but the principles are the same. Short wire run can be very small in size or short in time duration.

[MJR]

TObject has it right. Inrush current and speed of the relays contacting means that the relays will be smoking in short order. It only takes a moment to weld the relay contacts. Most do not realize that relays have two current ratings. One for start current and running current. The Bosch 40A would be start current.

Bosch does make a 70 A relay though.

A starter relay can handle the current though they are usually momentary but I have seen them used to isolate dual batteries when the ignition is off.

[Hellbender]

I think it is a fine idea.......... AS LONG as 2 of the 3 relays can handle the max. load, because those cheap relays will have a fairly short service life if near max load cycles (i.e. only 2 relays) is placed on them in a device that may cycle several dozen times a day (or more). Also, in the winter (for those of us that have it) a little ice buildup may make the fan exceed the 80 amp draw.

I'd replace them (as cheap as they are) every 15,000 miles or so to be on the safe side.

That must be a heck of a fan to have an 80 A. starting load, if your motor quits, just "Air-boat" it home.

What is Ford using to control them?

[Daless2]

Great discussion folks, and many great points. Sorry for the delay in replying. Been a busy day.

Maybe I can address some of my thinking and experience with this.


With the three relays in parallel if any one of the three relays should go south (bad or slow to activate due to binding or dirt or whatever) then the entire current flow would be going through the two remaining relays.

For use in the application of the Ford Taurus Cooling Fan this is not a problem.


Let me explain.

The Ford fan pulls 74.6 amps at start up and about 31.4 amps when in use based on my measurements on this fan installed on a friends TJ with LS1 engine last year. (Other's claim this fan pulls 80 amps, at start up, but I could never get it to draw 80 amps no matter what I tried including jamming the blades so it couldn't start right away.)

The three relays wired in parallel, as I have illustrated can handle a minimum of 120 amps at start up (3 relays times 40-amps = 120-amps) and (as I have tested) a maximum of 159-amps of continuous current draw before a failure occurs.

More then enough for an 74.6-amp application.


Any two of these three relays can handle a minimum of 80-amps of current draw, and again (as I have tested) up to 103-amps of continuous current draw. Once again, more then enough for this 74.5-amp application, all for the cost of $2.67 and a few minutes to wire them up.

The continuous duty cycle solenoids that are available from Bosch will work great just as suggested. However they are pricey; an order of magnitude higher then $2.67 and perhaps not readily available on the trail or my neighborhood at this time.

Also as Blaine has already mentioned, many folks mistake "Starter Solenoids" (i.e. Ford Starter Solenoids) for Continuous Duty. They are not.

They are more like 40 to 60% duty cycle depending on MFG'er. Yet they too are expensive and present a significant risk of fire to use in this application.

This is not to say you can't buy a Solenoid that will handle an 80 amp draw, That is what Ford uses to control the Taurus Fan. But at $68 per unit, they are pricey too.


Absolutely, Relays and Solenoids do have Duty Cycles. The Bosch 40 Amp Automotive Type Relay is a 100% duty cycle relay for it's designed amperage. Be sure to check this if you are using some other MFG'ers Relay. (Basic Frank Rule # 324876324; Buy Bosch and don't worry about it.)


Sergey, your point about the impacts of one relay closing sooner then the other two is valid, however the delay of BOTH other relays would have to be substaincail (in time, 50 to 75 msec) for this to have an impact. When one or all these relay close, the current flow does not go from 0 to 74.5 amps instantly. It gets there fast, but it ramps up. I don't have access to test this tonight but I will to see just how long it takes this application to ramp up to full current flow. The 50 to 75 msec is an estimate on my part, and certainly not a known fact.


In a sense having three relays wired in parallel provides a bit of redundancy as well, that would not be there in a one solenoid/relay implementation. If one relay goes out on the three relay system there is no outage as two relays in parallel will handle the job just fine.

If one relay or solenoid goes out on a one relay (or solenoid) system there will be an outage. But a recoverable outage, like this;


"Hmmmmmmm....... What happens if 80 amp relay (solenoid) fails while out wheeling?

Hmmmmmmm...... I know, "Maybe I can ask my friends to pull their Hella Light Relays." "Yes two of theirs and one of mine should do it nicely!"

10 Minutes to wire all three up in parallel and I'm back in business! COOL! (as in radiator!)

"Sorry for the delay folks, I'm COOL! (as in radiator). Let's get wheeling again!"



Certainly one relay can and does work. As I shared before this solution has been working for over a year with no failures. It is just another way of doing what can easily be done with one expensive relay / solenoid, for a whole lot less $$$$$$.

Hopefully something else to stick in your back pocket. Who knows, some day it may come in handy.

Have a great night folks!

Frank

[MJR]

I do not know of any automotive manufacturer doing this though. They may use two relays to control something but only one is switched at any given time and they are each rated to handle the load by themselves.

[mrblaine]

Originally posted by MJR
I do not know of any automotive manufacturer doing this though. They may use two relays to control something but only one is switched at any given time and they are each rated to handle the load by themselves.

Where can I find a 300 amp relay rated for continous duty?

[MJR]

Here is a 180A one I found on a UK site. 11.57 british pounds.

[Darrell C]

However, if you use it you'll have this uncontrollable urge to drive on the right side of the road

[MJR]

I found some other stuff.

Marine battery change over switch.
Phenolic moulding with solid copper contacts. 175A at 12v, 300A for 5 seconds. Selects between batteries; one, two, both or off.
Dims:137x137x72mm
Ref:60510...............?19.84



Electronically switched, double pole battery isolator. Phenolic moulding with 'Sheffield plate' contacts and studs. Includes terminal covers (not illus). 250A @ 24v, 2500A for 5 secs.
Ref:60532. 12v..?103.13
Ref:60544. 24v..?103.48

[Daless2]

Given the thought provoking discussion on these Bosch 40 amp relays I did some additional research and some more testing today.


First The Research:

The 40 Amp Bosch Relay I am using is part number 0-332-209-138-AS40150. Purchased at AutoZone.


According to the Bosch spec sheet on this commonly available relay the Normally Open contact (Pin 87) is rated at

40 Amps Continuous Duty,

AND

90 Amps In-rush Current.

They do not specify for how long the relay can sustain this level of in-rush current.

There is also a suppression resistor built into this relay, which explains why it can handle that level of in-rush current and all relay contacts are made of a Silver/Tin Oxide.



On the Test front,

I put an effort out today to time, with a test scope, how long it takes these relays to latch. I DID NOT do this under a 40 amp load, but that should not matter. (I just used minimal load (test light) across pins 30 and 87 and read the time delta between the relay coil current flow starting and the start of current flow through the relay load circuit.

I tests a total of 20 relays. (Yes I have more then that!.)

All 20 relays latched closed to complete a current flow between pins 30 and 87, in 14 to 16 milliseconds.

17 of the relays latched closed in 14 ms, 1 latched closed in 15 milliseconds, and 2 latched closed in 16 milliseconds.

Bosch does not publish any data on the speed of the relay latching closed.

The greatest time difference between any two of the 20 relays I tested was 2 milliseconds. Give this, and given the 90 Amps In-rush ability of this relay, and assuming testing 20 relays is a representative sample of all Bosch relays of this type, I feel even more comfortable (perhaps even sleepy) in my 3 relay design ability to handle an 80 to 120 amp circuit.

Frank

PS: Lots of good stuff there Mike!

[Blackjack]

Frank,
I know that I am dragging this from the dead, but I remember there was a lot of discussion on JU about using this arrangement (or how it would self destruct). So I was out at the junkyard grabbing a few fans for some club members and grabbed a factory fan controller while I was at it. Sorry for the crappy pic.



A quick disection reveals Ford seems to have the same idea as you. A quick look at the traces on the board revealed only two of the four relays (Bosch 40 amp 0 332 014 162) are wired to the high speed side of the fan.

I plan on swapping out my corsica fan here soon. When I do, I am going to run the high side with just two relays to see how it does. Most likely my four popper will not require running on high very often, but I will give it a thorough thrashing just to see what happens.

I do have one question though. Are you running freewheeling diodes with this arrangement?