Current going to Blower Motor can be higher than Headlights - why no relay mod?

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Current going to Blower Motor can be higher than Headlights - why no relay mod?

CountryBumkin
Just posting this for discussion purposes only.

I was looking at the circuit for the AC/Heater Blower motor and I see that it is a 30 Amp fused circuit (and that the "full power" to the Blower goes through the "AC/Heater Function Selector Switch".

So I'm wondering if sending 30A through the Selector Switch for the Blower is, or should be, a concern.

We are doing/recommending the "Headlight Relay" mod to keep the headlight switch from melting, and the headlight circuit is on a 16GA fusible link (which from my research shows that a 16GA fusible link is designed for a circuit with 20 (maybe 25) amps max).  Therefore the Blower circuit has (potentially) higher current flow than the headlight circuit.

If this is true why are we not seeing melting of the AC/Heater Function Selector Switch plug?

Why is a "relay mod" similar to the headlight mod not needed here too? Is it because the AC/Heater Function Selector Switch electrical connector is just better (heavier duty) than the headlight switch?
-Mike
1982 F250 4x4 XLT Lariat, 460cid, 9.5:1, TFS 290 Street Port heads, Crane Hyd. Roller cam (278/286 adv.,216/224 @.050, .560"/.586" lift), Edelbrock Performer 460 intake, MSD Atomic TBI fuel injection, MSD 6AL ignition, MSD Billet distrib., Hedman headers, NOS Cheater system, 4" Suspension lift, 3" body lift, 4.56 gearing, 17x12.5Rx37 tires MT Baja ATZ-P3.
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
Actually, there has been some discussion about that on FTE.  The thread heater recirculated air has a few posts about that.  Further, I am planning on a significant upgrade on Dad's truck's wiring, as discussed in Dad's Wiring.  And on post #12 of that discussion you'll find the diagram below showing how I'm going to do it.  (Yuk!  That didn't come across very cleanly.  )

But, as Jim said on FTE, the issue is with the small size of wire going to the fuse box.  So, while the current draw of the blower motor may be ok for the size of wire running from the fuse box to the motor and back to the switch, by eliminating the load of the motor from the original fuse box the load on the wire feeding it goes down and everybody is happier.

In my plans the power distribution box, liberated from a later F150, will go right behind the battery.  So there will be a serious power feed to it, and then the big items will branch off from there, leaving what's on the original fuse box to be fairly minimal.  Plus, most of the lighting will use LED's, so that reduces the draw even further.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

85lebaront2
Administrator
In reply to this post by CountryBumkin
First, look at the entire wiring diagram for the blower motor. On our trucks the blower motor is fed 12V through the function selector switch, the 30 amp fuse or breaker feeds both the blower and the A/C compressor clutch, yet on the heater only there is also a 30 amp fuse. The blower speed control is on the ground side of the motor.

If you want to add a "relay mod" you need it on the power side of the motor, but controlled by the function selector switch on A/C models. The brown/orange wire provides 12V to the blower motor so that would have to be connected to one side of the relay coil (85 or 86 on a Bosch relay) with the other side grounded. If you really want to unload the blower switch, the a second relay to ground the motor in high would be needed.

If someone really wants to go through all this, then I can give you diagrams and suggested locations for the relays. Only issue I have ever had is the plug inside the cab where the HVAC harness goes through from inside to outside, that plug gets hot enogh to become stuck together.
Bill Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F-350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 1994 Taurus LX Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

CountryBumkin
I just typed out a complete explanation of Dark Energy and Dark Matter and how they interact with the Higgs boson- but the forum would not let me post. It wouldn't let me sign it. So I'm starting over - and skipping the physics this time.

My thoughts on this are to limit the high current through the AC/Heat Function Selector Switch by installing a relay in the engine compartment near the Blower. The original circuit through the Function Switch would only be used a the control for the relay. This will protect the Function Switch - but doesn't do anything to protect the Blower Switch, but current through Blower switch is split between switch and resistor ( I don't think switch ever sees full current load).




-Mike
1982 F250 4x4 XLT Lariat, 460cid, 9.5:1, TFS 290 Street Port heads, Crane Hyd. Roller cam (278/286 adv.,216/224 @.050, .560"/.586" lift), Edelbrock Performer 460 intake, MSD Atomic TBI fuel injection, MSD 6AL ignition, MSD Billet distrib., Hedman headers, NOS Cheater system, 4" Suspension lift, 3" body lift, 4.56 gearing, 17x12.5Rx37 tires MT Baja ATZ-P3.
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

ArdWrknTrk
Administrator
In reply to this post by 85lebaront2
I've replaced the socket for the resistor *twice* in my truck....

No relay on the hot side is going to help that.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with an Edelbrock 1826 and Performer intake.
Too much other stuff to mention.
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
Mike - Where you suggested is where I have the relay planned for Dad's truck.  But, as you and Jim point out, that won't help the blower speed switch nor the relay.  However, I've not had problems with either of those, so I'll take my chances.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

85lebaront2
Administrator
If you use a relay on the ground (switch) side, it only needs to be on the high leg as that is the highest current. If you use a Bosch relay, and use 30 as the blower motor, 85 or 86 to the high position on the switch, other to a 12V when the system is on source, then 87a would be to the switch and 87 to ground. You will probably need a diode in the switch leg to prevent the system from sticking on high.
Bill Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F-350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 1994 Taurus LX Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

ctubutis
In reply to this post by CountryBumkin
Not to be the Devil's advocate, but...

I disconnected, cleaned & serviced each wiring harness and made it OEM+ quality again, and I don't have these issues, it works as designed.

Which is a crappy design, the power-distribution stuff they came out with in the later years seriously kicks our gen's ass.

And it's a fair amount of work to take all that stuff apart, it's not something one "just does" unless they have serious time on their hands. ;)
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
Yes, the power distribution systems on the later trucks were far better than ours. Placing the fuse/relay box by the battery and using small-gauge wire to/from the switches solved a lot of problems. And distributing relays around the engine compartment, as they did later, helped as well.

I don't believe it is going to be terribly difficult to upgrade Dad's truck significantly by using a later power distribution box. As shown, I have a plan but haven't executed it yet. We shall see.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

85lebaront2
Administrator
In reply to this post by ctubutis
The more I dug into Darth's factory wiring the more I became convinced that Ford's electrical design team was on something pretty powerful (I want some!). It's pretty bad when Chrysler's cheap salvation (K-car) had a better setup, even though it was full of fusible links.
Bill Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F-350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 1994 Taurus LX Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

kramttocs
This post was updated on .
In reply to this post by 85lebaront2
Can you explain more about the need/benefit of the second ground relay as well as the proposed location (near a connector number on an EVTM page would be great)? I saw the same mention of this over on FTE.

Edit - nevermind, was post happy and took me a minute to digest. I follow now.

Would like some more on the diode part though. Why would it stick on high?

Scott
Daily driver: 'Camano' 1986 F250 Supercab XLT Lariat 460/C6
Work truck: 'Chanute' 1980 F350 C&C 400/T18 - Gin Pole
Various parts trucks
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
Scott - I'm going to jump in as I think I know why, but want to see if my understanding is correct.

I believe it is because the motor will act as a generator when the power turning it taken away, so current will be fed back into the truck via the relay's contacts.  And as those contacts open the arc created by the current doing its best to flow can weld the contacts together.  So this diode is in series with the contacts such that current can flow TO the motor but not FROM the motor.

If that is correct then the diode has to be a rather substantial one as it has to be big enough to handle the full current of the motor for extended periods.  (That's as opposed to the snubber diodes that just have to carry current very briefly as an inductor discharges.)

Bill - Is that correct?
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

85lebaront2
Administrator
First item, the blower motor will draw a substantial current load, where I have seen problems is in the plugs where the system power flows through. The main feed to the HVAC control panel, the feed connection through the firewall and the resistor block. These are the places where corrosion of the contacts (which are only about 3/16" to 1/4" wide) can create so much heat that the connectors become partially melted. A snubber diode across the motor feed/ground like the one used on the AC compressor clutch will solve the arcing problem on opening the circuit.

The problem comes in the way these trucks (and many other vehicles are wired) the blower speed control is on the ground side of the circuit, power is fed to the blower whenever the main control switch is moved to ON with AC, on a heater only truck, the blower is fed directly from the fuse panel and is grounded through the resistor and switch. GM used a direct feed for high blower on their vehicles, because they control the speed on the power side of the circuit and use the high relay to "unload" the switch, their feed from the panel is also a smaller wire since it only handles L M1 and M2, H being the relay coil.

The 30 amp fuse or circuit breaker is a pretty common size for the blower feed on most US vehicles and the wire is usually #12 in size. If you want to eliminate the high current draw through the switch panel, it will take two relays, one to power the blower, the second to provide the high speed ground. If you want to completely remove the current load from the switches, it will take 5 relays for AC and 4 for heater only, one for power and one each for the speeds. The interesting thing on these, the standard Bosch style mini relay is only rated 20 amps on the contacts, the micro relays (the ones with two large and 3 small pins), are rated 20 amps on the NO contacts (the ones that would be used) and 10 amps on the NC contacts.

Sorry this is rather long and involved, but, the short answer is a blocking diode wouldn't be needed and is it worth the complications for the problem that is usually caused by a bad or dry bearing motor issue?
Bill Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F-350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 1994 Taurus LX Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
Bill - Let me see if I understand.  You are saying that a snubber diode would or could be used across the motor's terminals?  So if/when the polarity of the motor's output reverses it'll take it to ground?  Why would the polarity reverse?  I thought the motor just became a generator.  

I'm missing something that is probably obvious.

Also, I'm wondering if a High relay wouldn't increase the voltage to the motor significantly, thereby getting more flow of air.  I'm not sure it is worth the hassle, but I'm just thinking.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

85lebaront2
Administrator
Actually, on cutting power I believe it will generate a reverse pulse, if it is a forward pulse then there really isn't anything easy to do for it.
Bill Profile

"Getting old is inevitable, growing up is optional" Darth Vader 1986 F-350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator Wife's 2011 Flex Limited Daily Driver 1994 Taurus LX Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

kramttocs
This post was updated on .
Thanks guys. Follow most of it but a little unclear on the best solution. Sounds like it may be the relays on all paths but not sure of the ROI of that.

The relay specs are a good thing to note - the iso 280 micros I am using are 35/20.

OK, so the diode is not needed in any situation? Whether you do the hot side relay only or relays on all paths (+ and -)? So that can be disregarded completely?

Obviously the hot relay only is the easiest to implement for people and sounds like it will have benefits without adding any new drawbacks that aren't already there from factory. Correct?

OR will this increase the chance of the speed switch failing since increasing the + will also increase the - side through the switch?

So ultimately, is just putting the hot relay in at C603 BR/O and making no other changes an acceptable, safe, worthwhile modification?

Or as was mentioned earlier and in Mike's diagram, is a relay on the + side and one on the High ground side the best solution after relays everywhere?
Scott
Daily driver: 'Camano' 1986 F250 Supercab XLT Lariat 460/C6
Work truck: 'Chanute' 1980 F350 C&C 400/T18 - Gin Pole
Various parts trucks
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
My opinion is that you are right when you said "ultimately, is just putting the hot relay in at C603 BR/O and making no other changes an acceptable, safe, worthwhile modification".  It won't increase current through the switch as the relay will carry the load in High.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

kramttocs
Thanks Gary. That's the route I would like to take unless it cause any problems and more needs done.

May try and test it out today. Hoping to add some wiring to the relay block and it will be the easiest since the blower wiring is all right there.
Scott
Daily driver: 'Camano' 1986 F250 Supercab XLT Lariat 460/C6
Work truck: 'Chanute' 1980 F350 C&C 400/T18 - Gin Pole
Various parts trucks
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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

Gary Lewis
Administrator
How 'bout checking the voltage drop to the motor with the switch vs with the relay?  That will tell us how much it helps.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI

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Re: Current going to Blower Motor can be higher than Headlights - why no relay mod?

ArdWrknTrk
Administrator
But I think you need to check this using the same ground in the harness.
If you check voltage against a chassis ground the data is next to meaningless.

I intend to implement a hot side relay as soon as I get back into my little PDC.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with an Edelbrock 1826 and Performer intake.
Too much other stuff to mention.
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