'74 with a/c Blower Motor Current Draw ??

[Gold Rush]

Does anyone know what the actual current draw is supposed to be? Out of curiosity I checked mine and it was pulling 11.56 amps in high. The fuse holder was getting pretty warm so I'm wondering if that old motor is starting to drag some.

 

[72RoadrunnerGTX]

While not able to locate the particular specification for that in the manuals, I can tell you the one in the current project here, freshly rebuilt-oiled, is currently pulling 12 amps directly- no resistor pack. Pretty sure when normally configured, there is some resistance in play while high speed is selected at the switch.

 

[1 Wild R/T]

Exactly why I add a relay to the blower high speed circuit.. The switch lives longer, the blower spins much faster & if the relay fails replacement is cheap & easy...

Add to that the reduced load on the bulkhead connector since the main power feed no longer supports the blower (or headlights) so the whole electrical system works better..

 

[Gold Rush]

Adding the relay may be the best option. Especially in light of the first response. Easy to do and sure would make the fuse block happier. In medium speed I'm pulling 6.2 amps and the fuse holders are cool.

 

[Nacho-RT74]

12 amps sounds about right.

I added 4 relays to the A/C- heater blower system, one for each speed ( heater gets its own speed when is AC equipped )

 

[Gold Rush]

Thanks for all the responses. Now I have to get busy and do something..

 

[Nacho-RT74]

I installed relays because the blower lever switch tends to get SO HOT that melts out the plug on back

 

[Gold Rush]

I installed relays because the blower lever switch tends to get SO HOT that melts out the plug on back

That stands to reason. As hot as the fuse block gets that connector must be getting equally as warm. I'm going to do some investigating to see how warm things do get in the low and medium blower positions. If they get very warm at all I think the multiple relays is a good idea.

 

[Nacho-RT74]

I can post later the info and diagrams about what I made. Although is partially posted on my thread about the wiring and alt upgrades at DC.com.

I actually installed 6 relays... the other two are for low and high beams. Everything sourced from ammeter post on a hot line I run from side to side of the dash. All relays are hidden down the dash. Lights are on back of the kick panel

 

[Gold Rush]

I'd like to see what you did Nacho. No rush but when you have time THANKS in advance.

 

[Nacho-RT74]

first some pics on the job I made at least for the low and mid AC blower speeds.

I didn't want to cut any wire after I restore all my harnesses. To keep the blower speeds working, the relay must be BEFORE the resistor. What I made is remove the tan and light green wire terminals from the resistor plug ( which are low and mid speed coming from AC control ) into the terminal 86 being the trigger for the relay. Then inserted a new piece of wire of SAME COLOR between the 87 terminal of relay and the resistor block plug... relays got hidden above the glove box liner, attached to the AC box bracket support.

If you look down the dash nothing is visible or noticeable on wiring, untill you pull out the harness itself, after loose 7/16" the AC box bracket nut to get free the relays. Same nut and bracket provides the ground for the relay. You can see one of the relays with a cut to be slided on bracket stud without need to remove completelly the nut. Relay plugs are slide in plugs one to the other, so they are like a plugs block. I'd like to find same kind of plugs but with the mounting bracket built in to get the plugs fixed on place and relay removable without attach them.

The red wire comes from ammeter stud and used a bullet terminal for easier access or mantenience, or whatever future upgrade without need to cut the red wire buss coming from ammeter

Pics are low quality because was on those days where smartphones were not on market

 

[Nacho-RT74]

On Heater and High speed I made the relay upgrade on steering column area ( relays are attached somewhere around the steering column brackets ) but idea is the same. On these what I made due the diff location is:

Heater speed comes from AC main control unit, not the blower lever speed. The wire running to resistor block ( coming from switch ) is this time inserted into the 87 terminal of resistor, and the wire section to replace the one coming from control switch is also same color and gets jumped between control unit switch plug and 86 terminal of relay... this wire is brown.

High AC speed is about the same but wire on 87 terminal is the dark green wire coming out from blower lever switch, and a same piece of green wire was used to feed the 86 terminal of relay to trigger it. Need to say this wire DOESN'T RUN to resistor block but to bulkhead. The green wire on resistor block is just the output from block up to bulkhead. Both dark green wires are spliced into the bulkhead terminal both to feed the blower, BUT one is used as a resistot block output and the other one a straight full feeded coming from relay for the hight speed.

Allong this process SEVERAL wires are hot on several stages, but just the one with less resistance ( feeding with more voltage ) is the one what will get the load to feed the blower. Saying this, the low speed relay will be ALLWAYS activated no matter the speed you select ( except on Heater ) because the input to the blower lever speed uses this network to feed every speed. Meaning with this, if you disconect the blower lever speed, the blower will work on low speed allways.

Same about the high speed... even dark green wire will be allways hot due the power coming from resistor block, the max speed is activated JUST when coming from blower speed lever ( hence the relay there ) out from the resistor bloc... but still the resistor block gets hot lines there coming from low speed.

 

[Nacho-RT74]

and about lights... I made exactly the same.... no wires cut, just untapped, pulled out the necessary wire lenght to install the relays on the right spot, and added the "jumper wire" ( same color to make it unnoticeable after retappe them ) up to dimmer floor switch

I'm searching a diagram about the headlight relays job to be located on kick panel area I made but can't find it. Will edit the reply and post when I find it. It shows more clear the job made on this kind of job without Hack up the harnesses just untapping and installing all relays into the cab, out of heat, enviroment elements, oil etc... and showing your car as a pure stock piece from outside

 

[1 Wild R/T]

So your still feeding the blower power from through the bulkhead connector..... You really haven't gained much... You've taken the load off the switch but all the power is still entering & exiting the car through the bulkhead connector and passing through the ammeter...

 

[Nacho-RT74]

If you really follow me out, would know why and how I made it on that way! ;)... sure my electrical system is safe, trust me ( yes, including ammeter ).

BTW, while the engine is running, the higher output alternator I have is what feeds the system, not the batt, so my ammeter is safe. Problems begins when alt is not enough to feed the system... which is not the case on my car. I'm feeding this buss ( wire ) from alt side of the ammeter ( Black end )

 

[72RoadrunnerGTX]

The diagram indicates the use of a fusible link at the ammeter for your relay power. Fusible links should not be used inside the passenger compartment for any purpose. Should it blow while the vehicle is in motion, it will generate a great deal of smoke very quickly. Engine compartment only for fusible links.

 

[Nacho-RT74]

yeap... but I wasn't really sure about use a fuse there, because headlight relays are feeded on same line. I'm pretty sure will never blow out, but is just an extra protection. I could change later for a thermo breaker maybe

 

[72RoadrunnerGTX]

Not only recommend resetting circuit breakers, separate breakers for lighting only, separate breakers for high and low beams as well.

 

[383man]

GM used a relay for the blower high speed for many years as I know their cars of the 60's and 70's did when I worked on them. And it makes sense as high blower speed and draw from 10 to 20 amps on some cars. Fords in the 70's used a 30 amp fuse or circuit breaker on their blower motor. And I had to replace many melted blower switches that had high blower speed going through them. Ford would have been better off using a high speed blower relay also back then. Ron

 

[Nacho-RT74]

I was debating on use one relay for every speed or just for the high speed. Opted for feed each one with individual relays after dissasembly the blower speed switch and note how weak are the points inside, ALTHOUGHT, low speed doesn’t go through the lever switch, is a direct feed from main control switch.

Also to guarantee a good power source from batt/alt on every speed. It was very tipical note an rpm increase on blower when giving throttle. With relays this become more stable.