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

[kramttocs]

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.

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?

 

[Gary Lewis]

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 - 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?

[85lebaront2]

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?

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?

[Gary Lewis]

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 - 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.

 

[85lebaront2]

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.

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.

[kramttocs]

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.

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?

[Gary Lewis]

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?

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.

[kramttocs]

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.

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.

[Gary Lewis]

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.

How 'bout checking the voltage drop to the motor with the switch vs with the relay? That will tell us how much it helps.

[ArdWrknTrk]

How 'bout checking the voltage drop to the motor with the switch vs with the relay? That will tell us how much it helps.

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.