chengny
Full Access Member
- Joined
- Feb 22, 2012
- Posts
- 4,086
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- Location
- NH
- First Name
- Jerry
- Truck Year
- 1986
- Truck Model
- K3500
- Engine Size
- 350/5.7
With the relay plugged in, key on , I have 12 volts between the purple wire and the red...when the fan is switched to high speed....no power.
Those would be the expected (and correct) readings.
The red wire is straight from the battery positive and the purple wire goes to ground - by way of the blower motor windings. While in a non-high speed setting, the coil is de-energized position - refer to the dwgs at the end of this post. In that condition, the high speed contacts - the ones that connect the red wire (at terminal 2) to the purple wire (at terminal 65) are open.
So, because they are not connected when not using high speed, putting your probes on the red (at pin 2) and the purple (at pin 65), is essentially the same as putting one on the positive battery terminal and the other to the frame (or negative battery terminal). You would expect to see 12 VDC.
But, when you switch to high speed, the coil becomes energized by the orange wire. The relay pulls in and closes the pin 2 to pin 1 contacts. So now, what you basically have is, the battery positive connected to the blower motor - by a single lead. It may change colors (from red to purple) along the way, but schematically speaking, it is just one wire.
A voltmeter works by measuring the difference in electrical potential between two separate points in a circuit. And by "separate", they don't mean just two different spots on the same wire. There has to be something in between the measured points that causes the potential energy on one side to be lower/higher than the other side.
I have conductivity between alternate contacts, but NOT between the red (power in) and the purple. Is it the new relay thats bad?
As noted above, the won't (or shouldn't) be continuity between pins 2 & 65 - unless there is power on the system and the blower speed switch is set to high. Then, if those two conditions are met, the coil will pull in and pin 2 will be connected to pin 65. But, because it will then be a hot wire, you would not be able to test for continuity across the 2/1 contacts anyway. To confirm proper relay operation, just put your black probe to ground and - using the red probe - check for full battery voltage in on 2 and out on 65. If you find that there is voltage on both sides of the contacts, the coil and contacts are okay.
I noticed that the metal contact for high speed position was cooked and discolored. Im wondering if possibly a bad wire somewhere under the dash?
I seem to recall quite a few stories and questions about the high speed contacts in the fan speed switches failing. If your high speed contacts were as bad as you describe, I don't think you have to even look elsewhere for a bad wire. If they looked anything like the switch shown here:
That may well be the reason why you don't get high speed blower operation. Excessive resistance across the high speed contacts in the blower speed switch. The voltage to the blower for high speed doesn't come from the speed select switch - it comes from the junction block, to pin 2 on the red wire, through the high speed contacts (2/1), out of the relay at terminal 65 and then over to the blower on the purple wire.
The speed select switch only supplies the control voltage to the positive leg of relay's operating coil. That control voltage - to the positive leg of the coil - comes in on the orange wire that connects to terminal 52. Terminal 52 is then internally connected to pin 3. And, the only time the coil is energized is when the blower speed switch is in the high position. Maybe the high speed contacts in your speed select switch are so carboned up, that they are not allowing enough power on the orange wire to enable the coil to pull in (and make contact between pins 2 & 1).
Here are a couple of wiring diagrams marked up to show current flow for both the high speed and non-high speed operation. Because all your reduced speeds/voltages are working okay, the text for non- high speed operation doesn't get into how the variable voltage generated. It simply says that the output from the resistors is variable, comes in on the dark blue lead (which feeds pin 5) and is is regulated by the resistor stack.
Non- high speed operation:
High speed:
Those would be the expected (and correct) readings.
The red wire is straight from the battery positive and the purple wire goes to ground - by way of the blower motor windings. While in a non-high speed setting, the coil is de-energized position - refer to the dwgs at the end of this post. In that condition, the high speed contacts - the ones that connect the red wire (at terminal 2) to the purple wire (at terminal 65) are open.
So, because they are not connected when not using high speed, putting your probes on the red (at pin 2) and the purple (at pin 65), is essentially the same as putting one on the positive battery terminal and the other to the frame (or negative battery terminal). You would expect to see 12 VDC.
But, when you switch to high speed, the coil becomes energized by the orange wire. The relay pulls in and closes the pin 2 to pin 1 contacts. So now, what you basically have is, the battery positive connected to the blower motor - by a single lead. It may change colors (from red to purple) along the way, but schematically speaking, it is just one wire.
A voltmeter works by measuring the difference in electrical potential between two separate points in a circuit. And by "separate", they don't mean just two different spots on the same wire. There has to be something in between the measured points that causes the potential energy on one side to be lower/higher than the other side.
I have conductivity between alternate contacts, but NOT between the red (power in) and the purple. Is it the new relay thats bad?
As noted above, the won't (or shouldn't) be continuity between pins 2 & 65 - unless there is power on the system and the blower speed switch is set to high. Then, if those two conditions are met, the coil will pull in and pin 2 will be connected to pin 65. But, because it will then be a hot wire, you would not be able to test for continuity across the 2/1 contacts anyway. To confirm proper relay operation, just put your black probe to ground and - using the red probe - check for full battery voltage in on 2 and out on 65. If you find that there is voltage on both sides of the contacts, the coil and contacts are okay.
I noticed that the metal contact for high speed position was cooked and discolored. Im wondering if possibly a bad wire somewhere under the dash?
I seem to recall quite a few stories and questions about the high speed contacts in the fan speed switches failing. If your high speed contacts were as bad as you describe, I don't think you have to even look elsewhere for a bad wire. If they looked anything like the switch shown here:
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That may well be the reason why you don't get high speed blower operation. Excessive resistance across the high speed contacts in the blower speed switch. The voltage to the blower for high speed doesn't come from the speed select switch - it comes from the junction block, to pin 2 on the red wire, through the high speed contacts (2/1), out of the relay at terminal 65 and then over to the blower on the purple wire.
The speed select switch only supplies the control voltage to the positive leg of relay's operating coil. That control voltage - to the positive leg of the coil - comes in on the orange wire that connects to terminal 52. Terminal 52 is then internally connected to pin 3. And, the only time the coil is energized is when the blower speed switch is in the high position. Maybe the high speed contacts in your speed select switch are so carboned up, that they are not allowing enough power on the orange wire to enable the coil to pull in (and make contact between pins 2 & 1).
Here are a couple of wiring diagrams marked up to show current flow for both the high speed and non-high speed operation. Because all your reduced speeds/voltages are working okay, the text for non- high speed operation doesn't get into how the variable voltage generated. It simply says that the output from the resistors is variable, comes in on the dark blue lead (which feeds pin 5) and is is regulated by the resistor stack.
Non- high speed operation:
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High speed:
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