Yep. Checked and verified...still no high speed. We had to use the medium speed last night while on the way to a carshow...it sucks when it's 102 at 7pm.
Man, this is a real head scratcher. Most of this post is just me thinking out loud. You don't need to read my list of the things you have already checked or the stuff about how a SPDT relay works. You might want to skip right down to the procedure on how to check whether the relay is working.
Alright, let's go over the relay connections one by one.:
1. Power is being supplied from the relay to the blower motor windings (on
pin 1 via the
PPL lead). Also the blower motor windings are properly grounded. These assumptions can be made because the blower operates normally - on all speed settings except HIGH.
2. The necessary power supply required for high speed operation is available into the relay (on
pin 2 via the
RED lead) . You have confirmed this by measuring 12 VDC with a meter . The RED lead is connected straight to the firewall junction block.
3. You have also confirmed - with a meter - that when the blower speed is set to HIGH at the control panel, 12 VDC is applied to the positive side of the coil (at
pin 3 - via the
ORN lead) The ORN lead connects to the terminal labeled
H on the speed switch.
4. You have checked the negative leg of the relay’s operating coil - with a meter - and found that it has a clear, consistent path to ground (via
pin 4 on the
BLK lead).
This last one doesn't have anything to do with the "no high speed" problem - it's just to cover the function of all the relay terminals in my mind.
5. By normal daily operation of the system, you have confirmed that a variable voltage supply - for reduced blower speeds - is available from the resistor stack (at
pin 5 on the
DK BLU lead).
That pretty much covers everything external to the relay. Let's see if it is actually pulling in and closing the high speed contacts. If your hearing is good, you should be able to detect a slight click from within the relay housing each time voltage is applied and removed from the coil. That click is the sound of the contacts closing in each direction. We know the NC contacts are okay - because they are the ones that supply power to the blower motor for all speeds except HIGH.
Single Pole Double Throw Relay operation:
Without power to the coil, the contact arm of the relay is held away from the coil's core by a spring. This particular style of relay (SPDT) has two movable contact buttons attached to the arm – at the end furthest from the pivot. There is one contact on each side of the arm.
There are also two stationary contact buttons located on either side of the arm. They are a few millimeters away and positioned so that they are lined up with each of the movable contacts on the end of the arm.
When power is applied to the coil windings, it’s core acts as an electromagnet and exerts a pull on the contact arm. This causes the arm to move downward towards the coil. But the arm never makes it to the core. Before it does, the movable contact at the tip of the arm hits the stationary contact. This stops the downward motion of the arm and the impact of the contact faces produces a slight “click” sound.
When power is removed, the coil's magnetism is lost and the spring quickly snaps the arm back away from the core. This time the motion of the arm is stopped by the other movable contact hitting the stationary contact - and another click sound is produced.
So, you can check this relay by listening for a click. You should hear one each time power is applied & removed.
Audible check for proper relay operation:
You can do this anyway that is convenient, my method is only a suggestion. But, you want to do this test under real life conditions so don't pull the relay from it's socket.
You will only be checking the operation of the control circuit anyway, so
pull the purple lead from the terminal on the blower motor. The sound from the blower will only make it harder to hear the contacts clicking.
The coil's control circuit has to be made and broken repeatedly in order to produce the clicking sound. This can be accomplished on either the power feed or ground leg.
Since there is really no easy way to remove and apply power to the orange wire - without being in the cab and operating the switch – it’s best to use the ground wire. Plus, if you were in the cab you couldn't hear the relay.
Unbolt the ring terminal at the relay coil’s ground connection on the firewall – the one shared with blower motor:
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Go in the cab and
set the blower speed to HI - turn the
ignition switch to RUN.
At this point, everything is connected as normal - except the blower is disabled. So now, when you
touch/remove the coil’s black lead to/from a good ground, you should be able to hear the clicking sound as the contacts as they close in both directions.
If the coil isn’t moving the contact arm - and making that clicking sound:
1. You got another bad coil
2. There is no power being supplied on the ORN lead. Another possibility is
that there is 12 volts, but the number of broken strands in the copper
conductor is so great that it can’t supply current at a sufficient rate to
operate the coil.
To check whether it is the case that the ORN lead is not allowing sufficient current flow for coil operation, do like gmachinz suggests - run a good
parallel jumper from either the battery positive or the firewall junction block. You can use this parallel conductor method on the coil power lead (red from
FW JB to pin 2) and coil ground lead (BLK from pin 4 to engine block/firewall) as well.
3. There is no path to ground on the BLK lead. Or like the described for the
ORN lead, there is continuity, but the current carrying capacity of the
conductor is insufficient due to broken strands.
