Stupid, difficult electrical failure solved

[rich weyand]

Here's a strange problem. It took a while to debug this one!

I was sitting in the truck with the engine running on a very cold day this week, waiting in a restaurant parking lot for the missus to join me for supper. I had the heater blower running on LOW till the engine warmed up, then on MED, finally on HI. And then it stopped, like it had been turned off. All three speeds gone.

I waited until it warmed up a bit the next day to look at it. Logic says it has to be something between the fusebox and the blower switch on the dash, or between the blower resistor on the firewall and the blower motor, including the blower motor itself and its connection to ground.

I performed various tests, and determined the following:

- Connecting the blower motor directly to battery positive, the blower motor fires up and runs just fine. So the blower motor and its ground are OK.

- Taking the blower resistor connector off the firewall and injecting battery power into the blower motor contact, the blower runs just fine, so the harness between the blower motor resistor and the blower motor, including the arc-noise-suppression coil on the firewall, is fine.

- I replaced the heater fuse on general principles. The old one looked like it might have been original with the truck.

- Pulling the instrument bezel and measuring the voltage at the blower motor switch, I had 0V, 13V, and 14.5V respectively with the ignition off, the ignition on, and the ignition on with the motor running.

- Jumpering the connector positions on the blower motor switch connector (LOW wire to MED wire, LOW wire to HI wire) made no difference. No blower function.

WTF! That is all of the elements common to all three speeds.

It's very uncommon, but it could be a wire failure. I measured the resistance of each wire segment, and found that the wire from the blower resistor connector to the arc-noise-suppression coil was broken inside the insulation about an inch back from the blower resistor connector, where it made the bend from vertical to horizontal to go into the connector.

I had tested this in the second step above, but when I took the connector off and twisted it around 180 degrees to jumper battery power into it, the break reconnected. When I twisted it back around to plug it back onto the blower resistor, it disconnected again. I found that when I bent it a certain way, it would work, and when I bent it back, it would disconnect.

My surmise now is that when I got into the truck and it was cold, it was working OK, and as the truck warmed up, and the radiator electric fan started cycling while I was idling in the parking lot, the temperature change, plus the current through it when I switched to high, allowed the wire to move enough to disconnect, or blew out the final strand of wire remaining on a progressive failure.

I took another look at the connector between the blower motor resistor and the arc-noise-suppression coil. This is a weatherproof connector maybe 8 inches straight up from the resistor connector, and a couple inches from the coil. This weatherproof connector had the "bell" of the female connector pointed straight up.

So while it would shed most water, whatever water collected at the seam of the connector could work its way down into the connector, and into the insulation of the wire, and collect in the bottom of the wire, where it turned the corner. And over decades it rotted the conductor inside the insulation.

How's that for a tough one to find! Note that it is failure by design, and is an incipient failure whenever you have a weatherproof connector oriented to catch water instead of shed it.

Anyway, I put in a new wire. And I redid the connection with a normal crimp female connector so there is a heat shrink on the male part of the connector that hangs down over the female to shed water instead of collect it.

I also took the opportunity of having the dash open to replace the existing, broken and marginally functional heater control assembly with a repro version from a mail-order supplier (plug and play, looks identical to the original).

The first photo is the fixed wiring. I am pointing to the place where the wire broke internally. The second photo is the broken wire. It looks short because it is tipped toward the camera so you can see the bell of the female open at the top. This wire is broken internally at the 90 degree corner at the bottom.

[tucsonjwt]

I had a similar problem with the connector at the heater control switch. Wiggle the connector and the heater control worked. I replaced the connector/harness and all is good.

[J Knight]

I had a similar problem also. I would turn on the key, blower would turn on then when I would switch to hi it would cut off. After digging around I found it was the connection from the fuse to the clip holding the fuse.

[rich weyand]

Yeah, the post was just a reminder to people not to assume that all the wires are good.

Update: While I had the dash apart, I replaced the heater control assembly, lubed all the cables, lubed the pivots on the three air doors, cleaned up the clearance issue on the defroster air door, fixed the light for the heater controls, sealed up the 1/2" gap between the heater box and the defroster duct with metal tape, and replaced the (broken) shift indicator cable. All heater controls are silky smooth now, I have full speed from the blower fan, and I have a whole lotta defrost air.

I think that defroster duct gap is very common on these trucks. It may even have been designed in there for some reason. You should check. It's at the top of the heater box just above the transmission hump. The forward side is pretty much up against the firewall insulation, but it's pretty easy to seal the other three sides with some of that aluminum tape.

Also, if you have a skreetch when going into or out of defrost, the air door is scraping on the sheet metal forward side of the heater box. That sheet metal can be persuaded to be a little more forward and better clearance the door by pushing it forward (toward the firewall) with a long blade screwdriver and pounding on the handle with the heel of your hand. To get at it, remove the little air deflector that aims air to your feet. There is one hex screw installed vertically on the passenger side flange of that little deflector. Give the sheet metal forward plate of the heater box a hit every inch or so along the bottom edge until the defrost door has clearance.

Truck is all back together now.