Heater circuit question

[Vhyle]

I'm diagnosing why my blower motor switch isn't getting any power. Using a test light on the 12v supply wire, I'm getting nothing.

Fuse is good. It's a new fuse. The old one melted away, but I cleaned out the socket and replaced the fuse. Since GEN1 is working, I know the circuit is good.

I wired in a new blower motor switch since the old one was bad, and they're prone to melting so I just got a different one.

On figure F-22 in the -20 TM, on the 12v supply wire (brown wire coming from fuse box to switch), there's a break there. It says 2 BRN 50A, going into 2 BRN 50.

Is that a fusible link? And if so, where is it? I can't seem to find it so far. I haven't pulled the gauges out yet (since the wiring runs behind there), and I probably won't have time today to do that, so I figured I'd ask here first before I go any further. Is there a fusible link in this circuit? I'm not familiar with that part of the schematic, so I came here to ask.

If not a fusible link, what is it?

EDIT: When the truck is running, I turn on the blower motor switch, and the GEN1 light comes on.

 

[scottladdy]

If the Gen1 light is on when you turn on the blower motor switch, based on my limited knowledge, your switch is working. IIRC, this could be a sign that your fuse is not working. Does the same behavior occur when the truck is not running but in the on position? Does the Gen1 light come on when you first turn the key to the run position with the blower switch off? If not, I would start looking at the fuse block again for a bad connection.

Please let us know what you find.

 

[Vhyle]

The fuse is working properly. I checked both ends of the fuse in the block with the test light and there is voltage on both ends.

GEN1 light does not come on when I first turn the key. It only comes on when the blower motor switch is turned on, truck running.

I haven't discounted the fuse block itself needing a good overhaul. That may be what the problem is. I've disconnected it once before but it was briefly. I may just need to completely remove it and go through it with a fine tooth comb.

But my question is the brown voltage supply wire that goes to the blower motor switch - is it supposed to be constant hot? And I'd still like to know what that portion is on the wiring diagram in the TM.

 

[Warthog]

The connection that you are talking about is just a connector. The brown wire comes up from the fusebox the the harness to just above the heater control. Here are a couple of pictures that show the location. See the red arrows without the labels.

The connector is a male/female with a plastic housing

The Heater fuse is only hot when the key is in the run or start position. It is off all other times.


Also you might be intersted in this thread

http://www.steelsoldiers.com/showthread.php?50551-CUCV-Switches-Diodes-Relays

 

[Vhyle]

Awesome Warthog, thanks.

My next question - the supply to the switch is supposed to be constant hot? Because with the truck running, I wasn't getting any voltage to the switch. That's why I wanted to know about that connection I saw in the diagram. If it's supposed to be constant hot, I'll need to pull the gauges and check that connector and see if my voltage is dropping there or not.

 

[Warthog]

You will need to pull the dash bezel to get to the wires. Shouldn't need to remove the instrument cluster

It is only hot with the key on.

 

[Vhyle]

It is only hot with the key on.

In that case, I have a voltage drop somewhere. Test light to the supply wire on the switch shows nothing.

 

[319]

Just had the same issue last week. Same symptoms, Gen 1 light not lit when starting, and lit up when non-functioning heater switch was in on position. Fuse melted in the block, tried to pull it out and the blade snapped off the hot side inside the block. Your fuse may be good but you're not getting 12 volts to the Gen 1 and heater switch wires. You may have to splice the two into the orange wire from the ignition switch, through a new fuse of course.

 

[Vhyle]

Just had the same issue last week. Same symptoms, Gen 1 light not lit when starting, and lit up when non-functioning heater switch was in on position. Fuse melted in the block, tried to pull it out and the blade snapped off the hot side inside the block. Your fuse may be good but you're not getting 12 volts to the Gen 1 and heater switch wires. You may have to splice the two into the orange wire from the ignition switch, through a new fuse of course.

That's what you ended up doing? What size fuse did you use for that circuit?

This will be my last resort. I'm going to continue to diag and hopefully fix it in an OE state. If not, then I'll splice it.

 

[scottladdy]

Heater Circuit Testing/Diagnostic

The fuse is working properly. I checked both ends of the fuse in the block with the test light and there is voltage on both ends.

If you are checking the fuse using the "test ports" open on the back of the case of the fuse, you are only validating that you have a "good enough" connection between the hot side of the fuse box (IIRC, the passenger side connector in the fuse block) and the fuse itself, and that the fuse is not blown. If this is what you are doing to test it, you are not validating that there is a connection sufficient enough to provide the necessary voltage/current coming out of the fuse box itself.

Since your original fuse melted, the fuse block connectors may be distorted and not connecting properly.

GEN1 light does not come on when I first turn the key. It only comes on when the blower motor switch is turned on, truck running.

This is a classic symptom of no power coming out of the fuse block to power the Gen1 exciter circuit and heater blower motor. I would look to problems between the fuse block and the junction between the Gen1 circuit and the heater circuit first, as this is the point in the circuit in common to both ends of the circuit. Another way to validate this is to remove the new fuse. If the symptoms don't change then you know there is a problem between the fuse and the junction.

The wiring after the junction is most likely good, as the Gen1 light will only come on when there is no power to the circuit if the alternator sees a path to ground, which you are providing when you complete the circuit through the blower switch to ground. You can confirm this by checking the Delco-Remy alternator manual I attached.

My guess is your fuse block was damaged when the fuse melted.

You may be able to repair the fuse block, but depending on the damage you may also be left with bypassing it for this circuit, as was suggested by 319.

I would suggest you also look to repair the underlying cause of the fuse melting, which is most likely an over current condition due to failing components, low grade shorts, or dirty/loose connections. For example, NAPA sells a replacement blower motor which is a 10 minute $20 swap, part number 655-1076, which would fix any over current issues you may be having due to an old motor. You should also check the connections between the blower switch and the resistor located on the blower housing on the firewall, making sure they are clean and tight. Check for shorts in the multicolored wire running between the switch and the resistor. Check to make sure the noise filter is not shorted (you can safely bypass this if you want to).

Pictures would help us help you as you proceed.

I hope this information dump helps. Please keep us posted on your progress.

Happiest of New Years to all!

 

[Vhyle]

Thanks for the great reply, Scott.

Here's what I'm working with. Below is the replacement switch I'm using (Dorman on-off-on switch, so I can have two fan speeds). The other pic is obviously the blower motor resistor.

As far as the heater circuit - I've determined I'm just going to re-wire it as a new circuit. According to you guys and the schematics, the ignition wire feeds to the supply wire to the switch. I'm just going to re-wire them with a separate fuse holder, and place the 20A fuse there. I'm pretty certain that the fuse block is damaged, and is causing insufficient current flow. It'll be the easiest way.

Another test I did was measure the resistance on each terminal on the blower motor resistor. Going across all pins in any combination yielded about 19 ohms of resistance. Since they're supposed to vary, I can assume the resistor is also bad. There's no open circuits, but they all measure the exact same resistance.

Finally, I tested the blower motor with a jumper wire from the second battery. The motor jumped to life but in a split second, the wire **** near almost welded itself to the battery. Huge current draw.

SO yeah, that's my next plan of action.

EDIT: Oops, forgot to attach the pics.

 

[319]

My situation was a little different in that the fuse blade broke and was melted on the hot side of the fuse cavity. The following is my temporary repair: I cut the 12 ga orange wire coming from the ignition switch and spliced in a 12 ga fuse holder, and using a spade terminal, reconnected to the "good" side of the fuse cavity. ​Heater and Gen 1 work fine. For the permanent repair, until I locate a new fuse block, I will disconnect the Gen 1 and heater wires from the fuse block and via the 12 ga fuse holder, splice into the orange wire.

 

[Vhyle]

I picked up an in-line ATC fuse holder, so I'll be re-wiring that circuit very soon. Not tonight, though. Too cold for that crap.

 

[scottladdy]

Headlight Circuit Recommendation

Thanks for the great reply, Scott.

Here's what I'm working with. Below is the replacement switch I'm using (Dorman on-off-on switch, so I can have two fan speeds). The other pic is obviously the blower motor resistor.

As far as the heater circuit - I've determined I'm just going to re-wire it as a new circuit. According to you guys and the schematics, the ignition wire feeds to the supply wire to the switch. I'm just going to re-wire them with a separate fuse holder, and place the 20A fuse there. I'm pretty certain that the fuse block is damaged, and is causing insufficient current flow. It'll be the easiest way.

Another test I did was measure the resistance on each terminal on the blower motor resistor. Going across all pins in any combination yielded about 19 ohms of resistance. Since they're supposed to vary, I can assume the resistor is also bad. There's no open circuits, but they all measure the exact same resistance.

Finally, I tested the blower motor with a jumper wire from the second battery. The motor jumped to life but in a split second, the wire **** near almost welded itself to the battery. Huge current draw.

SO yeah, that's my next plan of action.

EDIT: Oops, forgot to attach the pics. View attachment 401722View attachment 401723

Sounds like you are on the right track.

I believe another area to watch for is the headlight circuit. These trucks were wired "old school" where the current feeding the lights ran through the switch on the dash. I recommend installing one of those "upgrade" wiring harnesses that bring this circuit into the modern era. These harnesses feed the headlights from a tap directly off of the front battery (in our unmodified 24v trucks of course). A couple of relays are activated by the "old" wiring, dramatically increasing the current to the headlights and reducing the load on the old wiring. Good versions of these harnesses are "plug n play" for the most part. You can also upgrade to higher current draw headlights (= greater visibility) after doing this mod.

 

[Vhyle]

I'll look into that, thanks.

I'm just gonna go ahead and plan on getting a new blower motor and resistor. All I need is a payday!

 

[scottladdy]

I need that and about 20 more hours added to each week.

 

[MT4222]

I had the same problem. Eventually my heater fuse melted along with part of the fuse block. I bypassed the fuse block for the heater circuit and added in a seperate 20 amp inline fuse.

 

[Vhyle]

^ Exactly my next plan of action. Seems to be another one for the list of common issues on these old beaters.

(I say "beaters" in a loving way, of course!)

 

[MT4222]

^ Exactly my next plan of action. Seems to be another one for the list of common issues on these old beaters.

(I say "beaters" in a loving way, of course!)

Make shure you use the plastic type inline fuse not the glass buss fuses. I found that alot of my m1009's electronic problems Were fixed when i removed all the fuses. Then i sprayed electrical contact cleaner in all of the fuse blocks ports. Then i used dialectric grease on all the fuses when i installed them. If i were you i would buy and install all new high quality fuses.

 

[Vhyle]

Make shure you use the plastic type inline fuse not the glass buss fuses. I found that alot of my m1009's electronic problems Were fixed when i removed all the fuses. Then i sprayed electrical contact cleaner in all of the fuse blocks ports. Then i used dialectric grease on all the fuses when i installed them. If i were you i would buy and install all new high quality fuses.

Yeah I will. I see that cucvelectric.com sells an entire fuse set - the good kind, too.