Wiring problem, Inop heater blower

[Bounty__Hunter]

Chasing down a wiring gremlin in my '89 MJ 2.5L, trying to diagnose an inoperable heater blower. I've had the blower motor out and it works fine when 12v is applied. Here's my ref material: http://www.bteventures.com//mj1988electricalmanual.pdf

I tested the positive lead to the blower at the blower and it showed around .7v and I tested the ground at the same location and it seemed fine. I jumped straight to the fuse panel beneath the dash and tested the blower feed. .7v at this location as well. I tested the two other items on the same circuit and the radio feed tested at .7v and the turn/BU feed tested fine at 12.5v.

Any ideas as to why I'd have 2 different voltages between the 3 items on the same circuit?

I'm not very experienced diagnosing electrical problems so all help would be greatly appreciated.

 

[82CJ7JEEP]

I am no pro, but have a question or two:
1. With the heater/blower activated, have you tested on DC volts between the blower/heater ground and the positive voltage wire to the unit? If you have +12VDC at the lead then your ground is good and voltage is getting to the unit. If not you either have a ground issue, or a bad relay/wire/connection at/ near the heater/blower.

 

[Bounty__Hunter]

I tested the hot wire to the blower with a known good ground & had a miniscule amt of voltage, which disappears when the key or blower switch is turned off.. So no power to the blower. The blower ground Ohm'd out fine.

The lack of voltage at the blower fuse troubles me the most.

 

[82CJ7JEEP]

I am not good at schematics, however, is there a fusible link inline somewhere that may be blown. Also, I had to jump power from an available/spare fuse in my fuseblock for my radio. The factory slot/position would not give me power. You have already determined the blower is good, that would lead me to the fuse block or possibly the switch for the blower motor.

 

[SteelHeadz]

It seems to me that the symptom is a partial short to ground. I know you probbably know by now that i'm half Asian but my eyes couldn't slant small enough to look at the schematic diagram very well. I thought i saw a splice from the ignition switch to the fuse panel. I think it say's Brown on the wiring color from the ignition switch and Violet/ Yellow from the fuse panel. What's the voltage at the ignition feed and what's the voltage after the fuse panel?

 

[Bounty__Hunter]

The brown feed from ignition is 12.5v, and 2 of the 3 fuses fed by that circuit tested about .7v, while a 3rd tested 12.5v.

That document should open in a pdf reader, and if you look it should have a zoom function. I zoomed to about 125% to read it clearly.

Does anybody know if the fuse block can be taken apart and cleaned/repaired?

 

[MudderChuck]

The brown wire from the ignition switch to the blower fuse in a known trouble spot. The brown wire where it comes out of the ignition switch connector often burns the connector kind of dark brown or black. After the brown wire comes out of the igntion switch connector it looks like it goes to a splice, a likely spot for trouble.
Most often the resistor pack for the blower motor shorts and starts the whole process of the cooked connector, sometimes it is the windshield wiper motor.
I never have had to mess with that splice, though I have had to deal with a partially melted connector at the ignition switch.
EcoMike has a fix for this problem if it is what is causing your grief. I can give you his E-Mail, nice guy, he will likely help. Or you can invite him over to Jeepz.
The path is brown wire from the ignition switch, through a splice to the blower fuse, there it changes to violet/yellow, then through the heater controls, then through the resistor pack, to the blower motor.
Do your windshield wipers work? I'm pretty sure the power to the windshield wiper circuit breaker gets it's juice from the brown wire splice.

 

[SteelHeadz]

If the low voltage is after the ignition circuit that has battery voltage, i would trace those low voltage circuits down to the component connector. With the voltmeter leads attached to the low circuits, inspect the wire splices closely, wiggle and move the harness while monitoring the meter for changes in voltage. Most often, loose or rubbed through wires could be isolated in this technique.

 

[Bounty__Hunter]

Mudderchuck, the brown wire has good voltage to the fuse block, so the blower circuit is getting a good feed from the ignition switch. Anybody know if the fuse block can be disassembled?

 

[MudderChuck]

The out wire from the fuse block for the blower circuit, is Yellow/violet ? Yellow in most Jeep circuits means current from the ignition switch (switched current) violet is the sub system (blower circuit).
I usually divide and conquer type circuit troubleshooting.
Start at one end or the other, then the middle.
I'd strip back a little insulation on the yellow/violet wire after it leaves the fuse block and check for voltage. If I found voltage there it is likely the problem is farther down line and not the fuse block connections. Next I'd go to the blower controls.
Taking the fuse block off is a whole can of worms, I'm always afraid I'll do more damage than good. I'm pretty careful when i work down there, stuff gets brittle with age.
I've always had auto trans, so the old problem of the clutch cylinder leaking down the firewall and the brake fluid eating up the fuse block, has never been an issue for me.

That 0.7 volts you are seeing, is usually what is called "standing current" what it usually means is the circuit you are testing has an imperfect ground. You can often isolate and measure a ground circuit for resistance, a little resistance is often common and often doesn't hurt much in 12 volt circuits (it can cause grief in sensor circuits, usually using very low voltage). The 0.7 volts is often from another circuit sharing the same ground and all the voltage from the second circuit not making it through the ground connection.

 

[54cj3b]

should be a resistor hanging above the steering column for the blower

 

[Bounty__Hunter]

This resistor is mounted in the ductwork near the passenger's feet, and only affects the speeds other than hi. Hi is a direct 12v current to the blower and I'm working way upstream from the resistor.

 

[MoparMatt77]

The brown feed from ignition is 12.5v, and 2 of the 3 fuses fed by that circuit tested about .7v, while a 3rd tested 12.5v.

That document should open in a pdf reader, and if you look it should have a zoom function. I zoomed to about 125% to read it clearly.

Does anybody know if the fuse block can be taken apart and cleaned/repaired?

I have had several older XJs that had Elect gremlins. The main fuse block wiring and main headlight wiring junctions have a tendency to get real coroded with the OE electrical grease. I have taken 2 of those junctions apart, cleaned all the terminals with a small detail brush, a file, and some electrical cleaner. Also Jeep, GM (say 83-94 S10??), Painless wiring, or Napa should have those terminals available seperately, Sample may be needed tho.