A/C and heater fans not working, but fuse is good

[Maherpmj]

As the title says...neither fan blows, but the fuse is good. Both fans blow when jumped to 12v battery source so it's not the fans. I tested the fuse box at the heater/ac terminal and it gets 12v on a multimeter which is strange. Even stranger is that the a/c fan and clutch works if I connect the A/C power wire to the 12v batt terminal on the fuse box. This is really confusing as it won't run if connected to the a/c terminal even though it's putting out 12v. I know there are resistors for each fan, but I thought they control fan speed and if bad then the fan will only blow high or not at all. Any ideas or suggestions would be appreciated.

[serehill]

Well

Have you tried putting the voltmeter on the connection while the wires are all connected & the switch turned on to see if you have voltage then?

[Maherpmj]

Just tried it with the ac power lead connected and the voltage goes to zero. It returns to 12v when the wire is disconnected. Any thoughts? If it was shorting then wouldn't it be the same when connected to the batt terminal?

[JeepJeepster]

Prime example of a bad connection! Gives good volts but cant carry current.

Trace backwards from the fuse box till you find the bad connection. Maybe check where the two halves of the fuse panel go through the fire wall (Im assuming a 77 has that?).

Personally I ran a new 12awg wire from a new fuse box I installed on the passenger side fender well all the way to under the dash. By passed the two halve fuse block and installed a relay under the dash for the fans. Ignition turns on the relay and provides power to the fans and the emissions crap under the hood (fused with a 5 amp fuse). It may be made for it but I dont like having all those amps pass through the ignition switch.

[serehill]

yeah No relays on these models.

One variable I've seen is the rivet where the fuse clip ties on to the bus bar get's loose & will provide a residual voltage when there I no load & when check as you did under load it will disappear. A solution for this would ne a 30 amp circuit from the battery side of the starter solenoid to the fan circuits Via a relay controlled by the ignition switch. The ignition switch controls the relay & turns the blowers off Via the key the way it used to work. put a 30 amp circuit breaker on the wire out by the relay for protection of the circuit. A great way to solve this problem if that is what you find is the issue. I didn't just guess that's what it was. the symptoms are common. Removing the blowers from the fuseblock is good engineering.

[Maherpmj]

The relay is a great suggestion and was what I was planning if I couldn't figure out the fuse box issue. It likely just as easy for me to bypass the fuse box via the relay w/30 amp fuse than messing around with a 40 year old fuse box. Thanks

[serehill]

The relay is a great suggestion and was what I was planning if I couldn't figure out the fuse box issue. It likely just as easy for me to bypass the fuse box via the relay w/30 amp fuse than messing around with a 40 year old fuse box. Thanks

Best of luck PM me if I can help.

The relay will relieve the fuse block & let it relax. The circuit can fatigue from the heavy load.

[darylb]

Check your ground first. A loose ground will allow voltage at low current but open the circuit when a higher current is passed through. I think these are direct ground through the body, so check your ground from the battery to the body, the chassis ground doesn't always make it to the body due to rubber vibration mounts. I once saw an old Ford where someone had put rubber gaskets under all of the accessories thinking it would cut the noises and rattles. It also cut the ground circuit so the guy who owned the truck ended up pulling all of the accessories off to remove the gaskets.

Same is true of the positive lead, check for loose connections back to the fuse panel.

Is this just the blower fans or do you have any other electrical issues?

[serehill]

Yeah but

If he connects the + wire up to the motor & they run negatives are fine.

[Maherpmj]

Agreed. If it didn't run, the ground would have been the next thing to look at, then the fans themselves. Still I appreciate the help. I'm thinking its something loose at or near the fuse box.

[wookiee]

For future searchers.

Check the $10 'Heater Mode Relay'. Located behind 4wd switch. About an inch x inch cube.

It's what switches the wiring when you select Heater or A/C.