Fan causing misfires

[KenHall1702]

My 88 B2 will misfire around 3000 rpm when I turn on the fan. AC compressor is NOT engaged, just the blower fan.

I don't know what the connection is, but it is very reproducible.

 

[Uncle Gump]

Is the charging system working properly?

The only connection is that both the ignition and fan are both connected to the battery.

 

[RonD]

Only on HIGH or any speed for Blower?

 

[KenHall1702]

Charging system is working - the new alternator puts out 14 v and the battery does hold a charge.
With regard to the fan blower, it is any speed. New tidbit, I get the same effect when I turn on the lights, and it is worse when I do both. So current draw seems to be the common point but I don't know where to look.

So I just did a visual on all the grounding points (this included removing them and steel brushing the area then reattaching), and I measured their resistance back to the negative terminal of the battery and every on is less than one ohm.

 

[Uncle Gump]

Weird...

Does the charging system put out 14v with blower or lighting load at the rpm you have ignition problems?

Maybe RonD has more to add?

 

[RonD]

Blower gets 12volts directly, key on
Blower motor speed is controlled by it's ground thru a resistor block, HIGH bypasses resistor block, direct ground
Ground is in the cab

Headlights are grounded on the Rad Support, and get 12v from the main light switch which has 12v all the time

So there really is nothing in common, between these two and the spark system

Starter Relay on the inner fender should have one post with several wires attached AND the Battery positive cable
This post is the 12v distribution system for the whole vehicle
Disconnect either battery cable at the battry
Remove and clean ALL the wires and battery cable on that one post, and clean the post, lol

 

[4x4prepper]

Pull the Dash Illumination fuse and see if it still happens, usually 5 amp, but, I do not have a repair book in front of me to cite which fuse.