I watched the thread on the through firewall amp wiring issues. Is the 85 w150 prone to the over heating issues at the gauge? I do have factory air.

 

81 and newer use a shunt type amp gauge which are junk and rarely work. power does not run through them. they completely changed how the trucks were wired in 81. if anything the only thing you really got to worry about is the hater circuit being overloaded. almost need to install a solid state relay to control the blower motor to take stress of the old electrical.

 

solid state relay just for the heater?
should take the workload off the electrical?

 

ya solid state relays are good for stuff with variable speed. the blower motor is one of the larger draws in your truck which is why it is fused at 30amps. the fan pulls about 20amps on its own. when everything is new the wiring and switches are okay with that but after 25+years most of the switches and wires are not so okay with it. i had the heater control switch on my 93 fail 2 or 3 years ago. using a relay take the 20 amps off the interior wiring harness and puts it on the relay and the wiring you add instead. i usually use STANDARD MOTOR PRODUCTS RY330K since they are pretty cheap for what they are. they are usually used on 96-00 caravans for the radiator fans.

 

I never had an 80's dodge truck but can anyone post a pic of the shunt? They use shunt's on industrial battery chargers and if you match the amp rating you could use one of them. This is what they look like.