B250
Member
- Joined
- Sep 19, 2019
- Posts
- 37
- Reaction score
- 13
- Location
- Rhode Island
- Ram Year
- 1991
- Engine
- 5.2L V8 MFI
OK, so a couple of weeks ago when the weather turned hot and I couldn't take driving without A/C anymore, I decided to get the A/C system on my 1991 B250 working. As this is the old R12 system and the pipes were all rusty, I wasn't really expecting too much from it. A quick check of the low-pressure switch above the receiver-dryer showed continuity (that is, the switch was closed) so there was sufficient refrigerant in the system for it to work. There are a number of switches on the system in order for the compressor clutch to engage. I found an old Chilton's repair manual in the back of the van, but this was far from easy to follow. I ended up simplifying the wiring diagrams into a one page drawing which I have uploaded to this post.
There are two sections to the A/C wiring - an output section from the ECU to the compressor relay, and an input section to the ECU from the various sensors. I don't know why the ECU needs to be involved in the circuit as the A/C system will work by bypassing it altogether. Just connect the Dark Blue/Orange Stripe wire from the A/C relay to the Brown wire going to the thermo switch above the receiver-dryer (cutting/removing the existing wires to/from the ECU first, of course)
Assuming your ECU is working properly and you have refrigerant in the system, the rest of the system is straightforward enough. The order I would tackle the switches is as follows:
- check the A/C relay works OK. Relays wear out after a long time, and this one is a pretty-much universal BOSCH/ISO relay
- the low-pressure switch is (again) pretty-much universal but can be bypassed by connecting the Dark Blue/Orange Stripe and the Dark Blue/White Stripe wires out of the top of it together.
- the control head in the dashboard is also very simple. Pushing in the relevant button simply completes the A/C circuit by grounding it.
- the thermo switch is a little trickier. I couldn't find a direct replacement as it was a discontinued item. Simply connecting the Dark Blue/White Stripe wire to the Brown wire will put it in to an 'On' state which will get the compressor going, but unless it is a very hot day you'll find the evaporator starts to ice up.
I found a universal cycling switch off eBay for less than $10 - the UAC 6490C - which can be used as a replacement for the OEM thermo switch. However, it has two pins instead of the OEM three. The wiring diagram I uploaded shows how to use the third wire (12v supply) with a pull-up resistor to make your own replacement for the OEM. The 6490C switch in theory should also work just by connecting the Dark Blue/White Stripe wire and the Brown wire to the terminals on the switch (if Dodge have used pull-up resistors inside the ECU). The 6490C can also be adjusted to 'fine tune' the switching temperature.
One issue with the 6490C is that it has 24" of thermocouple which is best to coil around in a loop (see picture). It is important not to let the thermocouple (other than the tip inside the evaporator) touch itself or any other metal material as it will not work properly. Luckily the blower box is plastic. You could either feed the thermocouple probe through a hole in the plastic box and mount the switch outside somewhere, or mount it entirely within the box coiling the thermocouple round as shown (this would put it in the air flow from the blower fan, though)
If your compressor is on the fritz I saw some on eBay. I also understand that there is a modern R12 substitute, and some shops still have R12 to refill your system
The last resort would (for me) to upgrade the system to R134. I'm sure there are kits out there that will do the job. But simply replacing the compressor and receiver-dryer with ones off a newer Dodge model will not work - the R134 compressors have different pulleys for a start - and you'd have to flush out all the old R12/Oil before filling it with R134
Hope this helps anyone trying to get their A/C working for summer!
There are two sections to the A/C wiring - an output section from the ECU to the compressor relay, and an input section to the ECU from the various sensors. I don't know why the ECU needs to be involved in the circuit as the A/C system will work by bypassing it altogether. Just connect the Dark Blue/Orange Stripe wire from the A/C relay to the Brown wire going to the thermo switch above the receiver-dryer (cutting/removing the existing wires to/from the ECU first, of course)
Assuming your ECU is working properly and you have refrigerant in the system, the rest of the system is straightforward enough. The order I would tackle the switches is as follows:
- check the A/C relay works OK. Relays wear out after a long time, and this one is a pretty-much universal BOSCH/ISO relay
- the low-pressure switch is (again) pretty-much universal but can be bypassed by connecting the Dark Blue/Orange Stripe and the Dark Blue/White Stripe wires out of the top of it together.
- the control head in the dashboard is also very simple. Pushing in the relevant button simply completes the A/C circuit by grounding it.
- the thermo switch is a little trickier. I couldn't find a direct replacement as it was a discontinued item. Simply connecting the Dark Blue/White Stripe wire to the Brown wire will put it in to an 'On' state which will get the compressor going, but unless it is a very hot day you'll find the evaporator starts to ice up.
I found a universal cycling switch off eBay for less than $10 - the UAC 6490C - which can be used as a replacement for the OEM thermo switch. However, it has two pins instead of the OEM three. The wiring diagram I uploaded shows how to use the third wire (12v supply) with a pull-up resistor to make your own replacement for the OEM. The 6490C switch in theory should also work just by connecting the Dark Blue/White Stripe wire and the Brown wire to the terminals on the switch (if Dodge have used pull-up resistors inside the ECU). The 6490C can also be adjusted to 'fine tune' the switching temperature.
One issue with the 6490C is that it has 24" of thermocouple which is best to coil around in a loop (see picture). It is important not to let the thermocouple (other than the tip inside the evaporator) touch itself or any other metal material as it will not work properly. Luckily the blower box is plastic. You could either feed the thermocouple probe through a hole in the plastic box and mount the switch outside somewhere, or mount it entirely within the box coiling the thermocouple round as shown (this would put it in the air flow from the blower fan, though)
If your compressor is on the fritz I saw some on eBay. I also understand that there is a modern R12 substitute, and some shops still have R12 to refill your system
The last resort would (for me) to upgrade the system to R134. I'm sure there are kits out there that will do the job. But simply replacing the compressor and receiver-dryer with ones off a newer Dodge model will not work - the R134 compressors have different pulleys for a start - and you'd have to flush out all the old R12/Oil before filling it with R134
Hope this helps anyone trying to get their A/C working for summer!
