Retrofit W124 Monovalve in W123?
 

What are the benefits and issues related to retrofiting a W124 mono valve in my W123?

Issues (That I see)
1. I believe the brackets that attach to the firewall do not match so there might be some mounting hurdles to overcome.

2. Do the electrical plugs match? I think they do?

3. I don't believe the W124 has an auxillary water pump. But that may not be an issue on a retrofit.

4. Are there be functional problems that are likely to arise?

Benefits (That I see)
1. The monovalve kit/set up for the W124 is more reliable than the W123 system. Specifically the MTC replacement for W123's.

2. Not having to waste money on MTC kits that don't work.

Conclusion: Please give me your opinion as to whether this is worth trying. I'm sure I could find a cheap W124 unit at the junkyard. I am fed up with the quality of the MTC replacement (as I know many of you are as well)

Background: One of my w123 monovalves is currently stuck open (heat overrides AC in summer), it needs a new coil. The other w123 monovalve is stuck closed (heat goes cold on highway in winter). I know why these "issues" are happening and what it takes to correct them. But I would prefer to retrofit something more reliable. Haven't decided to do the manual cock valve (another option) yet. I was hoping to stay with something more OEM.

Electrically you should be fine. Remember that the plugs on 123 and 124 can be opened and the contacts transferred over so you may not have to cut wires.

The operational setup is the same for 123 or 124, apply voltage to shut off the flow.

I would still put an in line fuse on the valve coil and aux pump leads. This is due to a design flaw that has caused 123s to catch on fire if the aux pump seizes or the valve coil shorts.

The 124 does have an aux pump. However on the 124 controller they put internal overcurrent protection on the actuators inside the controller to prevent the fire issue.

I went with the 'Monovalve & Circulator Pump Eliminator' on my '83 W123 and it is perfect. Roasting hot heat when called for and no heat to the heater core when AC is on. No more worries about an electrical fire from a seized coolant pump. My previous symptom was no heat at highway speeds. Coolant pump was working.
Something to consider as the NLA quality monovalve (if you can find one) costs more than the kit.