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Hamilton Motor Company Climate Control Servo

Started by sutekh, 18 September 2022, 01:43 AM


As most with late production w116s have faced, I needed a reliable automatic climate control solution vs. the deeply flawed, Chrysler-designed ACCII servo & amplifier. The OE servo installed in my car when I acquired it (likely replaced at some point in the vehicle's life) was only intermittently operative, and was showing signs of coolant leakage around the heater valve. What to do?

1. Replace the servo with a remanufactured, aluminum-bodied OE / aftermarket hybrid? There's an authenticity argument to be made here, but the servo's flaws run deeper than just cracked valve body plastic. Pass for me.

2. Unwired Tools' modern microcontroller / solenoid driven retrofit? For years, these were final word in effecting a permanent solution to this tedious w116 shortcoming. I'd have bought one of these but for...

3. Hamilton Motor Company's relatively recently introduced unit that's very similar to UWT's above? After reading through the install docs, Hamilton's unit appears to be a lower-cost, functionally identical alternative to the UWT part. After getting swift and competent responses to my pre-sales questions, I took the plunge, and thus began my saga...

I knew going in that I wasn't going to be satisfied with Hamilton's packaging. Their website show's a sturdy aluminum housing, however it isn't sealed. It also isn't what they're currently shipping. The latest revision is installed in a plastic project box instead with a section of the lid cut away to reveal the electrical connectors (as well as the bare, un-potted or coated PCB.

This was among the questions I asked them prior to purchasing, however they claim they've never had problems with environmental issues, but that some users choose to install the unit behind the glovebox for this reason. Hmm... That'd probably work on an r107, but not the w116. I wanted something that a) wasn't going to short out the first time I steam clean the engine room, or b) require zip ties (or worse, drilling holes) to mount. As luck would have it, I'd just purchased some oil-resistant PETG filament for my 3D printer!

I retained Hamilton's project box body, as the PCB was clearly designed for its geometry and standoff holes, but discarded the lid. In its place, I designed, printed, and installed a replacement lid that seals completely and mounts to the holes in the OE servo's angled bracket. Since this factory electrical connectors are no longer plugging directly into the board, that required some wiring harness pigtails and custom connectors with the correct pitch. I also routed the vacuum lines out using some threaded 1/8in 90deg bulkhead connectors.


I wish that was the end of it, but remember when I used the word "saga"? After install, the unit worked intermittently and unpredictably at best. I pulled it apart a few times. Checked and re-checked everything multiple times. I abandoned my modifications and connected it directly. All to no avail.

I went back and forth with Hamilton's support team, and I frankly think they thought I was crazy. That it must be something specific to my car, despite my lengthy missives outlining my troubleshooting process and everything I'd tried. "Who is this nut job and how can we get him to stop calling / emailing us!?" They finally agreed to send me a replacement unit, but it took over a month to ship and their replies to my inquiries became less frequent. Meanwhile, I suffered through the better part of a record-breaking hot summer  >:(

The replacement didn't work either. Same behavior :'( Clearly this must be a problem with my car you're thinking, right? Me to. Maybe the packaging modifications I'd made? Solar flares? I started reverse engineering their board and it's pretty simply really. Finally found the issue and am frustrated at myself that it took me as long as it did to question their part and not my car!

In the pic of their board below, note the circled jumper I soldered in place of a component labelled "D2". Hamilton's engineers implemented a reverse polarity protection diode at this location (a perfectly sensible thing for any self-respecting EE to do) where +12vdc comes in on pin-2 of J1. Trouble is, whomever they outsourced manufacture of their PCB to installed the wrong part in their latest run. Specifically, a grossly underrated for this application BAT41 (rated at 100ma continuous drain in a circuit that draws 3x that!)

I think I trust myself not to switch pos & neg, so I just jumpered it vs. installing something like a 1N4001 to avoid any voltage drop :) Instant resolution. Suddenly the solenoids and relays can draw enough current to operate properly and I get proper vent, heat, and A/C response. Halleluiah!

Given the amount of time invested to that point and with a clear root cause identified, Hamilton did me a solid and refunded me the purchase price and invited me to keep the RMA unit to boot. At the end of the day, their engineers were great to work with and  they didn't really cause this. I imagine they also now have a recall mess on their hands trying to figure out how many of these bad boards they've shipped. If any of you on this forum are potentially on that list, take a long, hard look at D2!


Hey, thanks for documenting your journey with this. I also bought a Hamilton unit and went through the whole rigamarole of the initial unit and replacement. The current one "works" but won't keep pressure on the hot water valve sometimes. Was that one of the issues you faced? Definitely considering jumping the board!
1979 300 SD "Liesel", 282k Miles


Quote from: amilio on 20 September 2022, 09:33 AMThe current one "works" but won't keep pressure on the hot water valve sometimes. Was that one of the issues you faced?

Absolutely yes. Constrained by that incorrect diode, the board is underpowered which means the solenoids can't overcome high vac pressure reliably. Sometimes mine would work, but most often I'd be getting blasted with heat. I setup a test scenario with my MightyVac and a bench power supply, and at anything under ~13.3vdc at the board and more than ~10inHg of vac, the solenoid wouldn't open and close the heater valve.

If you're worried about any warranty implications, you may want to contact Hamilton first. Otherwise though, assuming you're handy with a soldering iron, I'd yank that BAT41 and replace it with a 1N4001.


I installed the UnWired kit earlier this summer. I wish the vacuum line for the main vacuum supply (yellow, I believe) was longer, but notwithstanding that their customer service was great with a few minor questions and the unit seems to work without issue. I am very pleased.

I haven't fully tested the system yet as I need to have my AC system rebuilt, but at least for the time being the heating functions work well and very fast. Thanks for the heads up about Hamilton...
1980 300SD | 684H Pastel Beige | 204 Bamboo


Quote from: CA116 on 22 September 2022, 05:54 PMThanks for the heads up about Hamilton...

The takeaway here shouldn't be "Stay away from Hamilton" IMO. As mentioned, their support team was responsive and they made things right. Mostly I wanted to highlight the packaging improvements I made and promote awareness of a simple fix in case others were similarly afflicted.

If I needed another one, I'd likely still buy the Hamilton, replace a $0.15 component, and pocket the $$$ delta. Aside from their recent contract manufacturing problem, the unit is otherwise well enough built and designed.


Quote from: sutekh on 22 September 2022, 09:57 PMThe takeaway here shouldn't be "Stay away from Hamilton" IMO.

Fair enough! I'm glad it worked out, and thanks for the detailed description / info.
1980 300SD | 684H Pastel Beige | 204 Bamboo


I should update this thread for posterity, having been messaged a few times about the diode swap I reference above. The 1N4001 swap reduces the overall voltage drop at operating current and helps make operation more reliable, but doesn't solve the problem completely. I continued to be plagued by intermittent opening of the solenoid valves and did some further testing of the parts Hamilton was using. Under vacuum, they didn't release until receiving 12v, leaving almost no margin of error!

I purchased a few potential alternatives for testing, and ultimately settled on this part, which instead opens consistently at ~10v:

Using those, you don't even really need to bother with the diode swap (but I still would). While you're at it, you can choose whether to buy 3 valves if you want the unit to operate like the factory ACCII system (Hamilton only ships with 2 solenoids and combines panel & leg vent operation, but the PCB and microcode has accommodation for the 3rd solenoid). Speaking of Hamilton, after discussing this issue, they've updated their parts manifest and are now using different solenoid valves since sometime last year. Newer units shouldn't have the same issues.


I have been down this road as well  My aftermarket aluminum-bodied unit failed after about 30,000 miles, so going with UWT or Hamilton seemed like a good option.  My mechanic went with UWT.

This hasn't worked out.  UWT Support has tried to help and we've worked this problem for a while, but I only get one speed in each speed setting  UWT documentation says I should have 4 speeds in Auto Hi and in Auto Low.


I reached the bottom of the rabbit hole with Hamilton. Replace the solenoids if you're having issues and it should work great. Mine has been working flawlessly for the past year. DM if you want full details.