W123 Monovalve disassembly and troubleshooting - Page 2 - PeachParts Mercedes-Benz Forum

funola · #16 11-11-2016, 07:17 PM

Quote:

Originally Posted by Demothen

I can subjectively tell you that yes, it's weaker - though since I deformed the housing opening it up, the weight doesn't slide smoothly in the housing anymore.

You do not need the housing to test the spring. Drill 2 holes in a larger washer and put it on top of the small brass washer above the spring and hang the bag of water from the drilled washer.

Demothen · #17 11-11-2016, 08:30 PM

Good call. I will try that.

No I have been driving the car every day this week and by the time it has cooled down enough to work on, I havent wanted to go work on it. Tomorrow I am getting a new windshield, and either later tomorrw or friday I will do some more diagnostics and swap to the core that I triee to get any rust or calcium out of.

funola · #18 11-12-2016, 10:43 AM

Have you checked the monovalve control voltage characteristic? Here's a video https://www.youtube.com/watch?v=74fyla3aG6U of what it should look like. I have an LED connected across the monovalve coil, out the hood in front of windshield.

LED on = monovalve plunger extended= no coolant flow.

sequence:

1. temp dial @ max cool (detent)= LED constant on = no coolant flow through heater core.

2.temp dial @ max hot (detent) = LED constant off = full coolant flow through heater core

3. temp dial @ 26 C = LED cycling on/off= partial coolant flow through heater core. note the longer on time than off time

4. temp dial @ 20C = LED cycling on/off. note the longer on time compared to 26C = less coolant flow through heater core

5. temp dial @ 18C= LED cycling on off. LED on time is much longer and appears to be constant on. If I let the camera run, which I didn't in order to make the video shorter and less boring, you'd see an off blip at around a minute or 2 further (to let a little coolant through the heater core)

Demothen · #19 11-12-2016, 11:31 AM

Not yet. That will be part of my diagnostics when I get time to work on it. Right now I am waiting on my new windshield- which is a muuuuch needed improvement

Demothen · #20 11-12-2016, 01:36 PM

So they found a small rust hole under my weatherstripping. Will need to be patched before they can install the glass. Car is stuck at the glass place for now.

Demothen · #21 11-16-2016, 04:20 PM

I haven't had much time to work on the car this week, and it's not drivable until I get the windshield frame patched and the glass reinstalled.

Last night I did get an hour or so to mess with the monovalve with the help of a friend.

The first thing was to pull the unit out of the car again. I had installed the spacer in the wrong spot - which means that the solenoid wasn't able to extend as far before sealing. I'm 90% sure this was the problem. When I did some earlier testing using a food scale and trying to measure activation force I realized that the extension force varies substantially over the range of extension (which makes sense, as the steel core is moving within the magnetic field). I suspect that it was not extending with enough force to hold back water pressure at high temp, so it was getting forced shut.

The second thing was to remove all of the rubber parts from the core, then heat it up to see if it started to bind. We got it to well above boiling, and were still able to actuate both cores smoothly. We could have used boiling water, but it was easier to use a heatgun.

Next, we reassembled the unit - this time using the core that had been soaked in rust release agents as well as some vinegar. We made sure to have the spacers in the correct spot, then installed it in the car.

Finally, we rigged up some wires and a light bulb to test the electronic control logic. I had similar results as your video and tests, so I'm pretty confident that electronically speaking the system is working as intended.

I wasn't able to run the car long enough to get it up to full temperature, since it's not drivable. Hopefully I'll get the car into the bodyshop soon, then the glass reinstalled, for final testing. I didn't have time last night to get into trying to test spring strength, but both myself and my friend agreed that the Bosch unit is noticeably stronger than the MTC unit in that regard.

I will need to find another MTC unit at some point, as I destroyed the screen on my good diaphragm unit removing it. I don't think that screen will dramatically impact the functionality of the part.

Demothen · #22 11-17-2016, 10:19 AM

Somehow it just clicked that the difference between the car at lower temperatures and full temperature is that the thermostat should be opening near full temperature. I'd bet that's what's changing the pressure against the monovalve and forcing it open.

funola · #23 11-17-2016, 10:20 AM

Quote:

Originally Posted by Demothen

............. I had installed the spacer in the wrong spot - which means that the solenoid wasn't able to extend as far before sealing. I'm 90% sure this was the problem. ........

Disclaimer: not nit picking, just trying to understand what you did.

Where did you install the spacer or did you leave it out?

Demothen · #24 11-17-2016, 10:29 AM

I had it behind the plunger assembly for some stupid reason. Now I have it between the plunger and the snap ring, like in your picture. That means I gained the thickness of the spacer in opening stroke before the plunger makes contact with the plastic housing and forms a seal.

funola · #25 11-17-2016, 10:59 AM

Quote:

Originally Posted by Demothen

I had it behind the plunger assembly for some stupid reason. Now I have it between the plunger and the snap ring, like in your picture. That means I gained the thickness of the spacer in opening stroke before the plunger makes contact with the plastic housing and forms a seal.

I don't know what having the spacer there would have done. I do not yet fully understand what goes on inside the core/ behind the diaphragm. In addition to the rubber plunger and diaphragm, there is a 3rd rubber seal on the shaft under the diaphragm. I do not see it on your cut open MTC core shaft. Did you remove it or does the MTC unit not have this 3rd seal?

You can see this 3rd seal, a small tapered rubber plug on the shaft in this video https://youtu.be/AUKvBorGYxg as the shaft is pulled out. From what I can see, it seals the bushing opening in plunger/screen/diaphragm assembly when the solenoid is energized (extended).

Demothen · #26 11-17-2016, 11:24 AM

I had that third seal also, it was just removed from the core I took apart.
I will try to get some pictures using my spare unit to better explain it. I can visualize stuff easily but have a hard time explaining it.

funola · #27 11-17-2016, 12:36 PM

Quote:

Originally Posted by Demothen

Somehow it just clicked that the difference between the car at lower temperatures and full temperature is that the thermostat should be opening near full temperature. I'd bet that's what's changing the pressure against the monovalve and forcing it open.

I do not know what coolant flow pressure from the water pump and the aux water pump generates but I don't think it is very high, my guess max 10 psi. Is that enough to force the monovalve open or closed if all the passages are not blocked? If you look at this coolant diagram, flow through the heater core/ monovalve circuit does not matter whether the thermostat is open or closed. There is always flow through the heater core unless the monovalve is stopping flow by having power applied to it.

Below is the only diagram I can find (2nd version starting Feb 1979), but the monovalve, item 12 is in the wrong place. It should be on the output side of the heater core. The aux pump is also not shown in the diagram.

funola · #28 11-17-2016, 01:28 PM

Quote:

Originally Posted by Demothen

.......... I can visualize stuff easily but have a hard time explaining it.

To help visualize, I made this poorly drawn diagram. Can you check it for accuracy? Maybe make a better drawing using CAD?

Demothen · #29 11-17-2016, 04:28 PM

I certainly can't prove that the thermostat open/close pressure is different, it's just the best explanation for the behavior that I'm seeing that I can think of.

Nice drawing! I'll try to show you tonight what the difference I saw was- Essentially I had the spacer (not labeled in your drawing) between Rubber seal 1 and rubber seal 2. Which means that rubber seal 3 was pushed further down into the seating position.

It's funny you mention CAD stuff, I've been looking around at laptops with digitizers built in to do some CAD structural designs and some 3d modeling for a non-MB automotive project I've been thinking about. Will probably wait until the black Friday sales to buy anything, if I decide to. My home PC is converted to an HTPC - which makes CAD work miserable.

Demothen · #30 11-18-2016, 11:30 AM

Funola,
Here's some pictures of my spare monovalve in both configurations (incorrectly assembled and correctly assembled) with some measurements taken. This is all spare parts (thus the torn diaphragm - and i'm out of snap rings). I'll explain the pictures a bit later, short on time right now but I wanted to get them uploaded.