Hello, looking for some thoughts regarding my heat in an 85 300TD.

Since I replaced the disintegrated and mostly missing foam tube to the sensor behind the glove box, my temperature control has worked flawlessly. I set the temperature wheel, put the fan speed on automatic, and then only change where I want the air to blow.

It has worked flawlessly except for when I started to inspect/replace the monovalve and one of the pins broke off in the plug to the solenoid. With one wire connected, I had nothing but full heat. I did not replace the valve because the screen looked pretty good and it was stuck fairly solid in the solenoid. I managed to solder some copper wire inside the broken pin, put the plug back on, and left it alone. Worked flawlessly again for about 2 years, until I smacked it with something replacing the trap oxidizer with Rollguys pipe about a month ago. This time both pins were broken. I stuffed a few short folded pieces of wire in the hollow pins in the solenoid and plug, and again the temperature control was perfect. . . until about a week ago when connection failed and I had full on heat again.

In the mean time, expecting a connection problem, I managed to find a working solenoid with the plug intact (I'm not about to try to pull it off. I've seen two other used ones with broken pins) and put the new valve I never used in it. The valve popped open and shut when the wires were touched to the battery terminals.

It worked only so-so driving around town for two days, but today on the freeway for an hour and a half at a time each way, the heat would barely come on, or slowly diminished to almost nothing by the end of each trip. Each way, I had heat when the car started cold. And when I got back to town at the end of the day, I started to get some heat. After stopping for burritos, when it started again, I had heat until it was up to heat, and then no heat again.

Since I know everything else was working with the old solenoid and valve until four days ago, and the new solenoid and valve work out of the car, it seemed like the valve must be sticking closed (no heat) unless somehow twisting the cut wires together could alter something, or two days of the temperature control box trying to control temp. with broken connections to the solenoid could have fried something.

I just read a thread on this forum where someone said he had bought a monovalve kit where the valve stuck. Has anyone had this problem, if that's what my problem is, and is there a way to keep the valve from sticking? Trim the rubber on the valve or clean the plastic pipe? A little synthetic grease? I could buy another valve kit, but they are probably all from the same manufacturer. Any other thoughts about other reasons I don't have any heat after the initial warm up?

Any help would be greatly appreciated. I'm in Western Washington, and it looks like it won't stop raining this year until June.

It also looks like a weak spring in the new valve kits (I bought mine several years ago and don't remember who made it) could cause high coolant flow at high rpm's to suck the valve closed to the heater. I did not have heat at about 70, 3100 rpm.

Fixable with the addition of an o-ring:
http://www.peachparts.com/shopforum/diesel-discussion/381589-mtc-monovalve-no-heat-issue-solution.html

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

I have all of TDHAwks symptoms (intermittent and random heat) and after reading these two threads i wonder if it is possible that the bore for the plunger might be the actual fault; is this cleanable?

i cant check since my car is buried in the barn behind two feet of snow still...

From what I've read, I don't think you can take apart the replacement kit valve without destroying it. It doesn't matter how loose or tight it is in the solenoid. I checked the valve I replaced. The diaphragm isn't ripped. Tonight I'll clean the screen and reinstall. I checked the source of the new one I bought 3 or 4 years ago. All it carries is MTC.

No need to take apart the replacement kit valve.

As shown in post 4 of the thread (below), the o-ring is placed on top of the diaphragm to create a thicker and more pliable rubber seal.
http://www.peachparts.com/shopforum/3647769-post4.html

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Problem solved. I may have been a bit quick to seek advice, but I was little impatient after 4 hours on the freeway with no heat Monday, knowing I have to make the trip again tomorrow. Lots of info on mono valve problems of all sorts on this forum. Before I fixed the problem, driving around town I determined the MTC repair kit would shut off the heat at anything over an idle. Thus, I figure the only way the o ring solution would help me is if it picks up the valve enough that it can't ever close all the way. Since my temperature control unit works great, I wanted a fully functioning valve.

I put the old valve with the slightly rusty screen back in, and it works perfect again. With both of them out, I could not discern any difference by hand in the spring strength. Neither did there seem to be much difference in side to side play with both extended. The difference I did notice is that the diaphragm on the MTC is just plain rubber like material, that was unsymmetrically bunched up with the valve retracted. The problem with the valve was obviously that it would not stay retracted at more than an idle. The old, and probably Bosch (or Mercedes) valve had a diaphragm that appeared to be more like a rubberized fabric of some sort. In the retraced position, it neatly folds up toward the top of the cylinder, like a partially open or closed umbrella. Maybe this configuration "catches" the increased flow of coolant like a parachute, helping hold the valve up and open until the solenoid applies the current to extend and close it. In any event, another thumbs down for MTC, and the old valve worked turning on and off the heat at 75 on the freeway. As fast as I go.

Thanks for the input.

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85 300TD 359K Daily driver 2,500 miles a month for the last 3 years