No Heat at highway speeds, W123
12-23-2018, 05:09 AM
#1
Newbie
Thread Starter
Join Date: Mar 2018
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
1984 300d turbodiesel
No Heat at highway speeds, W123
I have a 1983 300D Turbodiesel. At highway speeds there is no heat from the vents. Around town the heat blows very hot but occasionally disappears altogether. I have replaced the monovalve with the MTC unit because the original one was ripped. I currently have the vacuum lines through the firewall plugged because of door lock leaks. Is there a vacuum component to the heating system? My AC doesn't work either.
The temperature gauge sits just below 80 at operating temperature.
1)Am I overcooling?
2) Is it the climate control Unit?
3) is it the mono valve coil
4) Is the MTC unit defective?
We purchased this car in March of this year and my wife is using it as her daily driver. Driving it is a wonderful experience.
martino
The temperature gauge sits just below 80 at operating temperature.
1)Am I overcooling?
2) Is it the climate control Unit?
3) is it the mono valve coil
4) Is the MTC unit defective?
We purchased this car in March of this year and my wife is using it as her daily driver. Driving it is a wonderful experience.
martino
12-23-2018, 07:58 AM
#2
Member
Join Date: Feb 2013
Location: Beaverdam, VA
Posts: 207
Likes: 0
Received 37 Likes
on
27 Posts
W123 300CD, R107 560SL, W211 E320
Congratulations! You have a great car that just needs a little work. I too have been daily driving my 1983 300CD for 8 years so far. 350k miles and no sign of flagging.
1. You are not overcooling. That is where mine operates.
2. Your problem may be but is probably not the cc unit. If you have temp control it definitely is not.
3. The loss of heat at speed is a typical failure attributable to the MTC monovalve core. Some say it is because the MTC solenoid does not have the power to operate the valve at the higher coolant flow rates. Maybe. The fix is to replace the core again. The MTC units are cheap enough to replace periodically when needed. An OE core costs about $130. I prefer to keep an extra MTC unit on hand.
There is no vacuum component to the AC. Look elsewhere for your problem. I took mine to an AC expert when I found it did not work and had it converted to R134. Works o.k. but in a hot climate may struggle a little. If I had a do-over, I would have not converted it and instead looked for R12 or a synthetic equivalent. I am told that installation of a good cross-flow evaporator in place of the original would result in much better AC performance using the R134. Maybe some day.
For simplicity's sake I have eliminated my vacuum users except for brakes and transmission. This includes doors and the EGR. If you can get away with it emission-testing wise, I strongly recommend eliminating the EGR. It's main function seems to be gunking up your intake manifold. You will also simplify your vacuum system and create better access to engine components.
1. You are not overcooling. That is where mine operates.
2. Your problem may be but is probably not the cc unit. If you have temp control it definitely is not.
3. The loss of heat at speed is a typical failure attributable to the MTC monovalve core. Some say it is because the MTC solenoid does not have the power to operate the valve at the higher coolant flow rates. Maybe. The fix is to replace the core again. The MTC units are cheap enough to replace periodically when needed. An OE core costs about $130. I prefer to keep an extra MTC unit on hand.
There is no vacuum component to the AC. Look elsewhere for your problem. I took mine to an AC expert when I found it did not work and had it converted to R134. Works o.k. but in a hot climate may struggle a little. If I had a do-over, I would have not converted it and instead looked for R12 or a synthetic equivalent. I am told that installation of a good cross-flow evaporator in place of the original would result in much better AC performance using the R134. Maybe some day.
For simplicity's sake I have eliminated my vacuum users except for brakes and transmission. This includes doors and the EGR. If you can get away with it emission-testing wise, I strongly recommend eliminating the EGR. It's main function seems to be gunking up your intake manifold. You will also simplify your vacuum system and create better access to engine components.
12-23-2018, 10:00 AM
#3
Newbie
Thread Starter
Join Date: Mar 2018
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
1984 300d turbodiesel
Thanks Rocky
I replaced the monovalve only this year, if it truly is the problem it's better to replace with factory. I would happily pay 130 for an MB part but I got quoted over 300 when I contacted them here in Toronto Canada. Some of that is down to exchange rate but he admitted they've raised the price of older parts due to scarcity. I'm in upstate New York this holiday is there a dealership near buffalo that would have better pricing on this?
best
martino
best
martino
12-23-2018, 10:14 AM
#4
Member
Join Date: Feb 2013
Location: Beaverdam, VA
Posts: 207
Likes: 0
Received 37 Likes
on
27 Posts
W123 300CD, R107 560SL, W211 E320
The following on-line parts source offers OE and MTC monovalve inserts as well as an insert repair kit. The repair kit provides new rubber parts for the OE insert but the source is URO which has a bad rep.
peachparts.com
peachparts.com
12-23-2018, 11:28 AM
#5
Member
Join Date: Feb 2013
Location: Beaverdam, VA
Posts: 207
Likes: 0
Received 37 Likes
on
27 Posts
W123 300CD, R107 560SL, W211 E320
I have long ago eliminated dealerships for parts for our 35yo cars and instead rely on internet sales. If you buy from a dealer they will have to special order the part anyway and we can do that ourselves far more efficiently. BTW, the peachparts.com source also offers a complete OE monovalve assembly (seldom required) for about $453.00.
That site also has a good tutorial on the difference between OE, OES and OEM. Major differences.
That site also has a good tutorial on the difference between OE, OES and OEM. Major differences.
12-24-2018, 08:55 AM
#6
MBWorld Fanatic!
Join Date: Jan 2016
Location: In the Shadow of the Tetons
Posts: 2,180
Received 660 Likes
on
477 Posts
2013 ML350 Bluetec
Been there. The monovalve literally blows shut when the engine RPM's get up there due to the weak solenoid, as stated above. The valve simply can't stay open given the volume of fluid pushing it closed.
My fix, after trying a couple different tricks (shave the rubber flange of the monovalve seal so it doesn't catch as much water, install a flow restriction to reduce fluid flow through the valve) was to remove the OEM valve assembly altogether and replace it with a 3/4" valve assembly consisting of a 90 degree barb/mpt fitting, a 1/4 turn ball valve, and a mpt/barb adapter. Basically turned the automatic valve into a manual one, and had heat pushing 160 degrees coming from the vents at full tilt even on the coldest of days.
My fix, after trying a couple different tricks (shave the rubber flange of the monovalve seal so it doesn't catch as much water, install a flow restriction to reduce fluid flow through the valve) was to remove the OEM valve assembly altogether and replace it with a 3/4" valve assembly consisting of a 90 degree barb/mpt fitting, a 1/4 turn ball valve, and a mpt/barb adapter. Basically turned the automatic valve into a manual one, and had heat pushing 160 degrees coming from the vents at full tilt even on the coldest of days.
