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Old 02-23-2005, 09:43 PM
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Join Date: Feb 2005
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300sd heater trouble

i;m new to the forum guys...so thanks in advance for any comments..the heater on my recently purchased 1984 300 sd is not producing any heat from the heater...blower motor blows but no heat....replaced the mono valve and still no heat....any suggestions......this vehicle is definitly different from any i owned.......i got a 1987 nissan pickup with 526,000 miles on it....original engine.....thanks guys

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Old 02-23-2005, 11:08 PM
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Join Date: May 2002
Location: Blue Point, NY
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Welcome to the forum.

Couple of things to look for:

1) Is the coolant filled all the way up? Sometimes these systems get some air in them and it takes some driving to get it all the way out. Make sure the coolant is right up to the "fill cold" line.

2) Do you get any heat at highway speeds? If you get heat at high rpm's but not at low speeds, this indicates a problem with the auxiliary water pump. The pump is sitting near the monovalve, between the two firewalls. It's a couple of hours to get it out of there and see if it is functional.

3) The monovalve should be fully open if you disconnect the electrical connection. Try this and drive the vehicle and see if the heat returns. If it does, the problem may be with the CCU in the middle of the dash.

4) Sometimes, getting heat is as simple as dialing up the dial to a higher number. They sometimes are not accurate enough to provide heat when you set them to 20°C.

5) You can also try and push the defrost button (all the way to the left). This sends full heat to the glass. See if this provides anything. You are trying to determine if you have hot water in the heater core and it's just not coming out the vents for due to a control problem.

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