Heater not working after radiator switch, engine coolant temperature gauge also not working

Tiny
MICHAEL EMULATE KAI
  • MEMBER
  • 1996 DODGE DAKOTA
  • 3.9L
  • V6
  • 2WD
  • AUTOMATIC
  • 218,000 MILES
I found a crack in my OEM radiator at 200,000 Miles. So I swapped the radiator this morning. No major issues until I went to burp the system and the coolant temperature gauge stopped working and the heater stopped blowing hot. I did a little bit of troubleshooting and replace the coolant temperature switch but it's still not reading anything. My heater was working fine and I don't believe that there's actually a blockage or anything in there. Will try a flush if I need to but want to get the gauge working first to make sure it's not the ECM not sending hot coolant to said heater core. Please advise not sure what to do next other than test the wire itself that connects to said switch. Trying to take this truck across country and need the heater to stay warm until the summer. Thank you, thank you.
Wednesday, January 20th, 2021 AT 8:34 PM

15 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
Hi,

On this truck, either you have an air blockage or the blend air door actuator has failed. Do this: Start the engine, turn the heat on high, and allow the engine to reach operating temperature. Once there, feel both heater core hoses. Both should be hot. If they are, the blend air door actuator is likely the cause. Also, look under the hood and make sure you didn't by accident damage or disconnect any vacuum hoses. The mode actuators function from engine vacuum on some of these. Others are cable driven.

Let me know what you find.

Joe
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Wednesday, January 20th, 2021 AT 8:49 PM
Tiny
MICHAEL EMULATE KAI
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Going to check all of that now. Feel I should note the gauge did move after an hour of driving around. A tiny amount in the picture. There has to be some issue with the wiring between the gauge and the computer?
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Thursday, January 21st, 2021 AT 7:26 AM
Tiny
MICHAEL EMULATE KAI
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Image.
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Thursday, January 21st, 2021 AT 7:27 AM
Tiny
MICHAEL EMULATE KAI
  • MEMBER
  • 23 POSTS
Okay, at temperature the heater core hoses were warm and flowing.

I did replace the coolant temperature switch. I don't have good pressure in the lines though. Any of the radiator lines. Wondering if the thermostat is stuck open? I have a replacement but know getting at the housing is difficult. And I've already blown through way too much coolant I'd rather not have to buy another 3 gallons.
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Thursday, January 21st, 2021 AT 8:13 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
If the hoses are hot, then heat is getting into the vehicle. The problem is likely bad or misadjusted blend air door cable. Take a look at the attached pic. A cable from the controller on the dash runs to that lever on the HVAC box under the dash. Confirm it is moving when you adjust temperature. Also, make sure the shaft is moving on the lever.

Let me know.

Joe
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Thursday, January 21st, 2021 AT 9:12 PM
Tiny
MICHAEL EMULATE KAI
  • MEMBER
  • 23 POSTS
I did some testing on the wires for the temperature gauge problem. The connector to the temperature gauge switch on the passenger side of the radiator hose has almost no power to it when it's connected to a test light it barely shows but not bright like it should. Does this indicate that the gauge itself has gone bad or that the wire has gone bad or rather the connector at the wire in the engine bay? Please advise. Will be checking the blend door as soon as I can get the front panels off I spent most of the good part of the sun today painting the truck bed with bed liner.

I am referring to the yellow and violet/purple wire that I've seen on a couple of forum posts connects directly to the gauge, supposedly. Unsure I have a 96 v6 Magnum. And there's no post for that specifically I found one video set on the temperature gauge for an 88 v6 Magnum but it's a totally different connector.
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Friday, January 22nd, 2021 AT 3:03 PM
Tiny
MICHAEL EMULATE KAI
  • MEMBER
  • 23 POSTS
After a small amount of silicone WD 40 the gauge reads maybe 20 percent better. Still not fully correctly but as though some of the excess resistance was at the connector (extremely dirty).

Still unsure but the heater issue was marginally better when the gauge read at a higher temperature. Are they systems connected? Next week I will be replacing thermostat (I think is stuck open) thermostat housing, and flushing the heater core while I'm in there. Can't hurt hoping that the thermostat being stuck open is part of my heater issue. I had read that it flows coolant too quickly through the heater core. Could be wrong. Could have a blockage, could be the gauge itself. Have to junkyard for a replacement in case.
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Friday, January 22nd, 2021 AT 4:54 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
The coolant has to be hot regardless of the gauge for heat to be created in the vehicle. The gauge and the heater are not dependent on each other.

A stuck thermostat can prevent the engine from reaching operating temp, so that can cause low heat. I would replace it and flush the heater core as well when it's apart. Here are a couple of links you may find helpful:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

https://www.2carpros.com/articles/replace-thermostat

When you flush the cooling system, disconnect the heater core hoses and flush water through there. Backflush as well.

Let me know how things work out for you.

Joe
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Friday, January 22nd, 2021 AT 7:25 PM
Tiny
MICHAEL EMULATE KAI
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Shaft is moving can't locate the blend door actuator. Unclear what need to be removed next.

This is the Console box and it's moving properly.
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Saturday, January 23rd, 2021 AT 9:48 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
Hi,

I attached a portion of your pic and highlighted the arm that will have a cable attached to it. If you follow that cable, it will take you to the blend door lever. That is what we need to confirm is moving.

Joe
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Saturday, January 23rd, 2021 AT 12:56 PM
Tiny
MICHAEL EMULATE KAI
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So it isn't the black cable attached to the arm? I don't see a cable running from that piece in particular. Will I need to remove the box?
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Saturday, January 23rd, 2021 AT 1:04 PM
Tiny
MICHAEL EMULATE KAI
  • MEMBER
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I want to say it is working and it's a core issue. I get good heat when the fan isn't on. I have a chemical flush now and will do the full system flush and a core flush ASAP. Hopefully that fixes the heat issue.

Still confused on the gauge.
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Saturday, January 23rd, 2021 AT 1:07 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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That makes perfect sense. Let me know if it helps or if you have other questions.

Take care,
Joe
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Saturday, January 23rd, 2021 AT 1:44 PM
Tiny
MICHAEL EMULATE KAI
  • MEMBER
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Okay, final update for me.

Drained, and used a chemical flush. Then drained again and back. Flushed the heater core. Not dirty but good to know.

In replacing the thermostat and housing I was right in that the thermostat was locked open. Thus causing the cascade of issues.

New gasket, new thermostat and a mistake of a spill on my part later and it's filled burped and everything works again.

Heater is easily twice as hot as before. The temperature gauge works again properly.

Worth doing the checks and appreciate the help but regular maintenance is regular for a reason.
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Monday, January 25th, 2021 AT 4:04 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,101 POSTS
Hi,

I'm glad to hear it's fixed. Honestly, when you told me both heater hoses were hot (before the repair), I suspected it was the blend air door. However, they were just not getting hot enough.

The 3.9 is a good engine. Not too MPG friendly, but durable if maintained. I've had many over the years. If you end up with a timing chain noise, make sure you install guides/tensioner (aftermarket) which it didn't come with from the factory. It's worth the few extra dollars.

Please feel free to come back anytime in the future if you have questions. You're always welcome.

Take care,
Joe
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Monday, January 25th, 2021 AT 6:35 PM

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