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Discussion Starter · #1 ·
Hi all

Long time lurker, first time poster.

Bought my 2001 MDX 2 years ago - 91xxx km on the original engine.

Three days ago I noticed that my temp gauge on the dash was about 3/4. Found this interesting as she has always showed a temperature of under half, no matter the load or the ambient temperature.

Popped the hood - coolant overflow was bone dry. Refilled to between min and max. Started vehicle - ran hot again.

Figured there might be air in the system? Emptied the rad using the petcock and refilled.

Tried burping the system, noticed no fans came on even though the temp gauge on the dash was showing extremely hot.

Touched upper and lower rad hoses - Top hose extremely hot, bottom hose cold. Turned on vehicle and maxxed the heat. Blowing very hot air at idle (assume this rules our the water pump?).

Replaced thermostat. Refilled, began to burp - fans never turned on. Noticed fluid was only swirling minorly. Squeezing top and bottom hoses pushed quite a bit fluid up into funnel i had attached for burping purposes, some air bubbles came out. Turned off vehicle.

Checked overflow reservoir, empty again? could not find any leaks anywhere. When vehicle warms up, pressure builds in the top hose, but does not flow into reservoir ?

Tested fan motor by connecting to battery - fans turned on. (Fan motors not burned out?)
Tested cooling fan relay using the same method - clicking occured. (relays good?)

Thinking it is the Fan Switch that is bad? Maybe Rad cap? but why wont the fans come on if it's the rad cap? Would the fans not working not allow fluid to enter the reservoir but allow it to be sucked out? Cannot find any reference material identifying location.

Any help or questions that I may have omitted, please ask. Really hoping to get some insight into this.
 

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Great job of troubleshooting, and sharing info. Many times, the call for help would consist of "help, my MDX is overheating... what's wrong?". ;-)

I think the key is that the electric fans aren't coming on. Yes, there could potentially be a blockage in the coolant flow, but you've already replaced the thermostat, which would be the problem 90% of the time. Without the fans coming on at all, the car will overheat, no doubt about it (particularly here in Arizona, where it's supposed to hit 120° or higher in a few days!!!).

Here's the diagram that shows how the radiator (and A/C) fan(s) come on...



There are some "other things" that could cause the fan to not run...
Fuses #41, 57 or 76
The air/fuel ratio relay (not quite sure why this is even in line with the fan)
Engine coolant temperature sensor
ECM

I'd check the fuses (since that's the easiest thing to do), then the engine coolant temperature (ECT) sensor. I'm not sure how tough that is... it's kind of obscured by the intake manifold, but you may be able to get to the plug, or worst case, just poke ohmmeter probes through the wires running to it...

 

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Discussion Starter · #3 ·
Hi,

Thanks for the reply!

I am working diligently to troubleshoot this! Appreciate the kind words about giving information that I have gathered so far, I am by no means a mechanic and this is the first issue I have had with the rig.

If you could confirm the following it would help me follow the above steps.

Fuses:

41 is the battery fuse (only fuse that is 120 A) - This fuse is not blown.
57 is the Cooling Fan fuse (30A) - This fuse is not blown.
76 - Not sure where I can locate this fuse. Can't seem to find the under-hood sub fuse box. EDIT: Found a picture on google showing location behind air intake attached to body below windshield, however there is no fuse box in that position on my vehicle?

The ECT sensor is the sensor that is connected to the metal just in front of the top radiator hose, engine side? It looks like it from the diagram. Prior to posting this, I pulled the clip off that thinking it was the fan switch. when I started the vehicle, the temp gauge never moved at all while the vehicle was warmed. This led me to believe that it was not the fan switch. I connected leaders from my multimeter to the sensor while the vehicle was going. The resistance when cold was 1400 ohms. I started the vehicle and kept the leaders on the sensor connectors. As the engine warmed, the resistance dropped. I only did this for 3-5 minutes as I didn;t want to have the vehicle overheat (not getting a reading from the dash temp gauge). The resistance had dropped down to 400 ohms by this point. Not sure if this helps. I managed to replace the thermostat without taking the airbox out lol, just removed the battery and cursed a lot trying to get that back screw.


Questions I have are as follows :
1. Is it absolute that if the vehicle blows hot air then the water pump is functioning ? didn't really see much movement in the radiator fluid when i was trying to burp it.
2. Is this the correct way to test the ECT?
3. If the ECT is functioning, relays work and fuses are not blown, is there such a thing as a fan switch? My research led me to believe that the ECT tells the Fan Switch to tell the Fan Relay to turn on the Fan. Please correct me where I have gone astray.
 

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Do the fans come on when the a/c is turned on? I believe the should come on, at least they always do in mine. If not that would point to a fan issue, either mechanical or electrical.

Sent from my SM-G950U using Tapatalk
 

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Discussion Starter · #5 ·
Fans do not come on when a.c. is turned on.

I just found a check for the ect sensor. When you turn the key in the ignition without starting (cold engine) the temp reads half way.

Left the key in the on position and disconnected the sensor clip. Temp gauge dropped to zero.

Correct me if I'm wrong but doesn't that mean my ect sensor needs replacing?
 

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Fans do not come on when a.c. is turned on.

I just found a check for the ect sensor. When you turn the key in the ignition without starting (cold engine) the temp reads half way.

Left the key in the on position and disconnected the sensor clip. Temp gauge dropped to zero.

Correct me if I'm wrong but doesn't that mean my ect sensor needs replacing?
Doesn't that mean it's good?
 

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Here is what the under-hood fusebox should look like - the heavy connection is to the starter, so it shouldn't be TOO hard to find...



Here's a more detailed diagram of the location of the ECT sensor:



As for your questions...

Yes, if the heater is blowing hot air, there is water moving through the engine, so the pump is doing something.

That's exactly the right way to test the ECT, and I'm pretty sure it's working as it should (I don't know the proper resistance levels, but it sure sounds "right" to me from what you wrote). The problem is that it's reading "hot" and the dash is interpreting the signal correctly, but the computer that turns on the fan(s) apparently isn't.

There is no "fan switch" per se, but the ECM (s small portion of it is shown in the schematic I sent last time) turns on the relays based on the reading from the ECT sensor, A/C mode, etc. I should mention that the A/C condenser fan MOTOR is mis-labeled on that diagram as the "A/C condenser RELAY. Just a typo.

Since the A/C fan isn't coming on with the A/C engaged, it might be easier to troubleshoot that circuit (since both are out, I'm guessing it's a common problem). You should measure 12VDC at #3, and read close to zero ohms to ground at #4. That ground at #4 is supplied by the ECM (let's hope the problem isn't there). The troubleshooting index suggests checking fuse #58 in the under-hood relay box, and #3 in the driver's side under-dash fusebox.

The only other common failure-inducing point might be the ground (labeled G201). It seems to be the same grounding point that's used for the low beam headlights, if that helps. But since you were able to get both fans to run with 12VDC on the hot leads, that's pretty much not a possibility.

I guess the next step might be to try using wire to jump across the contact points on the fan relays (which would eliminate voltage supply or wiring issues between the battery and the fans). If each fan runs with the #1 and #2 connections shorted, you know the problem is on the control side. If you can connect a thin wire to the #4 connection WITH THE RELAY INSTALLED, grounding the wire should turn on the fan for that relay. If that works, you know the problem is in the ECM, most likely.
 

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Discussion Starter · #8 ·
Update:

Replaced the ect sensor. Working better than before.

Comments on replies:
There is no sub fuse box as identified in the previous post on a 2001.

Replaced the ect sensor because all multimeter readings were showing good up to sensor as stated previously.

The Ect sensors resistance is the determining factor of the temp gauge reading.

Ect sensor resistance should be 2400 ohms when cold and between 200 to 400 when warm.
 

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Update:

Replaced the ect sensor. Working better than before.

Comments on replies:
There is no sub fuse box as identified in the previous post on a 2001.

Replaced the ect sensor because all multimeter readings were showing good up to sensor as stated previously.

The Ect sensors resistance is the determining factor of the temp gauge reading.

Ect sensor resistance should be 2400 ohms when cold and between 200 to 400 when warm.
 

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Please and thank you for your insight in advance.
I have the same issue the fans will not come on. I have done everything above and replaced the ECT and they still do not work. Relays are good, direct wire to the fans they work. While i had the front end off I replaced everything from the fans forward, modified a few mounting brackets to have everything line up. Also changed out the thermostat while I had it apart. The heater works the radiator hoses get hot both of them, and the over flow tank drained down from over full to with in the lines. Which i think says everything is working as it should.

I have figured out that if i unhook the ECT sensor the fans will come on but they will not shut off and when you do it the car has a hard time running. For what it is worth.

A little background like Barryvey above i had the low coolant level. My daughter rear ended a car with a trailer hitch and it took out the radiator then she drove home. Yes it was about 15 degrees that night we live in Colorado could be the only reason it still runs. Kids you have to love them. Anyway, i have a 2005 touring edition and in my search for parts I ended up finding a 2004 non touring radiator. The only difference i can see it it is a little thinner and it has a sensor on the bottom left side under the battery. Mine did not have a plug for the sensor or sensor so I figured it does not matter that is is not hooked up.

Any suggestions as to what else to check or replace??

Thanks,
 

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Do the fans come on when you turn on the A/C?

Basically, it's a relatively simple circuit (particularly after I've been spending more time working on my BMW). The ECT sensor drives the PCM, which drives the relays, which drive the fans. If the ECT sensor is doing what it should, the PCM should be tripping those relays - it would be worthwhile to see if they are being energized, but of course doing so requires getting the ECT sensor hot enough.
 
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