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Help diagnosing coolant issue

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87 views 9 replies 7 participants last post by  Anders S SLK02  
S
#1 ·
Slk 320 155000k miles. Replaced passenger side headlight bulb and soon after had low coolant level light. Seems I had disturbed the small hose to the overflow tank and it was loose. Topped off coolant, tightened hose no leaks.

A few weeks after, I had intermittent coolant level lights. These only occurred after driving for at least a half an hour. The light would come on briefly and go off for awhile and this would repeat. It would tend to stay off when at speed. Temperature gauge stays around 90c. Coolant level fine, washer fluid full, no leaks. No error messages. I ran the fan activation with sds and it visibly increased in speed.

Recently I was stuck in a traffic jam on 90f day for 45 minutes to an hour. Temp gauge rose to 120c and the coolant light came on. I was able to park the car and coolant was bubbling around the overflow cap. When it cooled, I topped off the minimal coolant loss and went home when it was about 15 degrees cooler out. Checked the coolant level the next day and no coolant loss. In the past week I have driven mostly short trips of less than a half an hour with no coolant light and no coolant loss. Temp gauge stays at 90c to 100c.

With this mileage it could be several things: Tstat, water pump wear(not leaking) fan controller, fan not running fast enough.

The fact that the coolant light comes on intermittently and the temp rose quite a bit when not moving on a hot day leads me to think towards the fan not moving fast enough but at this mileage the pump could be worn too.

Any thoughts on further diagnostic steps would be appreciated.

Thanks
 
#2 ·
fyi
coolant reservoir and washer reservoir share same wiring
they share the same wiring - different sensor and different warning lights. Because they share the same wiring if one goes faulty (usually the windscreen sensor) if can bring up the other warning light. Of course this doesn't happen if both are working properly



The Coolant Light is supposed to come on if you have a Cooling Fan issue or Low Coolant level, but it can come on if there is some other fault as discussed by @jbanks15 such as Washer Level Sensor, Coolant Level Sensor, the shared Wiring or even an Instrument Cluster issue.
 
L
#4 ·
My 2004 SLK320 exhibited pretty much the same behavior you described. I changed the coolant level sensor, and measured the voltage and ohms as described in the thread where I detailed my troubleshooting steps.


In my case the coolant sensor was not the issue. If I were you I would focus more on the fan than on the coolant and windshield washer sensors, especially since you have no apparent coolant leaks and the car overheats when driving in stop and go traffic.

I would check the following things.

1) Confirm that the fan is spinning when the the air conditioning is turned on.
2) Check F2 50A fuse in the battery pre-fuse box near the battery.
3) Spin the fan by hand to determine whether or not it spins freely. My fan was not. The carriers for the brushes were probably scraping against the armature.
4) Unplug the fan and try running it by attaching a jumper cable to the ground and a fused jumper cable to the +12v side.

I am assuming that you will take precautions like disconnecting the negative battery cable for #3 and #4. Make certain that the jumper cables to the fan (for #4) are secured to the plug with appropriate sized terminals and that jumper wires don't interfere with the spinning fan.

I noticed that your profile says SLK230 and your post references SLK320. I purchased a genuine MB fan (without shroud) for my SLK320. It was over $400, so somewhat pricey compared to aftermarket options. It was actually fairly easy to replace. It helps to have an extra set of hands to push the radiator hoses around when you extract and re-install the fan assembly.
 
S
#5 ·
The fan works I activated it with sds. It allowed me to ramp up the fan speed in actuations. If I understand it correctly the fan controller adjusts the fan speed in response to temperature. It comes on at a certain temperature and the speed increases as the temperature rises. I'm just wondering if the fan is experiencing occasional dropouts or is not ramping up to proper speeds causing the increase in temperature and activating the coolant light.

I had both a 230 and 320 at one time. Now just the 320 and an ML430. Should update my profile
 
#7 ·
I'm also missing the info whether the fan is blowing at all :)

Actuating the fan with SDS is not the same situation as when it gets controlled by the ECU during normal operation.

SDS is telling ECU to actuate the fan diagnostically, regardless what the sensors are reporting, and that seems to work. The question is whether the ECU is controlling the fan correctly while running normally.

The fan control module does not pay attention to temperature. Its job is to process commands from the ECU and from the A/C module. ECU is using a PWM signal to command the speed of the fan. When something is wrong (e.g. coolant temp sensor) then the fan is set to 100%. The fan control module is autonomously driving the fan at 100% when the PWM signal is bogus. Everything is kinda fail-safe.

Switch A/C off, drive until the engine is reaching its normal temp., then stop and let the engine run at idle while monitoring fan and temperature visually. It will take a while until the fan is starting to blow, so be patient.

A side note: the ECU is starting to limit the engine power at 110°C. Have you seen a P2019 DTC in SDS?
 
S
#8 ·
The fan is working. Today ambient temp was 85f after getting to operating temp fan came on and held 100c at idle. I didn't have extended time to see if the temp would exceed that. That seems to be a bit high temp for comfort. Will have to run another scan this weekend to see if anything shows up.
 
A
#10 ·
Some simple fanology:

Fan is supposed to enhance the natural flow thru´ both the AC and the engine coolers. That's why starting the AC should make the cooling fan start - has nothing to do with the engine coolant temp. This possibly verifies that the fan is working by itself and that the AC calls for fan flow, not much else.

Now, the coolant temp is governed mainly by the thermostat, as it calls for cold water from the cooler when the engine gets hot (="opens"). Usually (at speed) there is an abundance of nice cool coolant to get and the cooler accepts the same amount of hot water from the engine. This loop continues as long as the temperature in the cooler stays below a preset temp, thereafter (high temp) the fan helps with creating a airflow thru' the cooler. Soon, the temperature drops, and the fan is needed no more, and turns off. (Unless, ofcourse, the AC still is calling for fan flow to help the condenser to work.

So, if engine temp is rising, it could be the thermostat that doesn't open as it should. It could be the temp sensor in the cooler that doesn´t call for the fan to start, it could be a clogged cooler not cooling as well as it should and it could be a water pump, not pumping at it's best. Let's not delve too deep into clogged engine cooling galleries or wrong spark timing.

Sometimes, cleaning and rinsing the cooler and mixing a nice new coolant fluid is enough. But I would (shade-tree-mechanic style and with no scan to help), make the termostat my "best suspect"...
 
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