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portopa · 01-16-2025, 08:54 PM

Intermittent Low Coolant light on dash, despite the level being fine in the reservoir. I have a replacement sensor on order, but it occured to me maybe there is just a bad ground wire going to the sensor, and the sensor is fine. This system is the nylon reservoir with the sensor penetrating low on the side.

I pulled the 2-wire connector and assumed one or the other wire would ohm out to chassis ground, but neither did. Both read about .2DVC, fluctuating.

Anyone know for sure whether one of these wires should be going to a ground? I am starting to think maybe there is a module at the other end instead.

Ref 2 pics:

Thanks!

jacwjames · 01-17-2025, 07:49 AM

You need to look at a wiring diagram and see if it shows voltage to the sensor, is there a fuse???

On my rig my sensor is one wire and it actually requires coolant to make a ground. I have a Kysor low coolant module which requires a ground. If the ground is broken (low coolant) the alarm/light comes on.

portopa · 01-17-2025, 04:53 PM

Thanks, Jim.

Interesting idea on the fuse. I checked, and apparetly no fuse.

I'll try to dig thru all the wiring PDFs and see if I find anything. If not, I'll at least report back whether swapping the sensor fixes it -- it should be in in a few days.

Paul

Argosy · 01-20-2025, 06:32 AM

I'd bet it still works the same as Jim J described, just a wire to ground because of the plastic reservoir.

Check for voltage on the plug and the other terminal for ground. If you have both the sender is bad.

jacwjames · 01-20-2025, 10:45 AM

I have the Kysor low coolant alarm which does rely on the coolant to ground to. One wire sensor, no power on it.

Newer ones have a two wire plug.

Yours may be different.

If you can confirm via a wiring diagram if it has a power supply that will help.

scaledquail · 01-29-2025, 08:11 AM

On my Sig we replaced the plastic sensor with a brass one, don't recall which. The plastic ones are prone to cracking, allowing coolant to travel onto the wiring. This will result in a fault and if bad enough it can get to ECM causing major issues.

portopa · 01-30-2025, 06:46 PM

Update. Replaced the coolant sensor, but the issue remains. Can be OK for 2-3 min after engine start, then ECM shows coolant level go from 100% to 0% and throws the 111 Low Coolant fault. I have both a Scan-gauge and a Bluefire module. To be clear, coolant level is fine and temp is fine.

I found Maxxforce DIAGNOSTIC/TROUBLESHOOTING MANUAL EGES-455 2010 that indicates +5v should be present at the connector. I think I read 0VDC last week, but I need to go check again. Will double check for bad connector and spray it with contact cleaner.

I strongly suspect a wiring issue and need to try to check the ~8ft+ wire run from the rear of the coach to the side of the engine where the ECM is. Pretty far away, so not too worried that coolant has gone to the ECM.

portopa · 03-22-2025, 09:31 PM

Multiple sensors, and the issue remained. It seems maybe a tiny amount of coolant was somehow reaching the connector pins and causing the fault. Tried new connector; same issue. Original sensor was 7.5uF capacitance when dipped in coolant, so I used a 10 uF capacitor to loop the 2 wires back to the ECM, and issue is gone. This will require daily coolant checking, monitoring engine temperature, and watching for an engine fault light if temperature gets too high. Ultimate fix might be replacing the plastic coolant reservoir.

Note the voltage from the ECM to the sensor was a mere .7v -- just a sensing voltage meant to measure the capacitance of the immersed sensor. Per the manuals I found, I expected 5VDC. In addition, simply disconnecting the sensor, or looping the wires back to the ECM without a capacitor did not work. That is contrary to the maintenance manual, which says the ECM looks only for in-range faults, not shorts or opens in the wiring. In other words, either disconnecting or shorting the wires should have inhibited SPN 111 Low Coolant, but neither did.

d23haynes57 · 03-22-2025, 10:12 PM

The discoloration of that tank should be concerning. It is getting overheated. When the waring light comes on are checking the coolant level without stopping the engine? The system could be air bound or filled with combustion gasses. With the engine running the pump can be pulling the coolant down into the air space. When you stop the engine, the system will burp and refill the expansion tank. One of the reasons that there is the low coolant alarm is that if you should lose all the coolant or it stops flowing, the temp gauge will not work, and you will not see the over temp until after you make steam or melt parts.

jacwjames · 03-23-2025, 07:37 AM

My only suggestion would be to join the Monacoers.org forum, lots of sharp people on the forum. There are 3-4 electrical types.

I found a 2007 electrical diagram but could not find a low coolant sensor listed and/or a lamp warning light for it.

Different then my coach.

portopa · 03-26-2025, 12:17 PM

Dennis made good and valid points about the risks of fooling the ECM with the capacitor as I did. So the capacitor is not a good long term solution, but it does keep the engine from derating, which in my case made a long trip to a repair facility likely to have ended up with me on the side of the road or crawling.

I did confirm my reservoir does not empty when the engine runs, nor refill due to a burp after engine stop.