Hey guys,

So, one of the tests for coolant condition is using a multimeter. Stick one electrode in the coolant and another to a ground or negative battery terminal, and anything over 0.2-3V means the coolant has dissolved ions and is now an electrolyte, corroding the engine/radiator. This is one of the tests I do on my coolant every summer to monitor its condition.

Today however, I made a mistake. I didn’t use the negative terminal, but the positive battery terminal instead. I am baffled at how I got a reading of over 11 volts. Testing with the correct negative ground yielded about 0.02V. I thought about this all day and can’t understand it. If the coolant is not conducting any significant current, and is basically acting as an insulator, how can I possibly be getting 11 volts when I touch one of the multimeter probes to the positive battery terminal? What’s completing the circuit? Where is the voltage potential coming from? If the coolant is not conducting, I should get close to zero volts no matter what I use as my ground right? I need someone smarter than me to tell me what the hell is going on please and thank you!

 

measure the ohms resistance of the coolant, it's likely not a great insulator and is electrically conductive enough to ground via the engine block (guessing here). did you measure the coolant in the rad?

regardless I don't think this is a useful method of evaluating coolant, just change it per the maintenance minder and you will be more than ok

 

Sure, it’s not going to be a perfect insulator. Grounding to a negative terminal does give a small, but not zero, voltage reading. I also understand you cannot rely on this test alone to assess coolant health. It’s just one of the things you can do to give you a hint of whether something is wrong. It’s like doing UOA.

I suppose my question is just academic. I just want to understand the physics/chemistry of how I can get 11 volts connecting to a positive terminal and 0.02V connecting to a negative.

 

I'm not sure if I understand your question properly, but here is my explanation of what you are saying. I believe the confusion is based on the reference point used for the voltage measurement.

Let's say your battery has a voltage of exactly 12 volts between the positive and negative posts.

If you connect the negative lead of the voltmeter to the negative battery post and the positive lead to the coolant, you might find that the coolant is 2 volts positive RELATIVE to the negative post REFERENCE point.

If you moved the positive lead from the coolant, to the positive battery post, you would read a voltage of 12 volts positive RELATIVE to the negative post REFERENCE point.

That would make the positive battery post 10 volts more positive than the coolant, using the negative post as the reference point.

When you put the meter leads between the coolant and positive battery post you will read that same 10 volt difference.

Note that since you put the negative meter lead on the positive battery post, and the positive lead in the coolant, you would read -10 volts on the meter, where you had read +2 volts from negative post to coolant. The difference between the +2 volts, and the -10 volts, is equal to the battery voltage of 12 volts.

 

Just replace coolant if you are 50K miles or 6 years

 

I haven't hit those milestones yet, so for now I'm monitoring the coolant with a few tests annually.

 

Hi Rick, thanks for taking the time to reply! I think your explanation makes sense. I guess my point of confusion is whether coolant is an electrolyte at all. Suppose you pour in some fresh coolant, should there be a voltage reading between the coolant and negative ground at all? If the coolant (in new and ideal condition) is not supposed to conduct any current whatsoever, then the voltage should be negative whether you connect to the positive or negative end of the battery right? In all the videos that show testing the coolant in this manner, it's sort of implied that you shouldn't see any current at all and that good coolant shouldn't let any current through. Maybe what they are really trying to say is that a small current is actually normal (just not high enough to corrode), and in that case, your explanation would make logical sense.

 

My earlier explanation for why you read a higher voltage when connected to the positive terminal had nothing to do with testing coolant, but rather just understanding reference points when making basic voltage measurements with a multimeter.

To be honest I had never heard of this technique until I read your post, so I did some research.

It seems that coolant has additives to prevent corrosion of the various metals within an engines cooling system. These metals include steel, aluminum, and copper. When the additives become depleted, corrosion begins, and the metal particles mix with the coolant, and the coolant becomes an electrolyte. The galvanic reaction between the dissimilar metals creates a voltage.

This is the effect that occurs in a potato battery. By placing a copper and zinc strip into a potato, it will produce enough voltage to operate a digital clock.

The following is a video that explains the process in simple terms


 

Hi Rick,
Yup, I get these principles, and that's exactly why I'm confused. Suppose you have fresh coolant in there and it's not acting as an electrolyte yet. The additives are doing their job and no corrosion is happening. When you use a multimeter and put one end in the coolant and connect the other end to the negative ground, *ideally*, you should get zero voltage right? Or at least, that seems to be what's being implied. If that's the case, why does connecting to the positive battery terminal give you a voltage. If no electrons are able to flow through a non-electrolyte coolant, it shouldn't matter what you connect it to. There shouldn't be a completed circuit.

So I think the issue here is that it's never going to be a perfect insulator. Even when you have fresh coolant in there, it's going to act like an electrolyte to a very minor degree. So when you connect the coolant to a negative ground, you will get a tiny voltage, a few mV. It's not zero. So when I mistakenly connected the multimeter to the positive battery terminal, I got a voltage. This seems like the most reasonable explanation to me so far. I guess I was just surprised at how high of a voltage I got when I connected to the positive terminal. If healthy coolant is only a very very mild electrolyte, I would have expected the voltage from the positive ground to be just as low as the voltage from the negative ground; a few mV.

 

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​​​​​​Okay, I think understand your question now, and this is a possible explanation I came up with. At least it's my best guess.

Try this experiment (don't worry you won't get electrocuted)

1). Connect the negative meter lead to the positive battery post. (same as you did when testing the coolant)
2) Squeeze the positive meter lead between the fingers of one hand, and then touch the negative battery post with a finger from the other hand.
3) Despite your body being a pretty good insulator, you will read 12 volts on the meter, as if you had connected the positive meter lead directly to the negative battery post. Your body completed the circuit.

Chat GPT says that the resistance from one hand to another is 10,000 to 100,000 ohms, and the resistance measured from the filler neck to the negative battery terminal, with new coolant is ~20,000 ohms.

So when you measured from the positive battery terminal to the coolant, the coolant completed the circuit back to the negative terminal the same way your body did in the experiment.

I think you are assuming that any solution that conducts electricity is an electrolyte.

The definition of an electrolyte is.
An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons.

​​​​​​The movement of ions between dissimilar metals is the principle of how a battery works (think about a lead acid battery) and that is why you read a voltage in the coolant as it changed from a simple conductor (a poor one) into an electrolyte.

The 2 volts you measured initially in the coolant is the result of the galvanic effect, and indicates that the metals are corroding, which is an indication that the coolant has lost its anti corrosion ability.

With fresh coolant you should read almost full voltage from positive battery to coolant, but that voltage differential will decrease as it is offset by the voltage being created in the coolant.

 

Thanks Rick, I think that makes sense and we are thinking along the same lines. Coolant is not a perfect insulator (that was my incorrect initial assumption). Even though it might not be a great electrolyte (when fresh) and it has pretty high resistance, it's not perfect and so you can still a bit of a voltage when coolant is grounded to the negative terminal. But I guess in these youtube videos, what they are saying is that as long as the voltage isn't more than 0.3V, it's not enough to overcome the potential needed to corrode anything.

My coolant is actually okay as far as this test is concerned. I wasn't getting 2V from coolant to the negative ground. It was 0.02V, so 20 millivolts, well below the threshhold these videos mention where I need to worry. It's just one test though. I also keep an eye on the pH and glycol levels, so as one of the other members posted, one can't rely solely on this test to determine coolant health.

Also, I just noticed your location is central Ontario. I'm from Kingston myself. And while on the subject of fluid maintenance, and I realize this is a bit off topic, but have you noticed any fuel dilution for your RDX?

 

Hi,
I am located in Barrie. I have not had any analysis performed by a lab, but since the oil level has never increased, and the oil does not smell like gas, I assume that it is not a problem.

Decades ago I noticed that the oil in my wife's car would become white and creamy in less than a month. I knew this was caused by the very short trips that she made. Five minutes to work in the morning, a trip home for lunch, a trip back to work after lunch, then a trip back home at the end of the day, along with several more 5 minute trips each week to the grocery store.

I insisted that she start taking the scenic route, and do not shut off the engine until it had reached normal temperature for several minutes.

That solved the milky oil issue, and I have been following my own advice since that time. I believe that making sure the engine always reaches normal temperature has helped to prevent fuel dilution as well.

 

Thanks Rick. We also short trip our RDX frequently - my toddler's school is about 2 km away. So this is pretty terrible, especially in winter. The oil doesn't look milky when I change it, but it definitely smells of gas. I have been doing UOAs the last few changes to keep an eye on things. There are actually a number of threads on fuel dilution for our cars, but it doesn't seem to affect all of them, so I was curious what your experience was in our cold climate. Thanks for chiming in.

Thanks again for taking the time to reply. I hope you enjoy the summer.