Repeat heater core failures and electrolysis

By MACS Technical Thinktank

Repeat heater core failures are often thought of as a Ford or General Motors problem, but the best information we’ve seen indicates that the problem is cyclical among all makes. We do hear more about Ford repeat failures, we suspect, because of an old Ford service bulletin that mentioned the use of extra grounds to the engine and heater core as an “if all else fails” possibility for high voltage in the coolant. However, there were no grounds on the heater cores to start with, so installing one would not qualify as an “extra.”

At any rate, the recommendation (for a ground from the heater core inlet tube to the body) seems to have been a mistake that just got into print. Five years later, a Ford bulletin on basically the same subject was blunt, “Do Not Ground Heater Core. If the heater core is grounded, you have provided the electrolysis a path through the heater core. This would cause the heater core to become an anode or receiver, and it would promote the electrolysis, or any stray voltage, to use the coolant as the ground path.” So that should end the debate, although technicians have long memories.

In most cases, leakage occurs at the heater core tank or close to it, from the pulsation of a high coolant flow rate. Some heater cores have been designed with inlet plates that distribute the flow to minimize the problem. But the usual fix is a flow restrictor in the inlet hose, or perhaps the outlet neck on the engine. If the issue is identified at the factory, a flow restrictor will be installed in production, or released afterward with a service bulletin. And even if one wasn’t originally used or offered, you can obtain a restrictor and install it in cases of repeat heater core failures.

When a heater core leaks after short life, and the leakage is from pinholes in the core itself, well yes, it could be electrolysis. Coolant laboratory operator Ed Eaton, however, has said repeatedly that it’s a rare occurrence, possibly caused by the use of different metals in the cooling system. But if it’s an OE system, that issue was covered in engineering, he has added.

As a vehicle-in-operation issue, excess voltage in the coolant may occur when a ground is lost, or if the coolant has become corrosive. The rule of thumb is that electrolysis is possible with a voltage reading of 0.4 or higher, taken with the voltmeter negative lead grounded to the battery and the positive lead suspended in coolant (preferably in the radiator or an engine fill point), with the engine running at fast idle. This also presumes you have an accurate voltmeter (more on that subject later in this manual) and a car whose cooling system has a fill on the radiator or engine.

Will a good flushing eliminate the problem? It might, but before you start, check for the more likely possibility that the cause is a poor ground somewhere in the electrical system. Make a visual/feel check of the physical grounds on the battery, engine, and body. If they’re okay, but the voltage reading is 0.4 or higher, at least Ford recommends flushing the cooling system.

A cooling system flush sounds like a simple proposition, but we have good reason to believe that few technicians are doing it right. In most cases, it takes at least three drain-and-refills (with a cool-down and engine warm-up between each) to get out 90% of the coolant. Some drain kits that use a venturi vacuum device may handle the air pockets problem very well, but can’t compare with a good drain-and-fill machine with a pressure back-flush feature. If the system has a number of restrictions from debris, only a machine with an effective back-flush procedure will do the job, and you may have to do it several times.


If this doesn’t cure the problem, Ford’s recommended procedure to test for an electrical system issue is in the following steps – with the voltmeter positive lead suspended in the coolant in all these cases. Yes, it would be preferable if you could put the probe into an area of active coolant flow, such as through a radiator fill. But with many cars equipped with pressurized reservoirs and no other access, that’s where you have to go:

  1. Turn the ignition on (but don’t start the engine), then, with all accessories off, with the voltmeter negative lead, check the grounds for the battery, powertrain (engine or transmission, or even both) and body in sequence. If the voltage reading is higher than 0.4 on any of the grounds, it’s bad, even if it looks good. Some vehicles have body to powertrain grounds in underbody locations, so don’t stop with what’s on top.
  2. If all these grounds test okay, check the accessories. Although not specifically listed in the Ford procedure, we’d suggest you consider the possibility of accessories that are always on, such as antitheft systems, particularly aftermarket types, and those that take a long time to power down, such as airbags, electronic suspension, etc. Ford recommends that you check the accessories with the switches in “Off,” using a jumper wire from the accessory to ground. If the engine has a block heater, be sure to test with that plugged in.
  3. Recheck the battery, powertrain, and body grounds with the engine cranking.
  4. Run the engine at fast idle (about 2000 rpm) and recheck the battery, powertrain and body grounds with all accessories turned on (including rarely-used ones. Some old, luxury cars had cell phones, which might just be sitting unused in a console compartment, for example). If the voltmeter reading becomes high (0.4 or higher), turn off the accessories one at a time and see if the voltage drops. Remember, you must check all three grounds with each accessory, and you must keep the positive voltmeter probe suspended in the coolant. The reading is unlikely to drop to zero, but should clearly drop below 0.4 volt. The accessory with the drop has a bad ground.
  5. The radiator electric fans should cycle on and off during your engine-running tests, and you should separately check voltage with all accessories off except the fans.
  6. Switch the voltmeter to AC and see if AC voltage is 0.4 or higher. If it is, operate each accessory with an electric motor and see if the voltage drops with the accessory turned off. This includes the blower motor, an electric Automatic Temperature Control system, and the radiator electric fans. Some engine block heaters also may generate AC voltage, so connect and disconnect this accessory.

Allow some time between each voltage test, as the voltage may drop below 0.4 somewhat slowly. If this occurs, the ground may not be poor, but the wiring may be overloaded. Change to a thicker (numerically lower gauge) ground wire. This problem occurs most commonly when an add-on accessory uses an OE ground wire, or if an OE ground is just deteriorated.

Organic acid technology antifreezes may produce slightly higher voltage readings than other types, but don’t switch antifreezes in an attempt to reduce voltage.

When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.

If you’re a service professional and not a MACS member yet, you should be, click here for more information.

You can E-mail us at or visit to find a Mobile Air Conditioning Society member repair shop in your area. Visit to find out more about your car’s mobile A/C and engine cooling system.

The 32nd annual Mobile Air Conditioning Society (MACS) Worldwide Convention and Trade Show will take place January 18-20, 2012 at the Rio All Suite Hotel and Casino in Las Vegas, NV.

About macsworldwide

Mobile Air Conditioning Society (MACS) Worldwide Founded in 1981, MACS is the leading non-profit trade association for the mobile air conditioning, heating and engine cooling system segment of the automotive aftermarket. Since 1991, MACS has assisted more than 600,000 technicians to comply with the 1990 U.S. EPA Clean Air Act requirements for certification in refrigerant recovery and recycling to protect the environment. The Mobile Air Conditioning Society (MACS) Worldwide’s mission is clear and focused--as the recognized global authority on mobile air conditioning and heat transfer industry issues.
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2 Responses to Repeat heater core failures and electrolysis

  1. Pingback: Griffin Radiator in Gen2 Blazer - S-10 Forum

  2. Pingback: 96 eldorado heater core bursted... again..

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