Updated May 2017

Electrolysis corrosion in vehicles eats away engine and particularly cooling system parts such as water pumps. It is often caused by a broken earthing lead. These are often accidentally disconnected, or broken. If so, return current travels through components such as the radiator cooling system. Also metal around them. This corrodes parts that have dissimilar metals.

This article explains the causes of electrolysis corrosion in vehicles. It shows how to detect it. And how to prevent it happening.

corroded water pump

Corroded water pump. Pic: Hybridz.org

Causes of Electrolysis

Electrolysis corrosion in vehicles was rare until 1970 or so. It began particularly in vehicles that had owner-fitted spotlights. These often used a radiator bracket for earth return. Also, electric radiator fans incorrectly wired. Electrolysis is common if anything electrical is self fitted near the engine/transmission.

Electrolysis is rife in 4WD vehicles with winches. These draw almost starter motor current. They must be earthed to the vehicle chassis. Or directly to battery negative.

Electrolysis corrosion in vehicles lessened in recent years. This is mainly because it’s hard for owners to work on recently made vehicles. Electrolysis corrosion issues are hard to resolve. Few mechanics and electricians know how to locate and fix it. The main causes, however, are poorly fitted earth leads. And also metal bonding straps. Or those being omitted following front end repairs.

Coolant changes

Electron flow through alternative returns corrodes anything metallic on its path to battery negative. The most common damage is to soldered radiator cores. Also anything metallic through which coolant flows. It may also rust iron components. That in turn contaminates coolant. It also blocks cooling and heating systems. 

The risk of electrolysis corrosion is reduced by using high quality coolant. This needs to be at the correct water concentration. That is (usually 50%). The coolant gradually loses it effectiveness. It should be replaced every three years. Check using pH testing strips (replace if under 7.0 pH).

Electrolytic corrosion in vehicles is also caused by using rain water. Much contains acid fallout.

Top quality coolant is essential for today’s engines. They are designed to run hotter than before. Coolants thus have a boiling point higher than plain water. They also contain ingredients that discourage current flow.

Checking for electrolysis corrosion in vehicles

There is no industry-agreed acceptable level of electrolysis. There is, however, usually some. Damage occurs above 0.5 volt for cast iron engines. It occurs above 0.15 volt for alloy engines. Be concerned if it exceeds 0.3 volts and 0.10 volts respectively. The electrolysis level can be checked, and its source specifically pin-pointed via a multi-meter.

Checking electrolysis

Checking electrolysis. It can be done with a special test meter – or with a multimeter on the 0-10 volt dc range. Pic: source unknown.

Here’s how to tell specifically if it is excessive. Also to find and fix corrosion damage (e.g. to radiator cores and/or water pumps).

Switch a multimeter to an approximately 0-1.0 volt range. Connect its negative probe securely to battery negative. Insert the positive probe into the header tank’s coolant. Ensure it only touches the coolant. It must not touch any metal.

With the ignition off you are likely to see 0.5-0.7 volt. This is a surface charge. Leave the probe in the coolant. That voltage should drop back after a couple of minutes. Wait until it settles. If it doesn’t, ensure the radio is not turned on. Or a GPS unit or mobile phone etc connected.

With ignition off, connect a heavy jumper lead to battery negative. Connect the other end to the earth lead of anything electrical that’s been added. Or earth leads that have been removed/replaced. If voltage drops, you’ve located the faulty lead. Fix or replace it. See what the meter reads now. There’s likely to be more than one such faulty lead.

Checking for electrolysis corrosion in vehicles – there’s more

Once that’s fixed, turn the ignition on, and check again. If all is well, check carefully whilst cranking. High voltage here is an engine eater. It’s typically caused by the starter cable negative lead inadequately earthed. It is often also loose and/or corroded. Moreover, there may be tarnished ends of that cable’s connector. If so, fix it urgently.

Next, run the engine briefly. Check (as above) for leakage in charging circuits. If a fault voltage shows up then look for it as above. Check the alternator earthing lead etc.

Then, with the ignition off, turn on everything electrical. Especially spot lights and every ancillary device. Check the meter again. If still excess, locate the cause by progressively switching things off. It is not feasible to test for just one leakage path at a time. There may be several. A single one may be insignificant. Collectively, however, leakage can thus be excessive. (Doing the above requires a well charged battery.)

Checking for electrolysis corrosion in vehicles – earth returns

Negative earth returns must be attached to metal that has a secure electrical path to battery negative. The fault, for example, is often a cable ‘earthed’ via a loose screw. Sometimes a connection is sound, but to unearthed metal. If necessary, extend a heavy earthing cable from chassis to a common power post.

The book strongly recommends using dual core cable for RV wiring. This enables all negative returns to be directly to the battery. Or via a common negative terminal. 

Further information

Installing and fault finding wiring etc in RVs is too big a topic for article form. All you need to know, however, is in Caravan & Motorhome Electrics. Auto electricians use it as a working guide. It’s cost will be more than recovered by getting it right first time.

Our other books are the Caravan & Motorhome Bookand the Camper Trailer Book. Solar is covered in Solar That Really Works (for RVs) and Solar Success (for home and property systems).