Muckin_Slusher
****
A while ago, I came across an excellent and informative post regarding block heaters. This is copy pasta that I had saved. Not my post, but worth reposting.
If the original poster of this is on this board, thanks and you're welcome to claim credit.
Enjoy:
The heater is just a resistor wire inside a silicone sleeve inside a steel (hopefully stainless) tube. The silicone prevents the wire from touching the tube, and the tube keeps the coolant inside the cooling system - not against the wire. So if resistance is climbing slowly (as in: months, as opposed to just going to infinity/open-circuit), that means something dangerous is happening. Either the wire is corroding inside the silicone, or a connector is corroding, or the power cord is damaged. THAT's why it needs to be repaired/replaced; the fact that it doesn't heat the engine enough is secondary.
To test the cord, clean all the terminals (should be 6 for a single heater, plus 3 for each additional heater). Then test resistance from the service hot to the hot terminal at EACH heater's connector. It should be <5 Ohms (short/continuous/connected). Repeat for the neutral to each heater neutral, and ground to each heater ground. If it fails, repair or replace the cord. Once the cord passes, reconnect to all the (cleaned & tested) heaters, and re-test from the beginning. You should get a pass - if not, you missed something.
Next, use a special 110V test outlet that you've built with a common light switch in the ground circuit. Build it so you have access to the switch terminals with your meter probes; and with the service ground connected ONLY to 1 switch terminal, but NOT to the switch ground, or to the outlet box ground. The OTHER switch terminal should be connected to the outlets' ground & the box. When the switch is ON, the outlets are grounded normally (assuming you've plugged this setup into a grounded building outlet, which you should check before using it; do NOT attempt to use it with a GFCI since that will shut power off when you try to test the heater.) It's important to have a PLASTIC (insulating) face plate on the box so you don't touch any metal when you operate the switch. It would be a good idea to use a neon-illuminated switch, with the bulb wired across the switch terminals. Check your meter by setting to ACV and holding the probes together while watching the display. It should immediately drop to 0. If it shows anything else, this is your meter's "0" reading, and it should be SUBTRACTED from all future readings.
With the test outlet plugged into grounded service power and the switch OFF, measure AC voltage from one switch terminal to the other. It should be 0. Turn the switch ON (safe/grounded), plug in the truck's heater power cord, and turn the switch OFF. If the light comes on, the ENTIRE TRUCK and the switch box are HOT (110VAC), so flip the switch ON and unplug the switch box from service power. If the light is NOT on, re-check voltage across the switch terminals. It should be <3 VAC (ideally 0 as before). If it's higher OR if the light came on, there is voltage leaking from the heater through the silicone (due to coolant corrosion eating through the tube and soaking through the silicone) onto the tube/block/chassis, and therefore onto all the truck's electronics (even cell-phones that are charging from the truck while the block heaters are plugged in). If there are several heaters, disconnect ALL BUT ONE and repeat the test until you find the one that's leaking, and replace it.
This is NOT a test that can be conducted using ONLY a meter; the fault it detects is ONLY detectable with full service voltage applied, due to the high resistance of coolant. DO NOT begin with this test because it's the most dangerous, and if a defective heater can be identified & replaced BEFORE doing this, they should all pass this test with little or no risk to you or anyone else. Starting with this test COULD expose you or others to a mild shock (not life-threatening, except maybe to pacemakers). In any case, it only simulates a REAL-WORLD condition, and it's safer to test it in the shop than to find out that a truck driver got zapped.
If the original poster of this is on this board, thanks and you're welcome to claim credit.
Enjoy:
That tests MOST of the system, but not for leaks. And if you get a "fail" reading, you can't tell just from that if it was the heater, the cord, or a connection.
The heater is just a resistor wire inside a silicone sleeve inside a steel (hopefully stainless) tube. The silicone prevents the wire from touching the tube, and the tube keeps the coolant inside the cooling system - not against the wire. So if resistance is climbing slowly (as in: months, as opposed to just going to infinity/open-circuit), that means something dangerous is happening. Either the wire is corroding inside the silicone, or a connector is corroding, or the power cord is damaged. THAT's why it needs to be repaired/replaced; the fact that it doesn't heat the engine enough is secondary.
Begin with the truck & heater exactly as they would be if a driver was about to plug in; installed normally with coolant in the engine. First: clean all three prongs to shiny metal. Using a DMM set to Ohms, measure from the hot (smaller flat prong) to ground; it should be >10K (open circuit/not connected). Then from hot to neutral (the wider flat prong); the correct resistance must be calculated using this formula: R(esistance in Ohms)= V(olts specified by the heater mfr)² / W(atts, total of ALL heaters wired to that plug). If it passes, this was quicker, cheaper, and more-accurate than waiting for the heater to burn enough electricity to actually heat the block enough for a thermometer to detect. If it's substantially different, there's a problem. Disconnect each heater at the block, clean all the terminals, and retest each one using that same forumula. If it fails, the heater is bad. If all heaters pass, test the power cord (explained later). When it's all working, apply electrical grease (NOT dielectric, or chassis, or thermal) to all the clean terminals before they start to corrode again.
To test the cord, clean all the terminals (should be 6 for a single heater, plus 3 for each additional heater). Then test resistance from the service hot to the hot terminal at EACH heater's connector. It should be <5 Ohms (short/continuous/connected). Repeat for the neutral to each heater neutral, and ground to each heater ground. If it fails, repair or replace the cord. Once the cord passes, reconnect to all the (cleaned & tested) heaters, and re-test from the beginning. You should get a pass - if not, you missed something.
Next, use a special 110V test outlet that you've built with a common light switch in the ground circuit. Build it so you have access to the switch terminals with your meter probes; and with the service ground connected ONLY to 1 switch terminal, but NOT to the switch ground, or to the outlet box ground. The OTHER switch terminal should be connected to the outlets' ground & the box. When the switch is ON, the outlets are grounded normally (assuming you've plugged this setup into a grounded building outlet, which you should check before using it; do NOT attempt to use it with a GFCI since that will shut power off when you try to test the heater.) It's important to have a PLASTIC (insulating) face plate on the box so you don't touch any metal when you operate the switch. It would be a good idea to use a neon-illuminated switch, with the bulb wired across the switch terminals. Check your meter by setting to ACV and holding the probes together while watching the display. It should immediately drop to 0. If it shows anything else, this is your meter's "0" reading, and it should be SUBTRACTED from all future readings.
With the test outlet plugged into grounded service power and the switch OFF, measure AC voltage from one switch terminal to the other. It should be 0. Turn the switch ON (safe/grounded), plug in the truck's heater power cord, and turn the switch OFF. If the light comes on, the ENTIRE TRUCK and the switch box are HOT (110VAC), so flip the switch ON and unplug the switch box from service power. If the light is NOT on, re-check voltage across the switch terminals. It should be <3 VAC (ideally 0 as before). If it's higher OR if the light came on, there is voltage leaking from the heater through the silicone (due to coolant corrosion eating through the tube and soaking through the silicone) onto the tube/block/chassis, and therefore onto all the truck's electronics (even cell-phones that are charging from the truck while the block heaters are plugged in). If there are several heaters, disconnect ALL BUT ONE and repeat the test until you find the one that's leaking, and replace it.
This is NOT a test that can be conducted using ONLY a meter; the fault it detects is ONLY detectable with full service voltage applied, due to the high resistance of coolant. DO NOT begin with this test because it's the most dangerous, and if a defective heater can be identified & replaced BEFORE doing this, they should all pass this test with little or no risk to you or anyone else. Starting with this test COULD expose you or others to a mild shock (not life-threatening, except maybe to pacemakers). In any case, it only simulates a REAL-WORLD condition, and it's safer to test it in the shop than to find out that a truck driver got zapped.