[HDudrak]

I work on hd trucks for a living and I've been testing block heaters on heavy trucks by measuring resistance across the a/c terminals and checking to see if I'm open from the a/c prongs to ground.

So a 10 ohm block heater @ 110 v ac would be roughly 1200 watts and the wattage would drop according to how much resistance i have at the element and I usually change out a block heater that has over 50 ohms(250 watts) as its imo not enough for a class 8 truck that will see -50 temps.

If I have say 1.2 ohms from the ac terminals to the ground prong I would have a dead short correct?

Am I doing this wrong? Whats a better way to do this? I just got bitched out for doing this wrong and failing block heaters that work.

Would really appreciate some input on this

 

[ditchpig]

You are correct between 8 & 30 ohms would be 1500w to 500w resistive element at 120Vac measured between the hot & neutral prongs. Anything low resistance reading between power prongs & ground pin is a short (or partial).

Check at element & continuity of cord, I've replaced many cords as the plugs burn out.

Also depending on how long the run is the voltage will be lower at the element so its maximum heating will not be reached.

(corrections made due to alcohol induced computer operation)

 

[Muckin_Slusher]

Also depending on how long the run is the voltage will be lower at the element so the current will be higher unless larger gauge wiring is used to a point.

Think about what you just wrote.

 

[Muckin_Slusher]

A good way to test block heaters is to test how much current they draw. You could use a standard multimeter in series to get an amp measurement, but they're usually limited to 10 amps.

A clamp meter will do the trick nicely. I use a fluke 337a (AC/DC so it's useful for things like starter draw as well). You don't need an expensive clamp like that though.

Make yourself up a short extension cord using three separate wires. The clamp meter must only go around one wire (either ungrounded wire is fine).

The resistance measurement from the "hot" prongs to the chassis of the vehicle should be very high.

 

[HDudrak]

Thanks Ill make up a test harness with the three wires for measuring current.

Idiot shop foreman told me to test block heaters in the future by plugging them in and seeing if they get hot and using a hf "tester" because thats so much more accurate than a red seal tech with a fluke 88

 

[ditchpig]

Think about what you just wrote.

Thought about days later (when sober again) and made changes.

 

[steve83]

...testing block heaters on heavy trucks by measuring resistance across the a/c terminals and checking to see if I'm open from the a/c prongs to ground.

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.

So a 10 ohm block heater @ 110 v ac would be roughly 1200 watts...

That's a lot of heat to put into a cold block at one point. It would be better for the engine (& faster) to use 2 600W heaters on opposite sides.

...over 50 ohms(250 watts) as its imo not enough...

That's not the right way to look at it. 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.

If I have say 1.2 ohms from the ac terminals to the ground prong I would have a dead short correct?

Nearly, and if the power supply is wired properly, it would trip a breaker or a GFCI.

Whats a better way to do this?

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.

...if the connection at either end of its cord is compromised and consumes 0.5A each your current reading is close to the actual draw for the 500w element at 4.28A but not a true reading.

Connections do not "consume" current. Current is constant through a serial circuit. If a connection along that circuit is poor, then it increases resistance & DEcreases current through the WHOLE circuit, compared to what the current would be withOUT that resistance. A poor connection cannot "balance out" or hide a low-voltage problem the way you described. It would exacerbate it.

 

[Muckin_Slusher]

Thought about it makes sense so I'll elaborate.

The mains feed from the local utility is to be within +-12% of the service once stepped down to 120 single or 208/600 3 phase at the meter (main breaker usually mounted close to meter). The wire run from there can be hundreds if not thousands of feet (at CNTower 1800 feet) which requires us to have Buck Boost transformers to bring it back to 600V as its about 20% drop. On a 1 thousand foot run (straight up a tower) using 2 times rated wire we will have ~95Vac. Now figure in the run from the service panel through multipule connections & extension cords to the powered unit. My run at home from the main panel reads 125V and after another panel & 2 connections with a 15 foot cord ~250 feet to the truck is 107V which is in the spec (minimum 105.6V for 120V) the current draw for a 500w heater is 1A higher.

Measuring resistance at the block heater pins is the most accurate. Measuring current draw with cords & connectors (yes I do it for quick check too) can with failing connectors and break down of the conductor add current to the reading which will give a false reading.

So a 500w heater that's at 75% operation (375w) would draw 3.125A and if the connection at either end of its cord is compromised and consumes 0.5A each your current reading is close to the actual draw for the 500w element at 4.28A but not a true reading.

yeah, but:

What you said earlier would be true if the wattage remained the same, but it's the resistance of the heater that remains the same.

The resistance of the block heater stays the same whether you're plugged into a long circuit or a short one. The diagram above shows the circuit drawing 12 amps when connected to a short cable that has negligable resistance. In this circuit the heater is making 1440 watts of heat. In the other circuit with a long wire, the circuit is only drawing 10 amps, and the heater is only making 1000 watts because the wires themselves are heating 2000 watts.

So, with a longer run of cable your amps are decreasing not increasing like you said.

If you measure the voltage at the block heater plug on the front of the truck while it's plugged in, and you measure the current in the wire and multiply them, you're going to get exactly how many watts the block heater is using.

 

[Muckin_Slusher]

Blah, blah, blah.

/\ /\ /\ /\
This guy knows what he's talking about.:smokin:

 

[HDudrak]

That was bad ass. You sir know your shit :smokin:

Thanks for answering all my questions.

 

[68K20 x Drill]

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.

I tested this by just touching my truck while it was plugged in.
She's got a nasty bite, I guess I need some new parts

 

[steve83]

I tested this by just touching my truck while it was plugged in.
She's got a nasty bite...

Then your house (or whatever it was plugged into) also has wiring problems worth fixing. The breaker or GFCI should have tripped when the block heater went bad. Letting it run like that can damage the coolant, any metals in contact with the coolant, as well as any electronics in the truck. It could be bad for kids or pets that get near the truck, too.



BTW
The "special" outlet I described above SHOULDN'T WORK if plugged into a GFCI. As soon as the switch is turned OFF (creating a ground fault) with the heater plugged in, the GFCI should trip. But it should only be used for testing - not for normal operation.

 

[68K20 x Drill]

Definitely not GFI'd.
I don't know if you are old enough to remember when residential breakers were screw in glass things that burn out? I am not, but my barn has them.

Yikes, good thing it only freezes once or twice a year here or I could have done some damage. I think I've been hit harder by a fence charger on high, but the out of the blue surprise factor bumps it up a notch.

 

[steve83]

Yes, I'm that old, and I even lived in a couple of houses that I had to rewire because they had cotton/ceramic wiring.

 

[ditchpig]

You are correct Muckin Slusher and steve83

I removed comments that did not make sense.

I must have fixated on the joule heating wattage.

Long couple of weeks at work with little sleep (that's been repaired) and then maybe a little to much of the preferred beverages.

Hello my names Ron and other people think I have a drinking problem.:shaking:

 

[Muckin_Slusher]

Don't beat yourself up!

Cheers, :beer:

 

[Hiller]

A good way to test block heaters is to test how much current they draw. You could use a standard multimeter in series to get an amp measurement, but they're usually limited to https://peblueprint.com/bathmate-review the bathmate 10 amps.

A clamp meter will do the trick nicely. I use a fluke 337a (AC/DC so it's useful for things like starter draw as well). You don't need an expensive clamp like that though.

Make yourself up a short extension cord using three separate wires. The clamp meter must only go around one wire (either ungrounded wire is fine).

The resistance measurement from the "hot" prongs to the chassis of the vehicle should be very high.

Thanks for the helpful post Muckin, I'm going to give this a try now.

 

[Muckin_Slusher]

Thanks for the helpful post Muckin, I'm going to give this a try now.

Read post #7 as well, THAT is the way to test a block heater.

 

[barney12]

Definitely not GFI'd.
I don't know if you are old enough to remember when residential breakers were screw in glass things that burn out? I am not, but my barn has them.

Those are called fuses. Breakers are different.
Trivia: those screw shells are from Edison. He and Tesla echo through time.

It makes sense to test the effectiveness (heat generated) of this device by measuring current rather than the resistance (almost no current).
An even better test, but more difficult would be temp change of the oil over time. Difficult because the same heater would have a different delta for every block and starting temp.

This is an AC device, so voltage drop isn't much of a concern (as much babble it gets on the internet).

People might want to read up on Ohm's Law. Its only 3 variables so it isn't too tough. It explains electrical stuff.