2009 Mazda5 - block-heater health

[Number_35]

Jr and I were doing pre-winter maintenance on his 2009 Mazda5 yesterday, and per routine I checked the block-heater plug with an ohmmeter.

It was open, which was no surprise, as after I opened up the replacement end I had installed a few years ago, I found all the connections badly corroded.

I cut off the plug and stripped back the conductors in the block-heater cable. One side was quite discoloured, from salty water wicking up the stranded wire. The circuit was still open (i.e. reading infinite resistance). I kept going until the resistance got down to c. 375 Ohms. Way too high. Stripped it back some more until the copper looked good. Now the resistance was about 114 Ohms. That still seemed higher than I expected, but within the realm of possibility.

I didn't have a good plug, so spliced in a grounded appliance plug and part of its cord for the time being.

I have been unable to find the rated wattage for this block heater, so don't know if the resistance is appropriate.

Let's round that resistance up to 120 Ohms to make the math easy. 120 V/120 Ohms = 1 A. (1 A)^2 * 120 Ohms = 120 W.

I thought a block heater was usually 300 W or more. I'm a bit reluctant to plug it in for fear of heating up the cord internally if there is still high resistance in the cord.

Thoughts?

Perhaps I should replace the entire cord (which looks like it would be a real pain, but so be it).

 

[twX]

I've had a few block heaters on 4-cylinder engines and all have been rated 400 W, which would be 33 ohm at 115 V. If your block heater is rated 400 W but reading 120 ohms including the cable, that means the wiring would be dissipating most of the heat, and the heater itself would only be running at around 30 watts. Even if the cable doesn't melt from that heat, the block heater won't be doing much heating.

You could just keep cutting back the wire until the resistance is
If you do replace more of the cable, secure it well along the length with zip ties so that if your friend starts driving away with it plugged in, the plug end will take the damage and not the wiring connectors. I'd pack the wiring connectors with dielectric grease, then heat shrink or tape them up.

 

[B1974]

It's probably 400 watts. We had a van burn down in the parking lot at work about 15 years ago due to a bad block heater cord. I'd play it safe and replace the cord.

 

[daves87rs]

It's probably 400 watts. We had a van burn down in the parking lot at work about 15 years ago due to a bad block heater cord. I'd play it safe and replace the cord.

This. Too easy to go bad quick….

 

[Number_35]

It's probably 400 watts. We had a van burn down in the parking lot at work about 15 years ago due to a bad block heater cord. I'd play it safe and replace the cord.

That happened with a coworker's new Chevy truck about 15 years ago. That was with the thermostatic cord that stays open until -18°C. Quite spectacular!

About 10 years ago I went to unplug our work group's truck, and found the plug hot to the touch. Dodged a bullet on that one! Took it over to fleet services right away for a new plug - they said they see that a lot.

I found the cord for the Mazda online from Parts Avatar, about $40 delivered. Will call CTR tomorrow to see if they stock it. It's a Phillips/Temro part.

 

[Number_35]

I've had a few block heaters on 4-cylinder engines and all have been rated 400 W, which would be 33 ohm at 115 V. If your block heater is rated 400 W but reading 120 ohms including the cable, that means the wiring would be dissipating most of the heat, and the heater itself would only be running at around 30 watts. Even if the cable doesn't melt from that heat, the block heater won't be doing much heating.

You could just keep cutting back the wire until the resistance is
If you do replace more of the cable, secure it well along the length with zip ties so that if your friend starts driving away with it plugged in, the plug end will take the damage and not the wiring connectors. I'd pack the wiring connectors with dielectric grease, then heat shrink or tape them up.

I've decided to replace the cord. It will not be fun, but I think is the best way.

I like the idea of a strain relief as you've suggested. My workplace kept telling us to do a 360° walk around any vehicle before driving away, and I have encouraged my family members to do the same, but stuff happens.

It is common here during the winter to see vehicles driven by an oblivious soul, with an extension cord flapping, the cord being abraded to death on the roadway.

 

[Number_35]

I cut back the old cord about another 20 cm/8", and the resistance dripped to the expected 36 Ohms.

Had an appropriate cord salvaged from some past project, and spliced it in.

120 V applied to 36 Ohms works out to 400 W.

Old replacement plug

Additional section of original cord cut back:

New section of cord with sealed plug installed:

Lower resistance!

 

[twX]

I cut back the old cord about another 20 cm/8", and the resistance dripped to the expected 36 Ohms.

Had an appropriate cord salvaged from some past project, and spliced it in.

120 V applied to 36 Ohms works out to 400 W.

Old replacement plug

That should work well. I used one of those same yellow replacement plugs the first time I replaced a block heater plug (I'm one of those oblivious souls). They're rated for outdoors, but aren't sealed well and usually the cable sheath isn't big enough to fill up the opening at the back so they just fill up with dirt and road salt spray.

Splicing on a sealed plug is the way to go. At least the wiring connectors can be kept in the engine bay, away from any of direct spray.

 

[Number_35]

That should work well. I used one of those same yellow replacement plugs the first time I replaced a block heater plug (I'm one of those oblivious souls). They're rated for outdoors, but aren't sealed well and usually the cable sheath isn't big enough to fill up the opening at the back so they just fill up with dirt and road salt spray.

Splicing on a sealed plug is the way to go. At least the wiring connectors can be kept in the engine bay, away from any of direct spray.

Your description of the issues with the aftermarket yellow replacement is right on - the gap between the flat cable and the entry port is too large, and admits salty slush.

If I use another one in the future, I'll use some silicone or something else.

 

[B1974]

I've used these plug ends on block heaters.

https://www.homedepot.ca/product/le..._All Products__PRODUCT_GROUP_pla-300587868876

 

[Number_35]

I've used these plug ends on block heaters.

https://www.homedepot.ca/product/leviton-rubber-plug-3-wire-black/1000105474?eid=PS_GOOGLE_D00_Corporate_GGL_Shopping_PLA_All-Products_All Products__PRODUCT_GROUP_pla-300587868876

Does this style provide better sealing for the cord (than the cheaper yellow type)?

 

[B1974]

It's does seal better. I wrap a bit of electrical tape around the wire cover to get the back end to seal a bit more.