Time for new grid heaters.

[Doubletrouble]

 So here lately it's been getting colder and things have been working just fine. WTS light comes on, then off and she fires up just fine until.......

 When getting ready to leave work a few days ago I went to start her up.lile always but when I hit the key she didn't fire right away like normal. It hit one 1 or 2 cylinders and began to chug and huff smoke like mad. It caught me off gard so I immediately shut her dow to evaluate the situation. I listened for the lift pump, it's working, but I noticed the voltage gauge wasn't dipping like it should with the draw if the grid heaters. So I then cranked her over and let it run, it fired on a few cylinders again and after a few seconds smoothed out and all was good.

 Next morning after the truck sat out in 25°F Temps I Bagan diagnosing.  First with a test light then the DVM. The grids are getting power but making no heat what so ever. Didn't even get warm to the touch.

 I've ordered a new unit from Amazon (would have used DAP or Genos Garage but I need it quickly), should be here Tuesday.

 Not a difficult fix, just thought I'd share in case anyone runs across these symtoms. 

[Mopar1973Man]

Yeah like my conversation on the phone you need to ohm out the 2 positive terminals to the ground terminal. I know the ohms are super low value like 0.6 ohms on that grid heater. I know as the grid heater ages the ohm value goes down continually till the grid heater is like a dead short. I'm kind of interested to see the grid heater when you pull it off. Did the element crack and open or did something else happen? 

[Tractorman]

Make sure to verify that the ground terminal of the grid heaters is not problem before replacing the grid heaters.  Probably not the case, but it shouldn't be overlooked.

 

- John

[Doubletrouble]

 Replaced the grid heater today, all seems well now. Got it done just in time as we have a cold snap coming in this weekend. Saturday nights low will be 7°F. Gonna need that grid working! 

 Very straight forward repair. Simply remove and replace. 

 I put the ohm meter on the old one. Not sure what specs are but it showed about 21 ohms on each post to ground. Seems high to me but I could be wrong.

[Mopar1973Man]

Ohms decrease with wear and eventually vurn out being it reached near zero.

[Doubletrouble]

 I don't know what new spec would be on resistance. I didn't think to ohm the new one before install. I know the new one works though. She started just fine at work this morning at 30°F and I could see the dash lights dim when it cycled on, voltage drop on the gauge also.

[Tractorman]

23 hours ago, Doubletrouble said:

I put the ohm meter on the old one. Not sure what specs are but it showed about 21 ohms on each post to ground. Seems high to me but I could be wrong.

 

Your readings could very well be correct.  Heating elements can fail both ways - a shorted / partially shorted circuit or an open / partially open circuit.  Either way, there will be little or no heat generated by the element.

 

I would be curious for you to retest the old grid heater elements on the bench to verify your first test.  Also, do you make a practice of setting a specific range on your multimeter?  Do you first touch the leads together to ensure a "0" ohm reading with the leads before testing a component?

 

2 hours ago, Doubletrouble said:

I don't know what new spec would be on resistance. I didn't think to ohm the new one before install.

 

Even if you checked the resistance value for an new one, the information would not be helpful.  

 

Using Ohm's Law one can calculate the approximate resistance value, but only for a heated element that is probably glowing.  Under a load the applied voltage would be around 11 volts.  The current flow would be approximately 90 amps.  That would calculate to be around 1.22 ohms of resistance.  But, since that is a heated resistance, that resistance value doesn't mean much.  The resistance value would be much, much lower after the element cooled.  Our meters do not have anywhere close to the capability to read accurate resistance values that low.  Even if they could, an absolutely perfect electrical connection would be required at both meter connections and the resistance value for a cold element would have to be known.

 

Another way to look at this would be to consider that only one wire strand of a 20 strand wire was connected and the rest of the wire strands were broken and not connected.  The one connected strand would still give you the same almost "0" ohm reading, consequently, there would be no value to that reading.  With one connected strand and 19 disconnected strands, an electrical load could not function properly, yet the ohm reading would indicate that the circuit is okay.

 

- John