Rear Defogger "step down amps"? - Ford Truck Enthusiasts Forums

Notices
2009 - 2014 F150 Discuss the 2009, 2010, 2011, 2012, 2013 and 2014 Ford F150

Rear Defogger "step down amps"?

  #1  
Old 01-07-2019, 10:39 AM
HMG_Auto's Avatar
HMG_Auto
HMG_Auto is offline
New User
Thread Starter
Join Date: Jul 2011
Location: Dirty Jersey
Posts: 12
HMG_Auto is starting off with a positive reputation.
Rear Defogger "step down amps"?

I see the rear defroster is a common issue and I see that a cause is the amps too high. I keep seeing mention that the amps need to be stepped down, but I cannot find how this is done.

Is there a write up how this is done?
 
  #2  
Old 01-11-2019, 02:55 PM
P.Bronner
P.Bronner is offline
Elder User
Join Date: Aug 2011
Location: Chico,CA.
Posts: 834
P.Bronner has a good reputation on FTE.P.Bronner has a good reputation on FTE.
Well I stepped mine down to zero by pulling the ground wire. Seriously though, I think you're on to something. If the amp draw could be reduced the problem would largely go away. I don't know if the grid is heating the glass too rapidly or if the contacts overheating is the primary issue. I know quite a few of them have stemmed from the slider contacts getting hot due to corrosion.
 
  #3  
Old 01-12-2019, 10:53 PM
wtroger
wtroger is offline
Postmaster
Join Date: May 2001
Posts: 3,029
wtroger is gaining momentum as a positive member of FTE.wtroger is gaining momentum as a positive member of FTE.
Measure the resistance of the defroster grid in ohms and divide the voltage by the resistance. That will give you the amps it takes to run the defroster grid. You need to either reduce the resistance or up the voltage. A poor connection will up the resistance increasing the amperage. If the grid resistance is in spec for voltage then suspect a bad path to ground.
 
  #4  
Old 01-13-2019, 12:31 PM
Greg13
Greg13 is offline
New User
Join Date: Jun 2017
Posts: 3
Greg13 is starting off with a positive reputation.
Originally Posted by wtroger View Post
Measure the resistance of the defroster grid in ohms and divide the voltage by the resistance. That will give you the amps it takes to run the defroster grid. You need to either reduce the resistance or up the voltage. A poor connection will up the resistance increasing the amperage. If the grid resistance is in spec for voltage then suspect a bad path to ground.
Not exactly....
if you measure the voltage across the grid when it is on, and divide that by the resistance of the grid (measure that with it off and one wire disconnected from it) that will tell you how much current the defroster grid is using. To lower the current you need to INCREASE the resistance. If for example the grid has 11 volts across it and it has a resistance of 5 ohms, then you have 2.2 amps running through that.. 11volts / 5 ohms = 2.2 amps.
If you added a 2 ohm resister in series with the grid you would now have a total of 7 ohms in the circuit so:
11volts /7ohms = 1.57 amps.
Heres the catch... the small resister will get hot. In my example the 2ohm resister will need to be rated for at least 4.9 watts. I would use at least a 10watt resister to keep the resister from overheating.
A poor connection does not increase the amperage (it actually decreases it).. but puts a resistance in a very small place with no way to dissipate the heat.. thus it gets very hot.

I made up these numbers and have no idea what the grid resistance actually is, but if you can tell us the voltage across the grid and the resistance of the grid we could work out what value resistor would work.
 
  #5  
Old 01-15-2019, 02:50 PM
tractorboy924
tractorboy924 is offline
Junior User
Join Date: Jan 2014
Posts: 92
tractorboy924 is gaining momentum as a positive member of FTE.
since I first heard about this I need to just unplug mine. never had an issue yet but I just dont use anymore but there will come a day Im gonna want my mirrors to work and not the rear glass!!!
 
  #6  
Old 01-15-2019, 08:02 PM
2jhanna
2jhanna is offline
Senior User
Join Date: Aug 2006
Location: Double Oak, TX
Posts: 338
2jhanna is starting off with a positive reputation.
Originally Posted by Greg13 View Post
Not exactly....
if you measure the voltage across the grid when it is on, and divide that by the resistance of the grid (measure that with it off and one wire disconnected from it) that will tell you how much current the defroster grid is using. To lower the current you need to INCREASE the resistance. If for example the grid has 11 volts across it and it has a resistance of 5 ohms, then you have 2.2 amps running through that.. 11volts / 5 ohms = 2.2 amps.
If you added a 2 ohm resister in series with the grid you would now have a total of 7 ohms in the circuit so:
11volts /7ohms = 1.57 amps.
Heres the catch... the small resister will get hot. In my example the 2ohm resister will need to be rated for at least 4.9 watts. I would use at least a 10watt resister to keep the resister from overheating.
A poor connection does not increase the amperage (it actually decreases it).. but puts a resistance in a very small place with no way to dissipate the heat.. thus it gets very hot.

I made up these numbers and have no idea what the grid resistance actually is, but if you can tell us the voltage across the grid and the resistance of the grid we could work out what value resistor would work.

yes, good post. good info.

i may add, and I know your numbers were a guess and just for example, but I bet the resistance of the grid is less, making the current draw (amps) greater.

once the real numbers are measured and known, volts multiplied by amps, will give you watts. At least double the wattage rating of the resistor, and other than physical size, resistor wattage can't be to big, meaning having a 20w resistor where only a 10w is needed, won't hurt.
 


Thread Tools
Search this Thread

Contact Us About Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

© 2018 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: