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Old 01-10-2012, 03:54 PM   #1
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Default Rear defrost defogger relay - DO NOT USE FUSE - Explanation

I've seen many people suggest replacing the rear defrost relay relay with a fuse when the rear defroster shuts off after a short time.

I ran some experiments and found some interesting results. The relay only shuts off after a short time when the car is at idle or low RPMs. A typical alternator output drops to approximately 30% of maximum capacity at idle. Standard alternators are rated at 30-40 Amps at idle and about 105 amps at 1500 rpm. When you turn on the heater fan on full, have the headlights going and possibly the wipers and add in other loads like a radio, you can quickly exceed the idle output. When that happens the voltage drops from 14.2-14.5 down to as much as 12 volts. If I were to assume a typical 25 amp draw at 14.2 volts that equates to 355 watts (v*A=W). If I assume a drop voltage to 12.5 V. 355watts/12.5V= 28.4 Amps So as the voltage drops the current draw goes up.

As the car ages, the output of the alternator starts to drop. Add to that total current draw our phone charges, super stereo amps and other add on toys and we are making the problem worse with voltage drop. The relay itself also starts to get weak and trips at lower current draw. Popping in a 30 amp fuse that is relatively slow reacting and you will have too much current draw for the wiring and the wires will get hot. SO DON'T just pop in a fuse. Replace the relay.

To prove the point, if you have a relay that shuts off quickly - only a little heat then stops - wait to turn on the defrost until you are at cruising speed. You will find it either runs longer or does not shut off before it times out (10-15 minutes). Just remember if you slow down or stop for a light it will trip and stop operating. It has to be turned off, allow the relay to cool down for a few minutes and then turn it back on. A weak relay will just cycle on and back off it is not allowed to cool.

To really SOLVE the problem, I'd suggest replacing the standard 105 amp alternator with a police version 220 A along with the defroster relay. Idle output is also more than 2x. It bolts right in - belt may change though - i'm not sure yet - will be doing this myself soon. When you think about it, police alternators are sized to handle all of the extra police equipment power load at idle. If they didn't have the higher output, the battery would drain while idling!

Problem REALLY solved.

Anyone know if the belt changes with a 220 Amp police alternator on a 2005 Impala SS 3800 SC?

Last edited by Cheshire; 01-10-2012 at 04:02 PM..
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Old 01-10-2012, 09:00 PM   #2
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220 amp police alt? never heard of it........maybe the 145 amp AD244
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Old 01-11-2012, 09:05 AM   #3
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Quote:
Originally Posted by Cheshire View Post
I've seen many people suggest replacing the rear defrost relay relay with a fuse when the rear defroster shuts off after a short time.

I ran some experiments and found some interesting results. The relay only shuts off after a short time when the car is at idle or low RPMs. A typical alternator output drops to approximately 30% of maximum capacity at idle. Standard alternators are rated at 30-40 Amps at idle and about 105 amps at 1500 rpm. When you turn on the heater fan on full, have the headlights going and possibly the wipers and add in other loads like a radio, you can quickly exceed the idle output. When that happens the voltage drops from 14.2-14.5 down to as much as 12 volts. If I were to assume a typical 25 amp draw at 14.2 volts that equates to 355 watts (v*A=W). If I assume a drop voltage to 12.5 V. 355watts/12.5V= 28.4 Amps So as the voltage drops the current draw goes up.

As the car ages, the output of the alternator starts to drop. Add to that total current draw our phone charges, super stereo amps and other add on toys and we are making the problem worse with voltage drop. The relay itself also starts to get weak and trips at lower current draw. Popping in a 30 amp fuse that is relatively slow reacting and you will have too much current draw for the wiring and the wires will get hot. SO DON'T just pop in a fuse. Replace the relay.

To prove the point, if you have a relay that shuts off quickly - only a little heat then stops - wait to turn on the defrost until you are at cruising speed. You will find it either runs longer or does not shut off before it times out (10-15 minutes). Just remember if you slow down or stop for a light it will trip and stop operating. It has to be turned off, allow the relay to cool down for a few minutes and then turn it back on. A weak relay will just cycle on and back off it is not allowed to cool.
All great points, although V=I*R is probably the better equation to explain why current (I) would go up with a lowered voltage.

Circuit protection is a key safety feature on any piece of equipment and fuses do react differently than circuit breakers. The engineers put breakers in for a reason. (When you take in to account that these cars were nickeled and dimed during design and part selection, one could infer that there was a very good/compelling reason for picking the more expensive breakers.)

However - the circuit is designed to handle the load of the defroster and the power seat. As long as you aren't operating the power seat at the same time, the as-designed circuit will have enough overhead to cover this small increase in current.

Quote:
Originally Posted by Cheshire View Post
To really SOLVE the problem, I'd suggest replacing the standard 105 amp alternator with a police version 220 A along with the defroster relay. Idle output is also more than 2x. It bolts right in - belt may change though - i'm not sure yet - will be doing this myself soon. When you think about it, police alternators are sized to handle all of the extra police equipment power load at idle. If they didn't have the higher output, the battery would drain while idling!
Not a good suggestion. First, the size of the alternator isn't the issue here. Second, new and/or larger alternators cost money - which would be wasted because the size of the alternator isn't the issue here. Third, larger alternators suck more power from the engine and burn more fuel. This would be a waste because the size of the alternator isn't the issue here.

I realize that lots of people who don't need a larger alternator (some do) won't listen to this because bigger must be better - and everyone knows women judge us by the size of our alternators...

But don't take my word for it - remove your original circuit breaker and open the insides up. You'll see all sorts of soot and corrosion inside from use over the years. This creates increased resistance. Looking back at V=I*R, THIS in addition to the increased heat produced in the breaker causes a the trip.

If you just replace the worn-out breaker, the problem will be solved for a lot less effort.
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Old 01-11-2012, 03:03 PM   #4
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Another thing to add (which is prolly different for everyone) but my defrogger cuts off with no lights, no radio, and no other accessories operating besides the nessecary ones to run the car. And this is all at a constant between 1500-2500 RPMs all the way to work in the morning, even when it is freezing in the car. I am tended to believe it is more of what wssix99 was saying about how the years of build up and corrosion are the main issues here and the correct fix would be to replace the relay/breaker with a brand new piece.
I do however agree that putting a fuse there is not a good idea. If something catches fire I can see an insurance company possibly denying a claim if they find a fuse where a relay should be.
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Old 01-12-2012, 01:24 AM   #5
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I guess I've never read the defogger threads.
They use relays & circuit breakers for a reason.

It is true, the higher the amperage demand the lower the system voltage in a generator/alternator system.
The fact is you are demanding high amp load of the battery (everything is wired to it) The voltage drops & the alternator charges it, so there is a voltage drop under load. The current wouldn't flow @ the same voltage because of the capacitance in the battery.
A static fully charged battery is 12.6 volts. This is why the running alternator charging system voltage is 14.3 ish. Current flow is the real important thing.

As with igntion systems the higher the firing voltage the shorter the spark duration per event. Electical engineering is very interesting to me anyway. Alway predictable, mathematically consistant with the use of Ohms law.
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Old 01-12-2012, 10:14 AM   #6
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I have seen this...

It scared the **** out of me the first time, (JUST had alternator replaced), and it was the first cold morning of the winter.

I had my headlights, heater, radio, etc. & the rear defroster going, I pulled up to a light and looked at my gauges to see it start moving past 13.

I turned off the rear defrost and everything instantly got more power, (fans blew faster, lights got brighter, etc.)
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Old 01-12-2012, 11:14 AM   #7
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Wssix - great response and you are correct. The real issue is the relay. As stated by SJsingle1 - you are correct. 145 was maximum factory for police cars. There are few out there that are much higher and direct factory replacements.
You are also correct WSSIX that all things being right, a larger alternator is not required. What I am suggesting is two things. People are plugging more things into their vehicles to charge. Some leave them charging over night which drains the battery. Others add super amps to their stereos which suck a lot of juice. Depending on your circumstances and load, at idle - you may want up your alternator output and increase the size of the cables to the battery and alternator.

Relay first. If the problem comes back, you need to look elsewhere and load may the issue.
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Old 01-12-2012, 12:21 PM   #8
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Quote:
Originally Posted by Cheshire View Post
Wssix - great response and you are correct. The real issue is the relay. As stated by SJsingle1 - you are correct. 145 was maximum factory for police cars. There are few out there that are much higher and direct factory replacements.
You are also correct WSSIX that all things being right, a larger alternator is not required. What I am suggesting is two things. People are plugging more things into their vehicles to charge. Some leave them charging over night which drains the battery. Others add super amps to their stereos which suck a lot of juice. Depending on your circumstances and load, at idle - you may want up your alternator output and increase the size of the cables to the battery and alternator.

Relay first. If the problem comes back, you need to look elsewhere and load may the issue.
The plug-in accessories are a problem. A different alternator probably wouldn't help with battery drain, but a second battery in parallel, might. (I understand police cars have this type of setup.)

I started working with a low drain, switching voltage regulator that I can plug in to the car's system. Its supposed to have near zero drain when the accessories are powered off. Unfortunately, the regulator assembly I received was not at the rated voltage on the package. So... I need to find time to play with it and make my own assembly that puts out the correct voltage for my application.
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Old 01-12-2012, 12:41 PM   #9
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Quote:
Originally Posted by Cheshire View Post
What I am suggesting is two things. People are plugging more things into their vehicles to charge. Some leave them charging over night which drains the battery. Others add super amps to their stereos which suck a lot of juice. Depending on your circumstances and load, at idle - you may want up your alternator output and increase the size of the cables to the battery and alternator.
While alternator size isn't going to do anything to help with battery drain due to accessory charging overnight, it is certainly something to look at if you have added items that place additional load on the system when the engine is running.

Significant stereo upgrades are a great example. The car's charging system was designed to handle the stock load, but once you go beyond that it may no longer be sufficent. An aftermarket crank pulley can also reduce idle output of the alternator, unless you change the alternator pulley to something that will retain the stock rotation ratio.

I think sometimes people get nervous when they see the volt gauge drop when the electrical system is under heavy load at idle. But the fact is, even a perfectly functioning stock electrical system on these cars when they were brand new did not provide for 14 volts of charging at idle when every possibile factory accessory was running. I've never owned any car that provided maximum system voltage with every possibile accessory running at idle (especially automatic trans cars idling low(er) in gear).
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Old 01-12-2012, 12:41 PM
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