Hello brothers,

The rear defroster worked well for the first two winters, but when I started using it in the third, I noticed an abrupt drop in performance. Subjectively it seemed like it had half the heat it had before. There are no cracks or damage on the grid, and all wires heat evenly.....its just gotten so weak I can't even tell its working below about -10C. The dealer said "no fault found" but I suspect they just looked for heat and then passed it if they detected anything at all.

It wasn't until I measured the grid voltage at the rear window edges that I became more convinced something was amiss. 11V with the engine running, 14.4V everywhere else. As you know, electrical power is proportional to the square of voltage, so this voltage drop does align with the half heat observation. The 14.4V also appeared at the associated fuse and relay outputs. The only other thing I haven't checked is the wires around the rear hatch hinges, since they may have broken due to repeated movement when the hatch is opened. Still, one would think broken wires would lead to zero volts on the grid, not 11. The grid voltage is stable and consistent at 11.0 volts.

I fully intend to check the wires around the hinge area as soon as I find a diagram that describes how to remove the plastic, but now I have another suspicion. We all heard about the rear glass breakage fiasco, which is related to a defroster design issue, per Acura themselves. My RDX has had it share of dealer visits. Could it be they popped some kind of resistance or voltage converter into the harness while it was at the dealer? Thoughts?

 

I just did some voltage measurements on my own 2020 RDX.

Battery=14.3v (at idle)
Defroster (from extreme right to extreme left of the same strip) = 10.99v

​​​​​​These defroster grids consume a lot of current, and as you know, voltage drop equals current times resistance, and you know that a wires resistance increases with its length. So I am not surprised to see a 3.4v drop, considering the length of wire from the front all the way to the back of the vehicle.

I measured 19 amps being consumed, using a clamp-on ammeter around the negative battery cable. This test had to be done with the engine off, because with the engine running the alternator would provide the power, rather than the battery, and I wouldn't be able to measure it on the battery cable. I'm sure the defroster would draw more than 19 amps with the full 14.4 volt alternator voltage.

​​​​​​You could measure the temperature of the grid with a non-contact IR thermometer, but comparing that to my own readings would be meaningless due to different ambient temperatures, and the varying time that the defroster had been turned on.

 

Follow up.

I measured the 19 amps when the battery voltage was 12.16 volts, so the resistance of the defroster circuit is 0.64 ohms (12.16/19}

When the engine is running, the defroster would draw 22.5 amps (14.4/0.64)

Since you noticed a 3.4 volt drop, that makes the resistance of the wires, from front to back, 151 milliohms (3.4/22.5).

​​​​​​I don't think your voltage drop is abnormal or unexpected.

 

Thank you brother, your point is valid. I had a newer truck that measured several volts drop from battery to headlight socket. It actually prevented aftermarket HID ballasts from starting, and there was absolutely nothing wrong with the wiring or connectors in terms of the mother ships design intentions. We all know they have to save $$ wherever possible, and copper is $$$$$ nowadays.

I can't fault what you are saying about wire lengths etc but I still want to find an explanation for the very abrupt and palatable drop in defroster performance from winter 2 to winter 3.
Have you had the occasion to observe your defroster performance with a heavy layer of frost at say -20C or below?
I may pop into the dealer and ask if I can measure both grid voltage and resistance on a new RDX or two.

 

It would be pretty hard to measure the grid resistance directly with an ohm meter. My calculated value is probably half the resistance of your test leads, which means that a difference of 0.5 ohms would be a 100% error. Also, without unplugging the grid from the remainder of the vehicle, your resistance measurements would include other parts of the circuit that are in parallel with the grid, for example the timer and indicator light, which would be significant considering the small resistance you are expecting.

​​​​​​I have been through 4 Canadian winters with my RDX, and I haven't noticed anything about the rear defroster that seems abnormal. I do park in a garage overnight , and although the temperature does reach - 20C, I don't experience frost buildup on the rear window.

The problem I do experience, is when I get into the vehicle, the humidity from my breath condenses on the cold glass and fogs it up (much like the bathroom mirror after a hot shower). The rear defroster does eliminate this fog fairly quickly, as expected.