heated windshield on -> voltage drops to 12.7

Been chasing down a lot of electrical issues recently, while I was trying to figure out why my car voltage reads a constant 14.5v rather than 13.7-14.2 I discovered that when I turn on my heated windshield my alternator load will quickly go from around 30% to 100% load and pull around 100 amps rather than around 30-40, and then after about 30 seconds my voltage will slowly drop to around 12.7. I assume this cant be normal, is there a particular place I should start? The heating aspect of it works great, no issues but I know my driver side door mirror heating element is shorted and doesn't work. are these systems related?

I would appreciate any advice, thanks all.

I thought the windshield had two heating elements, one on each side, and each has a 30A fuse. The mirrors draw about 2A each. The blower fans can draw a good bit too.

I guess you could pull those fuses and see what the load is without (in case other things got turned on like the rear defogger). How are you getting your current draw reading?

ftillier said:

I thought the windshield had two heating elements, one on each side, and each has a 30A fuse. The mirrors draw about 2A each. The blower fans can draw a good bit too.

I guess you could pull those fuses and see what the load is without (in case other things got turned on like the rear defogger). How are you getting your current draw reading?

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IID tool, I could give you more accurate readings tomorrow. The heated windscreen works great, both sides work flawlessly. I just found it extremely odd that a factory component would tank my voltage that badly, I’ve read so many times that these trucks should not have voltage readings outside of 13.5/7 to 14.2. With the heat on full blast and the rear defroster on my voltage remains consistent, but even with the heater off and the rear defroster off it will still tank my voltage. I replaced the alternator about a year ago with the OE denso unit.

I’ve had some past electrical issues with this truck, one of them being the main ground strap to the engine. Basically the truck would run great but would take multiple cranking cycles to start. Found that the resistance to the coil grounds on the front timing covers were around 4 ohms. I just put a bandage on that by putting a ground wire from the engine coil grounds to a ground point on the body where the LH air box is. The voltage was normal but I used too weak of a ground wire. I believe I’m using 4 gauge wire now and Now voltage is extremely high in the 14.5-8 range. Tomorrow I plan on trying to clean the engine ground which I believe is somewhere in the RH wheel well, and take my ground strap off. hopefully this fixes that problem.

I’m wondering if this is all related or just some strange coincidence. To add to that, I tried to clean the contacts on the driverside mirror to get the heated mirror to work but I’m still getting a code for that, I need to pick up a multi meter tomorrow (my other one broke) but I’m guessing I’ll end up needing a whole new heated mirror.

All in all, is this voltage drop normal? When you have time, could you try the heated windshield in your rig and see if it drops your voltage down to the 12s?

Aren’t these alternators PWM? I thought we basically shouldn’t expect to read “normal” voltages on them?

But if you’re right about the 100A draw, it sounds like you have a short to ground somewhere. IDK why the fuse wouldn’t blow.

Your good at 14.8V. If the charge hits 14.9V, that is too high for the battery. The alternator puts out 150A so you are well within the normal operating range with the 100A draw.

I haven't monitored the voltages on mine, but had thoughts of using a 50A DC-to-DC Lifepo4 charger. I'm not sure there is enough headroom in the winter, probably not. The seat heaters kick ass so it probably draws 7-10A. The steering wheel warmer is nice too although its a bit weak and not warm enough to be able to unglove.

Typically, if the dynamic output generator is constantly running at maximum output, the available power of the main battery is starting to slide.
The battery is a fundamental part of the electronics system of a vehicle today.

One quick way of ruling this out: perform a battery voltage reading of a cold battery. This means, following a 6 hour rest (as in vehicle not running for 6 hours, battery not on any charging device for 6 hours), take the voltage reading. If you read 12.4v, your battery can only contain 50% of capacity... and that is far below spec. and requires replacement.