Lights Flickering On Cold Start

[EdHale]

My truck has had a slight flickering of the lights, headlights and dash, immediately after a cold start since new and I now have 133,000 miles on it. Cold to me means 20-30's. The flickering would only last a few minutes and seems to be the most noticeable around 1100-1200 rpm. Yesterday morning it was worse than usual, meaning more dimming of the lights. The flicker is about a twice/second, but only lasted the normal few minutes.

Is this the intake air heater working? Does this cycle on/off like I'm seeing because it seems to be an electrical load being turned on/off that is causing the flicker. Is something getting ready to die?

Everything else works fine.

Thanks guys.

[rapidoxidationman]

The glow plugs take a lot of amps to stay hot - you might be seeing the effects of them turning on and off for a short time after starting? Does the voltmeter show the same phenomenon?

[EdHale]

The voltmeter doesn't respond this quickly, but it does show lower voltage during this warmup period. After a couple of minutes the flickering stops and the voltage is back up to the normal range which has been the case all along.

As I said this has been happening since new but it was much more noticeable the last cold morning start. Of course maybe it was the fact that with daylight savings time it was dark and I just hadn't been able to see it in a while.

[DmaxMaverick]

This sounds like a failing/faulty relay, quite possibly the intake heater. Much the same way a starter relay/solenoid will chatter with a weak battery or poor connection.

[EdHale]

Thanks for the input guys. Since it is spring and I'm in the south I may have to revisit this later this year. I plan on keeping this truck forever so I will fix it when the time comes. It was just more pronounced the other morning than in the past, like something was getting worse.

[rapidoxidationman]

Maybe have your batteries load tested. At low RPMs the alternator isn't putting out much juice and a big load (intake heater or glow plugs cycling) could be pulling the batteries down a bit more than with fresh batteries. Does the starter still spin fast and easy like new?

[EdHale]

The starter spins normal. We had a few mornings of 9 degrees a few weeks back and no problems with the starter then (of course I did plug in the block heater at night). As I said earlier, this flickering has been going on since the truck was new, but just not as bad as the other morning.

Does the heater cycle on/off in normal operation? Do the glow plugs cycle on/off in normal operation? If so, is the cycle about a half second per cycle because that is what I'm seeing? Do both the heater and glow plugs cycle independently of each other, if they cycle on/off? If this is the case maybe both just ended up on the exact same cycle and that "double load" caused a much deeper voltage drop that I am seeing in the lights?

[More Power]

It's normal for the lights to dim in sequence with the intake heater and glow plug cycles. You'll also see it in the dash voltmeter.

However, once the engine start phase has passed the heater/plugs cycling, brightness of the truck's headlights and dash lights shouldn't change much at all with changes in engine speed from idle to off idle.

If they do, it could mean a weakness in the truck's alternator, batteries or battery cables.

[EdHale]

Is this on/off which causes the flickering in about half second cycles? I know what a load on the system will do to cause the voltage drop that causes the lower light levels. This truck has done this on cold starts since new, but was much worse the other day. Do the glow plugs cycle on and off in half second cycles and how about the intake air heater? If both do cycle and at the same time this would explain the heavier load that normal.

My batteries are good, cables are good and all connections clean, I have dual alternators and never had a problem. I tow a 35' 5th wheel with lots of lights on it and the voltage never changes when I turn everything on so I'm sure my system is working ok. The voltmeter on the dash will read about 14 volts or maybe a needle width under after a cold start, but as I said before it doesn't react fast enough to show the voltage change during the on/off cycles. After warmup is complete the voltage settles around 16'ish. Again, nothing has changed since new other than the other morning.