I have a 2005 Ford F150 and it has been serviced by a dealer all 78k miles.

I had brought it into a different dealer than usual due to the fact that the check engine light went on and started missing under hard acceleration. Also I noticed the headlights and dash lights dim to 1/2 half to a 1/3 their norm under hard acceleration.

The dealer repaired the check engine light issue (Battery , plugs and #5 Coil fuel system flush) but said they didn't notice the issue because it was light out. I guess the tech at this dealer did not know about a thing called a volt meter.

Brought it back and they replaced the alternator and belt.

Paid the bill and decided to test it out and the lights dim at 4500-5000rpm in neutral so it's not fixed.

So after I paid to have the alternator and belt replaced and they could not figure out what was going on so the dealer called Ford Motor Company and FOMOCO said this is normal?

I guess I am puzzed and would like to know if any 2005-2008 owners have noticed that your lights go dim interior and exterior (very noticeable) under hard acceleration (downshifts) or just running at about 4500-5000 rpm?

Is this normal , if it is I got soaked for an alternator and belt. If not I am getting a BS answer from some techs that don't know their stuff at this dealer.

Really could use some help verifying this.

 

Can't help diagnose the true issue, but I've run my lil 4.6 '08 STX pretty hard and never noticed and never once noticed an issue similar to this. IMO, it could be a weakened fuse that, once put under stress, can't properly handle the voltage passing through it (or lack of, since more power is being put to the engine when you romp on it).

I'm sure somebody will tell me that I'm wet behind the ears but I'm the first to admit that electrical issues aren't my strong aspect. Good luck finding the issue!

Update: after speaking to a friend on the phone, he's said two possibilities could be belt slippage or a wire somewhere might be getting pinched when you hammer down on it. Might wanna pop the hood and make sure nothing is getting crunched against the firewall when you stomp it....

 

Doesn't sound right to me, I've never had a car/truck do that. I think you're getting hosed. Maybe try another dealer's service department.

Either that or they didn't do the work they charged you for. I had a Dodge dealer do that to me back in the 70's.

 

That is NOT normal - and the most likely cause is a slipping belt or a bad alternator. If the belt were slipping that bad, I would think you would hear it screeching.

What you COULD check is remove the belt and see if any of the driven pulleys turn hard or rough.

 

usually when an alternator or battery is weak, the lights would be dim at idle, and get brighter with more rpms.

 

Not if the brushes start bouncing at higher RPM's - that's exactly what happened to my alternator - the idiot light would come on at 4000 rpm and the voltage would drop from 14 to 12.

 

I just got back from the Ford Dealer and now they said it was strange for me to "floor" my truck or accelerate that hard to see the lights dim.

I guess no one at this dealer drives on the expressway or tows anything?

I guess the service manager is going to drive it tonight and they have my old alternator and may put that back on.

I still believe that it may be a tensioner or something pertaining to the belt. I thought I saw a thread that said his tensioner failed but it never made noise it just slipped at high RPMs.

Also affirmed they had a rookie tech on this and never brought this to a more experienced tech to look at like I suspected.

I guess I'll have to wait and see.

 

Well, it looks like the rebuilt Ford/Motorcraft alternator replacement was bad. That fits what a lot of others have seen with aftermarket alternators so the rebuild failure rates look pretty high from what I have seen on this board.

The dealer is keeping it overnight and testing one more time in the morning.

Wish me luck ;-)

 

When my alternator started doing what I described, Ford didn't have any alternators in stock so they went over to O'Reilly's and got a reman with a lifetime warranty - it works fine.

 

alternator, my 97' did the same thing when i got it, then i replaced the alternator after it failed, and it hasn't done it since.

 

The 04-08's have finicky voltage systems and a lot of reman alts meet reman specs but not ford's OEM specs.....best solution is to only install a new alt not unless the reman meets or exceeds the Ford OEM specs....