1989 F-250 4X4 with 5.0

Alernator is pumping out at about 14.5, and I just bought a new battery. According to the gage in the cab, it sometimes is charging, and sometimes not. Drove it to the 'Zone for them to plug it up and check it out...all seems good. I don't trust the gage now, and that concerns me.

I picked up another two plug harness for the alternator at the local pick-n-pull, thinking perhaps there's a short in the wires, as they are a bit brittle. I'll install this later this morning. Hopefully this works, and gives the gage the ability to calm my nerves while driving (always hated dummy lights, and that's all the Lincoln has...my Mercedes has full gages)

Also, I hope to be able to track down why I have a retarded oil pressure gage...it, too, has the ability to drop down, causing the engine light illuminate. The oil pressure is just fine, never dropping. I think it may be a short in the wiring for the oil sending unit. It is electronic, isn't it??

Here's my question...am I wasting my time with the factory gages, or should I be investing my time in a gage set-up adding two new gages?

 

I would check for a bad ground or short in the wire(s) leading to the volt gauge. If you're reading 14.5 volts directly off the alternator then there's a discrepency with the gauge. Chase down potential gauge problems before switching out the plug.

Same with the oil pressure gauge. My truck is a 91 F150 but I have issues with my OP gauge reading low as well despite actual pressure being normal. I don't know for sure how it's done but you should be able to test the sending unit... or you could just go get a new one.

 

Ok...plugs are good on the alternator. I'm wondering if this problem could be in the relay on the passenger side wheel well? When i bought the truck, the relay was loos and the pos batt connection was loose as well. after tightening them, the problem started getting worse.

any help here would be great, as i have the next 2 days off and time to work on it.

 

I'd check the Yellow-white wire real good for corrosion, possibly at the fuse link.

The alternator needs battery voltage from that line to charge, the fact it charges but does so intermittently suggests a fault in that wire someplace.

The fuse link you're looking for at the starter relay on the fender is green. Peal it open and check the solder connections at each end of it, you may find one or both are corroded. Yea either way you'll have to replace that fuse link but its the only way to know for sure. Well you could wiggle it around and see if you can reproduce the issue.

If nothing found there further down the line that circuit joins with black-orange wire/s (2?), check the connection at that point.

You could use a meter and test it for continuity but with a intermittent problem you seam to have, it would likely test "good".

 

My 87 f150 5.0 efi did something like that about 12 years ago. My voltmeter would sometimes show about 14 volts or other times 8 or 12 or 11. I would check it with my test meter and it would be about 14 volts running. Clamp on dc amp meter would show the alternator working. It was the dash gauge. Ford place wanted about $150 bucks for that side of the cluster. I pulled out the instrument cluster and connected a 0 to 15 vdc supply to the input terminals. This showed the gage to be sticking or dragging. Probably caused by out gassing of the plastic or cleaning stuff i used on the dash or damping compound inside the gauge guts. I took the gauge out of the cluster and put some wd40 in a hypodermic sryinge and eased a drop on the shaft. I then moved the needle by hand a few hundred times up and down and round and round to work the wd40 in. I then followed with denatured alcohol to flush the wd40 out. then retested the gauge with the variable supply. I then used the hypo again to put another drop of wd40 on the shaft and let it sit over night. worked gauge hand again about hundred times and used denat to wash off the wd40. this time it tested fine. reinstalled it and its worked fine ever since. Which reminds me Its about time to do a calibration check to be sure its ok. You just have to be patient and hard headed to go this route. Its kind of like working on a watch. Best I can remember I did this over a period of about a week. I think I put the cluster back in except for the voltmeter. and then took the cluster back out when i fixed the voltmeter. Its easy but kind of tricky. the little black caps havw to be eased off the gage hand and then use a gage hand puller to remove the hand or needle. this is to get just the voltmeter out of the cluster and put the water temp part back in.

 

when you checked those plugs, the biggest one has a white plastic like dividers between where each pin goes, these should be soft, if they are hard and cracked or starting to fall off replace the plug. cracks in those dividers will allow sparks to jump around, could cause your situation. you can purchase those plugs at the parts store for around 5 to 10 bucks depending on which one you go to.