I have a proplexing electrical short on my 1989 F150. I first noticed it about a month ago. I was driving home from work after dark and had my headlights on. I noticed my lights were dim and the radio cut out. I looked at my amp guage and found it was drawn down to about 5 amps. The system was so taxed that my turn signals wouldnt blink. The next morning When I started the truck everything was back to normal.

That night when I got off work and started the truck all was well, till about half way home and again the same thing happened. The next morning all was well and it didnt happen again for a few days. It has since come and gone as if it has a mind of its own.

It only did it when the lights were on, until the other day. I took the brat to school very early in the morning and had turned my lights on and of course the "myserious short" once again returned but this time when I turned my lights off it failed to leave. The short was so severe my radio wouldnt work. I stopped for a coffee and parked on an incline, ( forcasting that the short may not leave when I turned off the truck). When I returned to the truck and tried to start it I found the draw on the system was too much and it wouldnt turn over. I popped the clutch and started the truck and as I sat there proplexed as to what to do I did what any self respecting man would do, I punched the dash. Wouldnt you know the amp guage retuned to "12" and all was well.

Since the coffee shop incident the short has continued almost always when the lights are on. I dont know where to start to track down this short but I am assuming it is in the dash somewhere. Anyone have any clue where I should start?

I have checked the harness on both sides of the firewall and everything is connected tightly

 

it sounds like you have an open in the circut, or a bridge to ground. I would take the bulb out of the headlight and look at it and the wires dirrectly behind it, make sure its sitting correctly in the harness and look at the first 6 inches to a foot behind the buld and make sure there isnt a cut or snag somewhere. Also do a quick check for corrosion on your battery, and take a look at your fuse box.
I would start with those things. b.c those are fairly simple to check out and especially the corrosion on the battery are somewhat typical power draws.
Goodluck, keep us posted,
-Patrick

 

Sounds like a connection at the battery or the cables.
The alternator is just barely able to put a charge back for starting.
When you use the headlites the alternator can't supply enough to them and the battery so the no start condition.
The fact that you got reaction from hitting the dash shows a physically poor connection close by even if it is cable or connector related.
Take some time to start the engine at home, turn the lights on and go around trying to produce the problem with firm tapping and wire/cable movments instead of getting stuck at the Donut shop.

 

It doesn't sound like you have a problem with your battery, or it's connections. It sounds like you have either an unintentional ground or a short circuit in the wiring. If you can get a friend of yours to help AND you have a voltmeter, you can have him watch the meter as you turn the lights on and off, or do whatever it takes to recreate the problem-which should be draining voltage from your battery. Then start wiggling wires under the dash in the hopes of finding where the problem is.

I know the older trucks had an instrument panel voltage regulator behind the gauge face, but I'm not sure if they still used them in 89. The dealer would know. Not too important in this case, unless it's fed by the ECM.

Since you noticed the problem affected your lights and your radio, try pulling the fuse for your lights first, if no change, try the fuse for the radio.

Take a look at the connector at the light switch and at your dimmer switch. If they both look OK, try disconnecting them. Is the light switch hot to the touch?

If possible, try disconnecting the wiring feeding your amp gauge, if the gauge went south, it can give you both a goofy reading, and drain power since it's tied to the alternator/regulator wiring.

Make sure that all of the wires going through the firewall have good, uncut insulation on them.

If hitting the dash solved the problem, even if only for a short time, it sounds like an easy repair, but NOT a particularly easy one to find. Good luck!

SL

 

Check the battery cables back into the insulation. I had a very similiar problem a few years back in a mustang, and the cables had coroded inside of the insulation 3 or 4 inches bbehind the battery.

 

Not to be negitive, but since his problem comes and goes, I don't see how corrosion ANYwhere in the cable could be causing his problem. Resistance from corrosion is a relative constant that slowly gets worse over time. It doesn't suddenly correct itself by smacking the dash or letting the vehicle overnight.

SL

 

Shorts and grounds usually blow fuses or circuit breakers or let the smoke out of a component somewhere. Since I have heard of none of the above in this thread, I vote for a bad connection. Does your truck have a fuse block under the dash? I have seen some interesting problems caused by bad connections on fuse blocks. However, I am more inclined to believe it is in a high current wire under the hood or the battery based on the available information. Let us know what you find.

 

Thanks for all the help and advice. Yesterday I pulled the dash apart while the short was occuring. I was wiggling everything I could find. I even pulled the headlight switch out and wiggled it too. I drove around the block hitting every bump I could find, tapping and smacking the dash as I went ( what the neighbors must think). Any how, I wasnt getting anywhere with it until I returned home. I pulled the instrument cluster out as far as the speedometer cable would let me and checked the two plug connectors on the back. When I pushed on the one on the right I looked at the amp meter and the short was gone.

I drove around for a while before I put the dash back together with the system fully taxed. I had everything that runs on electric turned on. I could not get the short to reurn. I put the dash back together and did it again and still no short. So here I am. Problem solved? ummm, lol. It appears that my mystery short is gone, at least I hope.

A little side note: my speedometer needle bounces around a lil now until I get over 25 mph. Looks like I need to pull the cluster back out and check the connector at the speedometer head. I thought I had checked the connection before I put it back together and It seemed tight.

Thanks again for the help!