Hey guys, my 1990 F-250 with the IDI is having some strange electrical gremlins, but we can't trace them.

Here's a list of the problems and solutions if we tried any yet:

Tach doesn't work (always works when engine bay is wet). We tried two different tach sensors but it changed nothing. We also traced the wire continuity from the sensor to the ECM and it was fine. The gauge bounces around at high RPMs, messing up the shift points, but during normal driving, the tranny isn't in limp mode.

Coolant gauge: occasionally works. Yesterday when I was driving, changing the tank selector switch made it move from the bottom of the gauge to a more normal running position.

Tank selector switch: rarely works (not to change the tank, but when it does work, it changes the fuel gauge from empty to whatever is in the other tank.

Any help is appreciated!

 

Loose, missing, or neglected grounds and cables can do that kind of thing.

 

Is there a list somewhere of all of the grounds in the wiring harness?

 

Has the engine ever been pulled? It's not uncommon for someone to forget one. You can sniff out a lot of 'em simply by eyeball. Anywhere a cable or wire bundle connects directly to frame or chassis, engine block, frame, etc. The fact that "works when wet" is a tell, that something is corroded anyway.

Everything needs a solid electrical ground reference. Real important. Especially with sensitive digital computerized gee-gaw sensors and solenoids and the rest of it. It pays to go through the grounds and cables, and the multipin connectors and such. 1990 was a long time ago.

Disconnect battery/batteries (good time to clean them,and the battery box, put on a charger while you do this. Hell, wintertime is pretty much in a week or two out there right?)

Grind any grounding point down to bright shiny metal, coat with NO-OX and tighten securely. I've seen these ground point connections get corroded in just a few years.

 

I don't know what NO-OX is. Should I use dielectric grease?

The batteries are about two months old and I cleaned the terminals before I installed the new ones.

I hope it's just the grounds as that would mean no expensive wiring solutions.

 

Dielectric is more of an insulator, NO-OX is a thick conductive electrical grease used for years by the phone company and railroads and such, it holds up better too.

The idea is to get a clean, bright shiny electrical ground connection and tighten securely, and have it sealed off from corrosion, like road spray grime and atmospheric pollution.

Dielectric is good stuff too, but it's proper use is to seal off around O-rings or packings and connectors from allowing grime and moisture in the connector. You can also buy fancy boutique electrical contact cleaners, but they are mostly petroleum solvents in a spray can. Naptha (lighter fluid) for example is a non residue cleaner that dissolves grunge and grime. Even the much despised WD40 will clear out skunge and displaces moisture, exactly what's needed.

Oftentimes simply disconnecting and reconnecting a plug or connector will remedy a "glitch", the NO-OX or dielectric and so on, is to help keep it from returning.

 

So do I just go into any auto parts store and ask for NO-OX?

 

I wouldn't worry about that part of it too much right now, see if you can find a schematic or wiring diagram of the main ground and accessory points, but when you have the time get out there and clean them up. Probably some ground points too, under the dash, behind kickpanels and such as well. Check SAE or weather-pac connectors and such, for evidence of corrosion on the pins and so on. Very common issues. Everybody knows about the battery cable terminals and posts, but the other ends of the cables and so on are just as important. They can be a real bitch to get at sometimes.

 

Went and wire brushed the whole connector, screw and surface of every ground I could find in the engine bay. This is me after doing it:

No change...

 

Have you tried some wiggle the wires testing? That and I'd focus on the "works when wet" angle, something is loose or corroded and the water is temporarily bridging a connection or something like that, for that to happen.

 

No, I haven't because it's only me . Any idea of which part of the wiring harness carries any tach signal? What are the chances water is seeping through the firewall into the dash every time it rains?

I have one other idea...take it to a car wash, make sure the tach is running and then just figure out where there is water on the harness and go from there?

 

No, you want to avoid that.. kinda the idea, but that could get outta hand.. permanently damage something if there is a major leak, maybe just get an old empty Windex spray bottle, and maybe try that? Intermittents can be a real pain. Some Fords have problems with water infiltration through windshield gasket or cowl or something, onto the GEM (general electronic module?) Not sure about your rig on that kind of thing.

Don't forget to use your mad internet search skillz. What I've found with tech issues, if I've had a problem with something it's very likely others have too. If you dig around carefully somebody else on other forums may have discovered the solution. May have to broaden the search terms to include other years and models, but still in the same "family". The intermittent tach and "works when wet " would likely be one that has hit others too.

 

I am currently chasing a similar problem on my '01 but the speedometer quits when wet, all the buzzers sound, the heater motor comes on along with the radio and if I am late to the show the battery runs down. All without the key. I believe it is water on either the ecm or pcm which also causes the OD light to flash as if there were a transmission error. This happened about 10 years ago but not every time it rained. Rock into the windshield and got new windshield and the problem stopped. A few months later talked to a friend who installed windshields and he said it had to do with a seam in the body at the windshield opening. The adhesive separates slightly and then water runs in and wets down the ECM. I am thinking my current problem is with the PCM which I believe is located behind the front left fender near the windshield column. This means removing the inner fender liner just to get a look and rust is my enemy. 2017 on order but I already sold the '01 to a friend and try to keep it up as I have always done in the past.

 

What about the electrical connector associated with the parking brake lever area in some Fords, is that a factor too on these coal burners? Well known problem causing dash issues. The shock of releasing the lever causes wire harness to become intermittent over time or somesuch.

 

My parking brake has been stuck since I got the truck. We had to cut it to service the drums.