1994 3.0 4x4 manual trans pickup with around 275k miles. Here is my saga with electrical issues.

Last winter I'd say Sept. my truck started blowing the instrument cluster fuse. It started by blowing once a month, then once a week, then constantly. I put a higher amp fuse in to see if I could figure out what was shorting but that started it smoking from somewhere near the ignition switch. Not wanting to deal with it in the winter I pulled the fuse and kept driving with no gauges. This spring I tried to deal with it by replacing the ignition tumbler since that was my best guess, I could pull the key out while the truck was still running so seemed likely. I found a great kit on Amazon, $99 for tumbler, and both door locks with 2 keys and a keychain . My door locks were also shot so I installed all that, happy day. Unfortunately this did not solve the issue. I replaced the negative battery terminal and this improved the situation a bit, instead of blowing the fuse straight off it blew after a minute. I had to drive from NC to CO and back since I'm moving there, drove without gauges using GPS for speed and carrying extra gas. No problems truck ran like a champ. In August I towed the truck back to Colorado with a RV that I needed to get there anyways, that was its own set of issues with the RV carb overheating but not really related. Made it. In Colorado I put a new fuse in, now the 4x4 light would stay on even in 2 wheel, although it flickered in 2 wheel and was solid in 4. Found out the guy I was subletting my NC spot to decided to stop paying rent, and after a angry call from my landlord I hauled ass back to NC to deal with that issue. I pulled the 4x4 sensor wire off the transfer case hoping that was the issue but no, still blowing the fuse although that fixed the always on 4x4 lamp obviously. On the ride back it got cold and I wanted some heat so I put the fuse back in (correct amps this time) and miraculously it didn't blow immediately and I got to enjoy some heat while cruising for the first time in almost a year. I noticed that the fuse while sometimes blowing for no reason would always blow if I put the truck in reverse so I avoided reverse as best I could. Got back to NC and had to throw out my friend who thought not paying rent was ok since he's a poor musician trying to make it ect ect. On the trip back the CEL started to come on intermittently after the speedo quit (needle jumped a few times then stopped forever) and I pulled the code which was 42 which made sense, code 42 is for no speed signal. I ordered a new VSS and tried to install it today, but somehow got the wrong one even though it said it matched my vehicle, perhaps it was for a automatic it was skinnyer then the one coming out. While it was out I completely dissasembled the VSS and I can't see anything obviously wrong with it other then being full of oil, unsure if thats its normal state. I put it back together and reinstalled but speedo is still broken. CEL is still blinking on and off while driving so I pulled the codes again, this time I'm getting 14,21 and 42 still. Truck still runs strong and I'd just go back to CO as-is but my registration sticker is expired now and NC requires a safety inspection every year and won't pass a vehicle with a CEL on. My issues are further compounded by the fact that I put in notice with my landlord and I'm supposed to be out by the 15th, although I could probably get a extension I really want to get out of here and get back to Colorado. I already moved a bunch of tools out there so I only have a skeleton set and stupidly I left my voltmeter there so I can't check much but I may be able to borrow one in a few days.

Sorry for how long this post is and how its kind of a diary entry the whole saga is intertwined in my mind.

My best guesses right now are there is still a short somewhere thats causing all this headaches, or the cluster has blown a cap or something or the ECM is going bad. I don't want to start dumping money into guesses so I'm hoping someone here can recognize a pattern that I'm not seeing in all this thats why I wrote it all out with as much detail as possible. The truck still runs strong except the idle now varies from so low it might stall to reving high, although it seems to be stuck in high lately. No other problems as far as I know.

Thanks for any help I have a week to sort this or I might try to disconnect the CEL idiot light, the safety inspection doesn't include a road test so they shouldn't know the speedo is broken they really just check lights tires and brakes. I'm headed back to CO with lots of expensive glassblowing tools and instruments I had in storage here so I'd really like to figure out the issue(s) ASAP and avoid a breakdown. Its been a tough summer vehicle wise and the truck has been rock solid since I bought it 7 years ago but I guess everything gives you problems eventually.

So thats where I'm at thanks for any best guesses or things to try.

EDIT: I *think* this might be a CA model pickup it has a insane amount of vacuum hoses that I'm guessing are emissions related although I'm not sure. Its got factory air but thats never worked and I pulled the belt to the compressor a while back when its bearing started squealing.

 

This part makes me think it could be the sensor for the reverse lights in the transmission.
The fuse for the gauges is also shared by the blower motor, 4x4 sensor and the reverse lights. The sensor for the reverse lights has a habit of shorting out internally and failing.

Try disconnecting the plug for the reverse light sensor (passenger side of trans, 1/2 way up), and jump the connector with a short piece of wire or paper clip.
Next put it in reverse and if the fuse still blows, its a short further down the line. If the reverse lights now work, then time for a new reverse light switch.

 

Ok iI'll try that tomorrow when its not raining. Thanks.

 

Ok, I replaced the VSS and the reverse light switch. Neither the speedo or reverse lights are working. Reverse lights stopped working at some point before I replaced the RL switch but replacing the switch did not fix it. Check engine light still coming on and off, on and off. I checked both bulbs on the tailights for the reverse lights both fine. Blew the fuse for the turn signals but replacing it seems to have fixed it. I'm at my wits end with this problem.

Thinking about ordering a used instrument cluster and ECM and just throwing money at the problem to see if that does it.

Is there a easy way to check what the alternator is putting out? That was my other idea maybe the alternator is putting out too much juice and is causing all this havoc.

Any ideas gratefully appreciated.

 

So I thought I'd give everyone a update:

Last week I was driving, still with the same problems and a relay under the dash started clicking crazy fast like a geiger counter (3.6 Roentgen Not great, Not terrible) then the 4x4 hubs started locking and unlocking with a huge THUMP! I pulled over and determined which relay it was, it was a relay under the dash labeled as a fuel pump relay but actually for the locking hubs. I pulled it out and kept driving. Today I found a parts truck near by and got a good deal on the relay and the instrument cluster, although the new cluster was the simple one so I lost my tach and some other gauges. I pulled the old one and the back of it was TOAST, burned copper lines on a bunch of the back. Amazing that only the speedo wasn't working and everything else was. I put in the cluster and everything seems to be working but the speedo still doesn't work. I haven't driven it enough to see if the CEL comes back on because of the no speedo but I'm betting it does.

My best guess now is that the faulty relay caused a short that burnt out the speedo circuit on the cluster which shorted the wire to the cluster since I've replaced the cluster and the VSS now. I guess I'm going to rewire the wires to the VSS from the cluster since I can't find anything obviously shorted.

Thanks for any help or ideas to try. Could this be the ECM or another relay somewhere?

 

That seems really unlikely. The relay under the dash that controls 4wd is the "ADD Control Relay," a SPDT relay. (I can't explain why it might be labeled fuel pump relay. Automotive relays are usually fairly generic, so someone could have manufactured a SPDT relay and just perversely labeled it that way. There is no "fuel pump relay" in your truck, the fuel pump is controlled by the COR and the EFI relay, neither of which would work as the ADD control relay.) That relay is actuated when the 4wd switch connects to ground. So the clicking you heard was almost certainly in the wire to that switch shorting to ground. While the 4wd switch also controls the light on the Combination Meter, it just grounds it. A faulty ADD control relay couldn't add any current to the Combination Meter. Your meter was scorched because there was a short IN the meter. That short usually blew the fuse, but as you kept replacing the fuse the short did more and more damage.

Since you have the electronic VSS1, if you have the correct sensor it will satisfy the ECM, even if the new combination meter doesn't read it correctly. Are you sure your new combination meter is for an electronic speedometer?

Not likely. No relay is causing your smoking, or your codes. The ECM is pretty much the very, very last thing to fail.

You almost certainly have a short in your wiring, probably in several places. Since the 4wd switch is on the transmission tunnel, I'd start there. That could explain why shifting to reverse caused the 4wd switch to short out. It's even possible a short in the VSS1 wiring (part of which is in the transmission tunnel) damaged part of the combination meter.

Last, I learned a (obvious) trick from the YouTube South Main Auto channel. Rather than burn up a box of fuses, replace the fuse with a relatively high current lamp, like a headlamp bulb. As you wiggle the wires and connectors, when you make the short the lamp will light rather than blow a fuse. The lamp also limits the current, so you're less likely to set your garage on fire.

 

Thanks for the reply. I've already spent some time looking at the wiring on top of the transmission looking for obvious burnt spots, I've found nothing. I neglected to mention that this issue burnt out the reverse lights completely at some point, I added a fresh wire from the rev light switch to the back taillights and got the reverse lights working again.

How possible is it that the instrument cluster was the source of all this shorting (it was seriously burnt, I'll see about adding a picture tomorrow) and that burned out the reverse lights, the speedometer wire and was causing the relay clicking and hubs to lock/unlock? I don't think the new cluster had a input for a cable but I'll double check. I'm considering running new wires for the VSS since that approach fixed the reverse lights but I don't know which wires are the correct ones to splice into on the cluster. Hopefully the wire colors stay the same from the VSS to the cluster.