I don’t know the technical details of how the cluster was setup, so I don’t know exactly how the drivers for the odo/speedo worked relative to each other. It’s possible they have separate drivers within the cluster.

A possible test is to check for AC voltage at the cluster end of the circuit. The engine will run with the cluster removed.

If present, compare the voltage to what it’s reading at the tach signal terminal (refer back to the wiring diagrams). If the readings are the same, the wiring is good and the cluster is still suspect.

If not present [or less than the alt. output] overlay the harness with a new wire, splicing as close as possible to the terminals at each end of the circuit. Solder & heat shrink is highly recommended, but the crimp-type heat shrink connectors will suffice if done properly.

 

I don't know anything about the cluster design either. But it seems beside the point if the repair folks put it on their test rig and it all checked out.

I also noticed that the tach needle comes to zero with key on, but then appears to drive downward when the trucks starts. I'm contacting the repair center again to see if they have any insight on the subject.

I like your test idea, and will keep it in mind. The big problem on that vehicle, as almost every other one, is getting to the wiring within the dash area.

Thanks.

 

Whether it’s DIY or a repair service, mistakes happen. The cluster also could’ve been damaged in transit when it was being shipped back to you.

I know the wiring is difficult to access but that’s why I mentioned that the engine will run with the cluster removed - because then you’re not fighting for access to test.

If the repair service has you ship the cluster back to them, it wouldn’t take much to check the circuit.

 

Well the fact that it works when stuff under the hood is wet to ME says it's not an issue with the cluster. He needs to trace that white wire from the alternator all the way back to where it goes through the firewall. Heck - it could be the plug on the alternator itself that is the issue. In the diagrams I saw, it is in a 4 pin connector that plugs into the alternator/generator. I am sure it goes through some other connectors before it goes through the grommet in the firewall and to the instrument cluster.

I never would have thought of the alternator being used to measure engine RPM, but I guess it makes sense, since the speed it rotates depends on engine speed. And maybe it is just a diesel thing too, since that white wire is just in the diesel wiring diagram.

 

It is just a diesel thing - at least, in 1993 it was. Gas engines of this era got the tach signal from the negative/control side of the ignition coil. The frequency of the coil being grounded (creating & collapsing a magnetic field) would generate a signal relative to engine RPM.

Obviously, diesels don’t have ignition coils. Since alternators generate voltage by creating/collapsing magnetic fields within the field coils, the AC voltage generated can be used to monitor engine RPM in the same way.

Whether it’s gas or diesel, the gauge requires calibration depending on the number of cylinders, and I’m sure there’s some basic differences in the motors used to control the gauge as well.

These days - pretty much since the introduction of OBD 2 systems - the clusters are fully stand-alone devices with maybe a few direct inputs (switches for low washer fluid, brake fluid, etc). They receive all information via data from other modules and simply change the gauge/display as needed to give the illusion of a real-time reading.

 

I have quite a story to tell. The problem has finally been solved!

The solution came about by something said by a diesel mechanic friend of mine. But it made me look at something I did not pay attention to before. Let me explain.

The facts I knew included the following:

• The original owners had an alternator fire shortly before selling the vehicle.
• Some electrical things did not work in the truck; including the tach, radio, and odometer.
• Washing the engine compartment at the manual car wash made the tach work temporarily.
• I was given the pin-out diagram for the alternator plug.
• I eventually found out that the tach is not mechanically driven.

These things will immediately bring to mind a concern about possible systemic electrical damage caused by the fire event. Heaven only knows if that is true, and it is extremely costly and time-consuming to check out each and every circuit to guarantee original operating specs. We will often keep driving a vehicle as long as everything appears to work.

I can observe that the alternator has a big cable connected at the rear (which is the main generated current output to the system) and a connector plug on the side.


In looking at some information gathered from the folks who make the alternator, it included a few diagrams I received from them with a couple in the form of tech bulletins. The pictures received included this (turned to show true orientation when viewed from side of alternator):

Please note the way that the alternator’s connector plug is laid out. We can clearly see that the plug has four pin locations. They are labeled as in the order of P, L, I and S, top to bottom. The P can also be ST, and the I can also be IG. It is important to know what the first three do. Once I focused upon that, everything became clear.

In our research we learn that the tach is not driven by any mechanical connection, as would best and most accurately be done. Instead, it is driven by pulses it receives. Being counter-intuitive to how things aught to be, the stator of the alternator gives off pulses and in this vehicle the stator is the source of tachometer signal. It is a somewhat silly design and is counter-intuitive to what you and I might expect to see; not being directly driven. (This sad fact even mislead another mechanic early on to look at another part of the engine.)

Note that the P stands for pulse and the ST stands for stator, the L stands for lamp, the I or IG stands for ignition and the S stands for sense. This information is key.

Now look at the picture of what I inherited. Note the use of a three-wire pigtail instead of a four-wire one, and the cobbled-together state of the repair. Therein lies the mistake:



Here is what I think happened. There was a fire at the alternator. The pigtail was burned. The people were traveling away from home and had to have a repair done on the road. There was no trust-worthy mechanic available, and so the repair was done as shown by someone. The reason for the three-wire pigtail becomes apparent when one visits RockAuto or Autozone or other parts supplier. We immediately see that there exists a long list of available pigtails for this vehicle, and they are all three-wire, except for one. The prices run relatively low for the three-wire, compared to outrageous for the four. Instead of five to thirteen bucks, the four can run upwards of seventy dollars! Worse yet, ignorance of how this vehicle works can be damning. And so it was.

The ‘mechanic’ who did the repairs made sure to connect the ignition wire to the I pin so that the alternator charge mechanism was excited and able to put out voltage. The tach wire was connect to the L pin, which did nothing. This was extremely stupid.

The solution included staring at the pin-outs of the pigtail, looking in the socket of the alternator and seeing the four pins clearly existing there, and finally grasping what they were for and how it should be connected together. The existing setup was misleading, and I needed to realize that there were only two wires from the truck harness which needed to be used.

(Special mention to @jfmorris for his insight.)

Taking the cobbled-together wire connections apart, and wedging the tach wire under the plug so as to make connection with the pin in the first position, immediately got the tach working.

My friend stopped me from ordering an expensive replacement pigtail by pointing out that plug assemblies can be taken apart and metal wire inserts can be move to the correct pin locations in the plug. I started looking at that and he showed up from his home down the road from me with his fancy set of connector tools. The foam-looking filler at the outside of the plug turned out to be a solid material that was poured into the plug to seal it and looked like something a mud-dauber insect would have made. It had to be dug out, the wire connector released from its position and plugged into the other position instead. (It can be sealed with some silicone later.) No replacement pigtail needed!

The S pin is evidently not used, and there appears to be no wire from the harness for it. I have no idea what the original looked like. (Please add photo to this post if you have one of your turbo diesel setup with tach.)

It is easy to be mislead by what we personally experience or by not paying attention to what we have in hand; whether facts we read or things we can see in front of our own two eyes. The original service manual on CD does not help. There is a dearth of information as well as a lack of clarity in wiring diagrams and sometimes even in mentioning the component in the first place. Learning what actually drives the tach in the first place is hard-won information, but we dare not forget the facts we know.

The alternator manufacturer knows what their connector pins are for and why they exist. However, they may not all be needed. The manual shows multiple dash displays for this vehicle, between gas and diesel, as well as basic and advanced offerings. To make it more confusing, there is a voltmeter and a ‘battery’ icon light on the advance model display on mine, which includes a tachometer. Since I can not remember that the battery icon has ever been seen lit up nor lights up during key-on engine-off, and the dash display was sent off for testing and repair, and since all components were tested on the bench jig (including lights), and since there appears no wire going to the alternator for it, I must assume that the battery light is also unused.

The tach works, as well as the voltmeter. What more do I need? Looks to me like doing proper repairs to the wiring connections will bring that part of the truck’s wiring up to specs.

Let this story be a cautionary tale for every shade-tree mechanic out there. The human fault of becoming fixated on the unimportant is a huge danger to us all. I sure hope my experience will save someone else unnecessary frustration, and instead direct one to quick problem resolution.

 

So glad you got it working!

What you finally figured out fits with what I saw on the wiring diagrams. They show the first 3 pins in use, but not the 4th pin, with the 1st pin being the one that ultimately drives the tach. This is from the gauge wiring diagram section of the manual on the charm.li website:



I guess you backed into that information.

I think that water must have been getting into the plug and shorting things between pin 1 and 2, when you sprayed water under the hood.

 

Yes, you are quite right. You found a connection between the 'generator' (as they call it in the service manual) and pin 121. Mine doesn't even seem to use the second, or lamp, pin. But to confuse it even more, note how they have the plug's holddown clip on the wrong side! It is all so very bizarre and it was quite the journey.