Upgrading to an Instrument Cluster with a Tachometer

[cc333]

I'm considering upgrading my K2500's stock cluster, which is the speedometer-only version, to that with a tachometer.

However, I don't know what, if any, compatibility issues there are, given that there was a fairly wide range of models that used various different engines, both V6 and V8. I have read that a tachometer cluster from a V6-equipped truck isn't necessarily interchangeable with a V8 truck, and vice versa, so it would seem that to ensure at least a modicum of compatibility, I should at least try to source a cluster from some sort of V8 truck, except if I'm not mistaken, there were several different V8's offered in addition to the 454, so full compatibility is still not guaranteed.

To that end, I'd like to know if there is a way to know for certain which engine a particular used cluster is tuned for. In other words, is there a specific part number I can look up which specifically denotes a cluster with tachometer for a 454-equipped truck? Barring this, is it possible to "re-tune" a tachometer to match my engine?

Secondarily, I'm curious about repairing, rebuilding or replacing the gauge motors in my current cluster, if only to extend their useful lifespan (and improve accuracy if possible) should I decide not to upgrade to a tachometer cluster. If anyone has any useful information on how to do that, I'd appreciate it.

Thank you!

c

 

[AK49BWL]

Taking apart the gauge cluster for motor access is actually really easy - there are four screws on the outside edges of the black bezel, two also hold on the bottom of the clear cover. Remove all those, then you can remove the bezel. From there, unplug the odometer motor cable from the back, then carefully start around the outside edges of the front and pull the gauges away from the white surroundings. The gauge motors are plugged into the circuit sheet on the rear so it won't just "fall apart", but the front face will come off with a little pressure. To replace the individual gauge motors, you'll remove two screws per motor, pop the needle off it, then it just comes out.

As for finding replacements, welllllll...that I can't help with lol

 

[cc333]

I see, that's helpful, thanks!

Is there something I can do to improve the old motors, like maybe a drop or two of some sort of lubricant? Will a lack of lubrication cause accuracy issues?

On that note, to touch on the speedometer accuracy issues I posted about last week, exactly how do I reposition the needle without knowing precisely where I'm putting it? For example, it reads 58 when I'm going 55; how do I reposition it to read 55 without the speedometer actually registering a speed? Am I making any sense here??

c

 

[AK49BWL]

You make sense. The only way I can think to do it would be to do it while driving with a GPS. That's how I did mine anyway... Just install it without the clear face.

Not sure how or if lube affects accuracy. I do know some guys use a sort of grease or something to slow down jumpy fuel gauge needles, but even that I don't know much about haha

58 at 55 really isn't that far off when it comes to the GMT400 trucks, either... That's about what mine reads.

 

[cc333]

Hmm, OK. I guess I'm used to all my other cars which read pretty much dead on (ie, 55 on the gauge = actual speed). Why would GM sell trucks with a speedometer so prone to inaccuracy? It seems like it could be a potential safety issue....

Oh, well. I'll see if I can get a cheap enough replacement non-tachometer cluster and play around with it until I know what I'm doing, so I don't wreck the one currently installed.

I have found what appears to be a tachometer cluster on eBay, and checking the VIN of the truck it's from (which the seller helpfully provided), it would seem to be for a 5.7L V8. Would that work with my 7.4L?

c

 

[DerekTheGreat]

Yes, it'll work. What year is your truck though? If it's a 1990.5-1991 you could have the rare unicorn cluster. which accepts the same harness as the '88 to 1991 trucks. Trouble with those is the internal DRAC module which processes the speed signal from the trans. If it's a '92 to '94 those are all interchangeable with no extra work. My wife has a '92 and we popped a cluster with tach from a '93 with no issues. Oh, I think '94 is the magical year where the oil pressure reads " 0, 40, 80 " where as earlier years are " 0, 30, 60 " so that's something to be aware of.

 

[Erik the Awful]

Why would GM sell trucks with a speedometer so prone to inaccuracy?

Tire sizes aren't exactly standardized, and inflation pressure can also affect diameter. Also, don't expect perfection from a truck that's over twenty years old.

 

[R422b]

Tire sizes aren't exactly standardized, and inflation pressure can also affect diameter. Also, don't expect perfection from a truck that's over twenty years old.

You know what I never thought about that. I like to keep my rear tires at 40 PSI on my Tahoe when I have it loaded heavy and I always wondered why my speedometer reads a little slow.

Talking about GMT400s using Tapatalk

 

[cc333]

What year is your truck though?

Mine's a 94, so I guess I'm lucky, as the 92-94 clusters are apparently much more common.

don't expect perfection from a truck that's over twenty years old.

I don't. I just wasn't expecting to have a weird speedometer.

c

 

[DerekTheGreat]

Then you might want to locate a cluster from a '94 truck, if you want it to be correct.