Upgrading to an Instrument Cluster with a Tachometer

[cc333]

When I was researching here and elsewhere earlier this year, my limited understanding is that ideally the gauge set has to come from a truck with the same engine and year to ensure maximum compatibility, but at a minimum, if it's within a year or so of the truck it's going in, and comes from a truck with at least the same kind of engine and number of cylinders, it should work mostly fine regardless of engine displacement (about the only thing is that the redline is in a different spot depending on what size V6 or V8 the truck has).

As for whether a gauge set from a diesel would work in a gas truck, I'd think that, aside from where the tach signal comes from and a few minor cosmetic differences, the features that are common among the gauge sets in all models (speedometer, voltage, temp and fuel, and most indicator lights) would work fine regardless of what truck they're put in, provided it's close to the same year so the wiring harness(es) and mounting points match.

c

 

[cc333]

OK, now that I'm back to this thread again, I can share the good news:

The new gauge cluster works!

I finally got around to installing it last week, and after zeroing out the gauges (basically by pulling the needles off and plunking them back where I thought they should be, using the old gauges' positions as a rough guide), everything seems to work as they should.

The speedometer, after I got the needle close to where it should be, reads about 1 MPH slow, but it's consistent across it's whole range, which is an improvement over the old one. It does waver a bit around the 50 MPH mark, which seems odd. Can I fix that with a drop of oil in the gauge's "motor", or does this mean it's on its way out?

The acrylic lens is all scratched up, so I ordered a replacement, which by some miracle I actually found. Next I'll do the illumination bulbs and reconnect the odometer (I forgot to do this before test driving it, so now it has about 12.5 miles that are unaccounted for), and it'll be all set!

A few questions about the tachometer, though:

1. What RPMs are normal for a 454 TBI? Once it's warmed up, it reads about 800 on the tachometer when idling in gear. In park/neutral, it reads about 950-1k.
2. At what RPM is the stock red line for the same engine? I think on this tachometer, it's somewhere around 5k.
3. How do I know it's even close to correct? Is there a cheap/easy way to check engine RPMs some other way to check the tachometer's accuracy?

It doesn't have to be perfect, as I only wanted it as a guide.

c

 

[someotherguy]

In the old days of points ignition, it was very common for a tech (we called ourselves mechanics, back then) to keep a tach/dwell meter in the toolbox for setting points. In other words there's an actual tool out there that will report the RPM to you so that you could compare it to your gauge cluster to determine if it's correct.

I can't recall the GM spec for idle speed on a warmed-up 7.4 TBI but it's probably around 700-800 in park. If your fan shroud emissions label is still legible, it should be on that.

Richard

 

[PlayingWithTBI]

I can't recall the GM spec for idle speed on a warmed-up 7.4 TBI but it's probably around 700-800 in park. If your fan shroud emissions label is still legible, it should be on that.

Yeah, 750 in gear. We were recently discussing this on another thread for an 88.

 

[cc333]

OK, it seems to read about half way between 800 and 900 in gear, so I's reading roughly 100 RPMs high. Oh, well.

I got the new acrylic lens installed, and now I'm preparing for my fimal upgrade step: the LED illumination.

Once that's done, I'll move to the steering column filler (the panel that's directly below the steering column). The original broke when I hit a particularly big pothole a couple months ago (the same day the alternator died, actually). I suspect it was because the trailer brake controller was bolted to it rather crudely using a pair of self tapping screws.

Rather than make that same mistake again, I'll have to figure out some other way of mounting the controller that doesn't stress the new part I just bought. Some kind of metal bracket, maybe. But where do I mount that?

Anyone got any ideas?

c

 

[DerekTheGreat]

Verify RPM with a scanner before you do anything. When I was new to the GMT400 game, my trial-by-fire vehicle was an '88 C3500 with a 454. The needle cluster I swapped in read 1000 rpm when hot idling, yet was only 800 rpm as indicated by the scanner. I didn't realize that though, and fought the dham thing for weeks. Lesson learned. Actually, my buddy said something which got me thinking, "Does it sound right?" Yep, it sure did. Did not sound like it was idling high at all. Got a scanner and then verified it.