1976 Factory Tach - Dead?

[76LongBox]

Hey guys...

I have a factory tachometer from another 1976 truck that I'm trying to hook up in my 1976 GMC Sierra Classic.  I bought the wiring harness for it, and it looks to be very straightforward, but I'm not sure my tach is any good.

I hooked the pink (12v) wire to one of the open IGN spades in the fuse block, ran the brown COIL wire through the firewall and to the distributor, and hooked the black ground to an existing screw under the dash.  My multimeter shows the correct voltage when probing the pink and black wires at the plug in for the back of the tach (with the ignition on), so I think it's wired right.

However, when I turn the key on, the needle jumps "backwards" to 5000 RPM on the tach.  Does this indicate that the tach is bad?  Or what else could be wrong?  It does not appear that there are any other connections from the wiring harness or anything else that have anything to do with the tachometer connections, so I'm at a loss.

Thanks in advance for any assistance.

[Captkaos]

The back of the tach should have where the wires go marked.  All you really need to test is a tach signal to it.
can you confirm the wires are going to where they are supposed to go?

[76LongBox]

Yes...it is wired correctly....hot wire (pink) running directly from the fuse block to the 12v plug, brown to the coil, and black to body ground.  When the ignition is turned on the needle wraps backward to 5000 RPM.

[sphinx]

I had this happen when testing a used tach.  Seems when the rpm jumps from around zero to more than half the scale the needle moves the shortest path to the indicated rpm.  In a real world condition, the rpm will slowly climb from idle to max not instantly jump from zero to say 5000 rpm.  These older techs are not stepper motor driven, they are more of relative position movement based in a set of energized coils.   So, it is possible for the pointer to move backwards if the signal is applied to fast.  I would check the tact 'filter' to be sure it is cleaning up the ignition signal.  Noise on the tack line may be causing the tech to jump to the wrong location.

[76LongBox]

I don't know anything about a filter being in the line from the coil to the tach.  The harness for the tach is very simple (unless I'm missing something they didn't include when I ordered it).  It has a brown wire that runs through the firewall directly to the distributor, a pink wire that runs to the fuse panel (I plugged it into one of the spades that is labeled IGN), and a black wire with a crimped on connector for running to a body ground (I used an existing location that had one already connected).

This is directly connected to the factory tach, and when the key is turned on it immediately jumps backwards to 50 on the tach (5000 RPM).  Cranking the truck has no effect...it just stays right there at 5000 RPM.

My guess is that the tach is just bad, but I really don't know how to test it to see if my assumption is correct, and I don't really want to spend the money to buy a reproduction that: 1) has the larger sized window for the "fasten seat belt" light, 2) might not even solve the problem.

[bd]

Electrical connections oxidize over time.  Since you installed a complete used cluster, check all the electrical connections very carefully between the tach head, the rigid PC board on the back of the tach, and the new wiring harness connecting to the tach PC board.  A pencil eraser works well for cleaning surface oxidation from the copper foil of the board. 

Visually inspect the components on the tach circuit board for evidence of heat damage (burning).  The missing tach filter should not cause the symptoms you described.

[76LongBox]

Thanks bd for the tips....

However, I don't actually have the new (used) cluster installed.  There is no cluster at all right now, so it's just the tach that I'm concerned with.  I did take the circuit board off of the back of the tach, so that I could get access to the connectors and clean them up a bit with emery cloth.  The harness for the tach is brand new and the plugs are nice and bright, but it still does it.

I really think the circuit board on the tach itself might be bad. 

[76LongBox]

Well, a little more looking with a magnifying glass, and now I'm certain that the circuit board on the tach is shot.  I found that one of the little resistors (maybe?) I actually broken.  Not having a good factory one to compare with, it appears that someone tried to fix this at one time by putting a jumper wire from one end of the broken part to one of the plug ins.  It looks like it's been there a LONG time just based on how stiff the wire is and also the appearance of the solder points.  I think it's safe to assume this thing is shot.

I guess I'll be looking to get another one.

[bd]

Can you post a close-up pic of the damaged component that clearly shows body shape and color bands?

[76LongBox]

Not really....Especially since I kinda messed it up more.

It was a little black part that has no numbers or color bands or anything on it.  There are 2 more just like the one that was busted and a couple of which are a little longer and have the number 052 80E on them. Using a large lighted magnifying glass, I can find no numbers or anything on the one that was broken (or the other 2 that look just like it), but it was completely cracked in two pieces.

[bd]

Black body with a silver band at one end?

[76LongBox]

Black body, but no silver bands on either end of it.  The slightly longer ones with the 052 80E number have that silver band, but the others like the one that was busted do not have any markings or color on them at all from what I can tell.

[Captkaos]

Generally when they start to go bad they register high and when the go bad they either don't register or peg out immediately.
Is this a points or HEI distributor?  If points is the TACH wire on the negative side?

[76LongBox]

It's HEI.

[76LongBox]

Just in case this might help the next guy, I found a place called Willcox Corvette who manufactures replacement boards for our trucks.  While they are not "cheap" @ $95, they are still less expensive than buying a reproduction (around $130), and a lot cheaper than sending it to tachman.com for a new board ($175). 

I just ordered one to fix my tach, and will report back once I receive it and get it installed. 

Their website is willcoxcorvette.com.  The part # is 975052T.