78 Burb - Original Tach Issues

[BlackTomC]

Anyone have any ideas why when I press my horn, my tach needle drops a bit, then when I let up, the tach needle goes back to it's original position?

PS - Tach doesn't work right, reads really low, unless I hose down the engine bay, then it magically works for a few hours (already replaced the distrubitor harness for it, no difference).

Thanks peeps.

[Irish_Alley]

still sounds like a poor ground. i know on my 91 i have a ground that runs from the aldl box (obd1) dracs box, pcm and speedo. its runs through the fire wall connection and turns into an engine ground. i dont know where it was connected but i know without it the speedo doesnt work. its just indicated as a black wire.

for those just tuning in here is his original post
http://forum.73-87chevytrucks.com/smforum/index.php?topic=30685.0

[BlackTomC]

Alright then, this weekend I'm ripping out all the wiring for the tach and redoing all of it. Stay tuned.

All the wiring

[Irish_Alley]

how do you have the ground and power ran?

[BlackTomC]

As it sits now, I've got 3 wires running out of the back of the tach

Brown is running straight down and out the firewall to the distributor.

Red/Pink is running down to the fuse box for power

Black is running to this 6 prong deal for ground (common ground is what I've heard it referred to as)

[BlackTomC]

That's off the top of my head, I'll revisit it when I get home just to make sure.

[Irish_Alley]

do you have another tach that you could hook up and see what its reading? the only other thing i can think of is running a new ground to the engine or directly to the battery and see if it helps your problem

[BlackTomC]

Unfortunately I don't, but as of now, the plan is to rip the tach out and manually run 3 new wires to it (1 ground, 1 power from battery, 1 straight from the distributor).

[blazer74]

Check the ground strap from the left rear head to the firewall and ground from batt to rad support.


Sent from my iPhone using Tapatalk

[BlackTomC]

Alright folks so I replaced the pigtail connecting the tach and distributor, ran a power wire straight from the tach to the + terminal on the battery/ and a wire from the tach ground to the battery negative. So far it's still reading low.

I started poking around the fuse block a noticed the pink power wire from the tach was plugged into the spot in the fuse block for the horn (probably explaining why the tach bounces down anytime I press the horn). I ended up moving it down 1 fuse position in the fuse block, so the tach still gets power but now the horn doesn't work at all. Anyone have experience with how this is is all supposed to work? I'm confused as heck at this point.

Also the pink power wire from the tach has a plug that plugs into the fuse block, HOWEVER it also has a wire coming off of it to some weird looking female connector..... also, can't seem to locate any wiring for the horn. Pls help.

[bd]

Conventional ignition or HEI?  Does the brown tach wire connect to a filter on its way to the distributor?  Crimp a new terminal end on the signal wire.  Is the wire connected to the negative coil terminal?

Check the fuses and make sure there is no oxidation or corrosion between the metal caps of the fuses and the fuse box clips - a common problem.

Horn wiring connects through a 3-prong relay that hangs out of the harness under the dash.  The horn button grounds the relay to energize the horn.

If you want help with the "weird looking female connector," post a pic.

[BlackTomC]

From what I know, it's got an HEI distributor. The brown tach wire connecting to the tach has a small little spacer on it, not sure if it's a filter or just rubber to protect it passing through the firewall.

Already crimped a new terminal on the end of the signal wire (pigtail going into distributor) (now has green and red wire coming off the distributor), didn't seem to help the issue at all. I did notice however when I rev the engine, the tach reads wrong until I let off the gas, then the RPM's shoot up, then back down.

From what I can tell, there is only one wire running from the distributor, to the tach, a brown wire.

Will check horn fuse in daylight tomorrow, would you happen to have a diagram pointing out which fuse belongs to the horn?

Picture coming soon.

[BlackTomC]

Can't upload high quality images here, 256kb slaughers image quality, uploaded to Dropbox, click the link and all the images should show up.

https://www.dropbox.com/sh/9v1qq8r8lwqlfq5/AAD5b8jt7LLaP3GzG1Uw0d3ea?dl=0


Not sure why you think it slaughters the quality, but this is the same pictures resized to 800x600...  Looks fine to me. - Admin

[BlackTomC]

As of now, the tach works (somewhat, still doesn't read accurately, reads low).

Horn doesn't work at all now, it did when I had the pink wire plugged into the socket above where it's at now, the socket that says HRN.

I think I understand this now, before I plugged the tach into the right spot, the horn was grounding out through the tach, causing the needle to dip... now onto where togo from here.


-This is the other 2.. -admin

[bd]

At the top of your 2nd image, above the gold flasher, is a little black box with red, green and black wires attached.  That black box is the horn relay.  Upon inspecting the wiring diagram, there is no fuse for the horn, so don't waste your time looking for one.  The red wire from the relay connects directly to battery through a fusible link at the firewall junction block over the back of the engine.  The green wire connects to the horn(s).  The relay energizes when the black wire grounds through the horn button in the steering wheel.  Does the relay "click" when you press the horn button?  Momentarily jump the red to the green and the horn(s) should blare, bypassing the relay entirely.

The 'weird looking blue connector' on the short pigtail is just a convenient location to plug in another accessory if the fuse box sockets are occupied by other connectors.

The pink power wire for the tach is located in the proper fuse box cavity in your image.  Interesting enough, the socket directly above the pink wire (where the pink wire was previously located) is electrically identical - that is both power tap sockets are protected by the same fuse located immediately beneath, the 15-amp "B/U LPS   DIR SIG" fuse.  That pair of 'stacked' cavities is marked "IGN FUSED" not "HRN."  The horn has no electrical connection with the fuse box.  Hence, this...

...before I plugged the tach into the right spot, the horn was grounding out through the tach, causing the needle to dip....

...is unlikely.  However, voltage loss due to other causes is possible.

Since you have already thoroughly checked the vehicle grounds (correct?), check the 12-gauge red wires and associated fusible links connecting to the starter solenoid 3/8" battery lug and the firewall junction block.  Give a gentle tug on the fuse links - if they stretch, they're toast and need replacing.  Any less than perfect connections in the main feeds entering the cab can cause a loss of voltage to the fuse box and distributor.  The greater the current demand of an appliance, such as the horn, the greater the voltage drop across an imperfect connection.  Don't make any assumptions when checking for poor connections and voltage loss.  The best method for determining voltage loss employs a volt meter.  Do you have one?