Factory tach not working

[fallguy]

Found a factory tach cluster on eBay. Putting it in my '86. Everything works but the tach! Bought new wiring harness from LMC for tach. One wire to distributor. One wire grounded. Other end had pigtail that didn't plug into my harness so cut off and spliced on male end plugging in directly to fuse panel. Plugged in to back of tach....nothing.

Thoughts? Bad tach?? No clue if it worked when I bought it, but was told it did.
Posted via Mobile Device

 

[J Knight]

Lmc now sells a replacement tach for original tach clusters.

 

[chengny]

Any tach needs 3 connections. A hot wire, an impulsed signal from the distributor and a ground.

These tachs (like yours) are not part of the printed board they have a separate plug type connection - like the OEM clock.

The connector should be in an "L" shape.

You must be registered for see images attach

The top of the long side of the L is the ground stab- run a lead to the common ground buss under the dash.

The connection under the ground stab is for a 12VDC supply - use any of the add-on sockets in the "I" row of the fuse block.

The last tab takes a signal from the distributor. Notice that like the other two tach leads in the dwg above, the white wire (121) is shown as a dotted line. That means it does not exist in your truck - unless it came with a tach. You will have to run a wire.

Next to the pink wire connected to your distributor (that is switched power to the coil primary) is a white wire. That sends an engine speed signal over to your ECM. If you are installing add-on/aftermarket tach, you will need to splice a lead into that white wire and fish it through the firewall and up to the tach.

You can make the splice at either the distributor connector or at the computer. If you aren't using your ECM just **** it off at the box, pull it back out of the sheathing and extend it.

 

[fallguy]

Any tach needs 3 connections. A hot wire, an impulsed signal from the distributor and a ground.

These tachs (like yours) are not part of the printed board they have a separate plug type connection - like the OEM clock.

The connector should be in an "L" shape.

You must be registered for see images attach

The top of the long side of the L is the ground stab- run a lead to the common ground buss under the dash.

The connection under the ground stab is for a 12VDC supply - use any of the add-on sockets in the "I" row of the fuse block.

The last tab takes a signal from the distributor. Notice that like the other two tach leads in the dwg above, the white wire (121) is shown as a dotted line. That means it does not exist in your truck - unless it came with a tach. You will have to run a wire.

Next to the pink wire connected to your distributor (that is switched power to the coil primary) is a white wire. That sends an engine speed signal over to your ECM. If you are installing add-on/aftermarket tach, you will need to splice a lead into that white wire and fish it through the firewall and up to the tach.

You can make the splice at either the distributor connector or at the computer. If you aren't using your ECM just **** it off at the box, pull it back out of the sheathing and extend it.

LMC wiring harness has 3 wires:

1. Brown to distributor
2. Black ground
3. Red to fuse panel

Tach still won't respond.

 

[crazy4offroad]

Make sure you are getting plugged in correctly at the distributor. It is impossible to see where the connector goes so it takes a large measure of "feel".

 

[Green79Scottsdale]

I think that LMC harness was created to go specifically with their new tach they offer. It may or may not work with your original.

I don't know electrical that well, but the wire you put to the fuse panel is getting powered correct? It should be an "ignition on" type power supply. Also, where did you make the ground connection? I have heard to not ground it under the dash, but instead to make the ground on the chassis. This gives it a better ground condition compared to under the dash. Again, that's just what I heard.

edit: Here is a pic of the harness from my '79 tach. Not sure if it will be helpful to you, but it will at least show what chengny was explaining.

You must be registered for see images attach

COIL - brown wire
12V power - red wire
GrouND - black wire

 

[fallguy]

Ok, here we go again.

New tach from LMC.
New tach wiring harness (HEI) from LMC

Brown wire (connector on harness verified sound) to firewall (back outside) port on distributor.
Pink wire to secondary "iGN" spot in fuse box. (Verified hot due to accessory flood lights plugged into other half of same port...lights work when turned on.
Black wire connected to one of the gas pedal mounting bolts.

Turned on for first time...tach twitched (old one didn't even do that).
Turned over engine...tach jumped up to 2k and then back down to flatline

What The Hell!?!

 

[chengny]

Brown wire (connector on harness verified sound) to firewall (back outside) port on distributor.

Not sure what this means, but the impulse lead to the tach needs to go to the white wire that comes out of the distributor cap. That is the front terminal and the cap is labeled "TACH":

You must be registered for see images attach

You must be registered for see images attach

 

[MrMarty51]

Brown wire (connector on harness verified sound) to firewall (back outside) port on distributor.

Not sure what this means, but the impulse lead to the tach needs to go to the white wire that comes out of the distributor cap. That is the front terminal and the cap is labeled "TACH":

You must be registered for see images attach

You must be registered for see images attach

Sometimes,not always,there will be a white wire at the coil that even has a terminal end on it,just for us tach guys.

 

[fallguy]

HEI makes the tach plug pretty simple as its the only port on the cap that is open and labeled "Tach", but I will check for an accessory wire to maybe plug into... I'm grasping at this point...

 

[fallguy]

Ok, ran test light. Started truck, probed TACH port on distributor...lit up. Probed fuse panel where I have it hooked up...lit up.

Up next...take apart dash...again...and probe other end of pigtail on back of tach?

 

[Green79Scottsdale]

Try a better ground point. A gas pedal mounting bolt probably isn't good enough. When I was reading the installation instructions to my SunPro Tach, it states... "Although electrical ground is available under the dashboard, grounding the instrument (the tach in your case) near or under the dash may cause it to operate erratically, as any ground connection other then the Negative battery terminal may be "electrically noisy" ."

edit: The part right before that says.... "Connect the black wire to the negative battery terminal, or a clean unpainred chassis ground..."

 

[fallguy]

Solved

Installed new tach after a final verification that the wiring was good. Didn't work. I made up a few words you haven't even heard yet! Started playing with the two tachs, comparing new circuit boards to old tach (found a dark did colored spot on old...it was fried).

Long story short, the circuit board can only go on one way. Then you put the little washer over each of the 4 prongs, followed by the little lock washer, then the nut. Next put on the plastic backing that screws into the cluster housing by 3 screws...then plug in wiring harness to the three prongs sticking through the backing. Screw plastic backing into housing, then screw in the tach itself.

My mistake? I put the nuts on the outside of the plastic backing to hold the tach to it...that was a bad idea apparently. The only thing holding that backing to the tach is the harness plug! Who knew!?!

 

[fallguy]

Ok, ran test light. Started truck, probed TACH port on distributor...lit up. Probed fuse panel where I have it hooked up...lit up.

Up next...take apart dash...again...and probe other end of pigtail on back of tach?

Wiring harness had broken distributor wire!