Ok, here is a weird one....

[Amc1320]

A little background for the experts

Rebuilt engine
All the vacuum hoses removed, exept for PVC, 4wd and vac advance
hei one wire dist

So, hooking my tach bask up, connected the wire from the tach place on the dist to the green wire on the harness

Tach works, sorta, BUT

Only when turn signals are blinking, each time the signals flash it jumps to the correct rpms then back to zero

Also I have power windows, so if you roll up the window but continue to press the switch after the window is all the way up the tach works great perfect, until you stop trying to "over" roll up the window!

What the......

Everything else electrical works fine

So if i want to know my rpms I just have to turn?

Thanks
Rob

[eaglebeek]

Check for ground for your tach...

[ammachine390]

Is this a stock tach or an aftermarket? Stock tachs ground through the negative post of the ignition coil. If it's aftermarket and has a separate ground wire and the trigger wire to the negative of the ignition coil, then I agree with eaglebeek, check the grounds.

[Amc1320]

It's the stock tach, I agree it's a ground somewhere, i don't think it's in the dash must be under the hood somewhere, after all it was all removed and replaced, musta missed one somewhere

It's just weird that's it related to the turn signals

[vangremlin]

The stock tach grounds through the upper left mounting screw where it attaches to the instrument cluster, and it looks like it shares a common ground with the turn signals and various other lights on the cluster.  Here is a picture of the back of the cluster, the ribbon on the printed circuit board goes to about the 9:00 position on the round connector.  Please don't mind the slippers 

PS that box in the background is your hood trim waiting to be shipped today Amc1320.

[rollguy]

Cool slippers! (disregard that, you said not to mind the slippers).

[eagleman]

Wish I had a pair of blue slippers like those! You said pay no attention to the slippers but I just had to respond there so  well you know, cool. yeah thats it cool.lol

[vangremlin]

Thanks, and those slippers rock by the way.  I don't think they make that particular style anymore thoush, so I'm trying to keep them going....kind of like our Eagles.

[Amc1320]

Ok, heres some further investigation

THe tach works fine with the headlights OFF, does not work when the lights are on??

huh?

[carnuck]

Try jumper cables from the battery negative to something clean metal (possibly bumper face) with the lights on and see if it works. That would be missing/broken/corroded body to battery negative strap. Headlights will also get brighter and battery will get fuller.

[framedoctor]

Ground issue. Good luck. My gauges top out if i turn headlights on. I cant find issue. several tried to help me but understanding electrical is not my skill.

[Trooper]

There is a ground point on the firewall in the engine bay between the back of the engine and the brake booster. There may be one or several wires grounded there. Make sure it is a good ground.

[mo.eagles]

A friend of mine had a Gran Torino that had an engine rebuild. When you opened the door the starter would start cranking. Found ground strap to body was cut. Electricity will do funny things.

[EagleSX4_5Speed]

You need to put a resistor in line with the tach wire. My dad also put a HEI distributor in and had the same issues with the tach until he added an inline resistor. He had to try a few before he found one that worked.

[framedoctor]

My gauge issues were solved, bad ground on firewall. Blinker and or headlights would max out my gauges.