Tach works when I hose down my engine bay

[BlackTomC]

So, my tachometer doesn't read correctly anymore, reads slow and usually way under what the actual RPM's are. Verified it's got a good connection in the cluster area, beyond that, I am puzzled (not a mechanic, trying to learn).

When I hose down my engine bay, the dang tach works for about a day, then back to crap again. Any ideas?

Also noticed that my horn stopped working... when it did, everytime I push my horn button, my tach needle drops, then goes back up.... so yeah, two questions.

[fxrsrider]

likely you have a bad ground connection(s) somewhere.  .....a wet chassis provides ground....which explains the "hose down" phenomenon you're experiencing.  This, at the very least tells you that your ground issue is likely located in the front end.

[BlackTomC]

Makes sense and probably wouldn't hurt to replace the existing grounds I do have... they don't look so good.

[FlatBlack77]

i agree with the grounding problem.

when i first got my K10 the horn, lights, gauge cluster, and even the starter all were very weak and just not right.

turns out it had a ground going from the battery to the alternator bracket and a ground from the passenger side head to the firewall and that was it. so i replaced those and added
battery to frame
battery to rad support
driver side front of head to frame
and dash to metal on the driverside kick panel

[BlackTomC]

This may sound like a stupid question but how do I ground the "dash" to the frame? As far as I can tell, the tach has a different ground than the rest of the gauges, radio, etc..

[enaberif]

This may sound like a stupid question but how do I ground the "dash" to the frame? As far as I can tell, the tach has a different ground than the rest of the gauges, radio, etc..

Behind the cluster near the driver side will be either a bolt with a star washer or a block with a bunch of tabs. This is your dash to cab ground point. This is where grounds should be terminated for the dash and anything else.

This ground isn't much use though unless you have the cylinder head to cab ground in place as well.

[BlackTomC]

Just checked, looks like I've got this 6 prong lookin thing with ONE ground running to it (from the tach). No grounds from head to frame, only grounds up by the headlights, 1 also from battery to core support and firewall to fender..... How does this thing even run?

[enaberif]

6 prong is your common ground for the interior which all your grounds should be going to. There should be a ground wire somewhere in the harness around that general area that will plug onto that.

Your tach should NOT have a ground at all. If you are using a HEI distributor the base has a connector which comes out and connects to the cap. The cap will have 2 additional wires one of which is for a tach output and the other for a 12v power input. You need to verify these 100%

Your general basic grounds should be as such..

1) 1 small gauge 14-18g wire from negative on battery to rad support or inner fender
2) 1 large from negative terminal to alternator bracket or engine block
3) Driver/Passenger head to cab
4) Ground behind cluster

If any of those are missing replace them as soon as possible. The truck will still run but perhaps that great.

[BlackTomC]

Interesting, nothing is grounded to that common ground except for the tach. This week I'll push the dash out and figure out what the heck is going on here, someone has messed with the wiring in this burb during it's previous years. As for the tach, the tach pulls out, on the back its got a plug with 3 prongs, and you're saying those are basically for power/tach input/output only?

I am using an HEI distributor, I actually replaced the harness (wiring connector) that plugs into the distributor so try and fix the tach problem, ended up fixing nothing except an old connector.

So basically, I know for a fact:

1) Exists but wiring is literally falling apart
2) Doesn't exist
3) Exists but wiring is also falling apart
4) Doesn't exist at all.

[enaberif]

Tachs usually have a minimum of 4 wires.

Wire 1 = 12v
Wire 2 = Ground
Wire 3 = Tach Signal from distributor
Wire 4 = Light

If you only have 3 wires then I'm going to say its 12v, Ground, and Tach Signal.

Regarding your grounds get the battery to engine on there and fix the other three.

[Irish_Alley]

are you talking about a stock tach thats in the dash cluster itself? or one thats aftermarket?

[Irish_Alley]

i just got this one and i dont know much about it. it would make sense theres only 3 wires since you have the dash cluster bulbs to lighten it up

[enaberif]

Yes and those 3 wires would be 12v, ground and tach signal from distributor.

Looks like red, black and brown.

[Irish_Alley]

brown would be tach signal?

[BlackTomC]

That's identical to my setup! Yeah, that tach reads ultra slow.. sometimes when I hose down my engine bay at the car wash it magically works for a day or two, then goes right back to junk. I'd love to find an affordable gauge (lens) cover for it, my old one is shot. I have yet to take a look at those grounds, work and school have me slammed up until Friday.