What the tach!

[Lastwagen]

The guy who had my truck before me installed a tach. I like having it because the truck did not come with one. The thing jumps around and does not give an accurate reading. The truck runs fine too, and I don't know sh!t about electrical. Any thoughts what I should do? Some times it works fine when I am driving it slow!?!?!?!! But when I am "gettin on it" it fouls up like I said before and does so for a while. Is it a conecction, or the unit?


-John

[87wildside]

I would check th wires where the pass through the firewall first and make sure there is a gromet' if not look for bare wires. Then trace the wires to the distributor. It sounds like a bad connection to me.

[04silveradomykk]

Did it just start acting erratic or has it done it since you've owned it? The reason I ask, you might need a tach adapter to get the proper reading.... depends greatly on your year truck & brand tach.

also check the Tachs ground, a weak ground can cause the needle to dip.

[Lastwagen]

the tach goes to the distributor?

[04silveradomykk]

the tach goes to the distributor?

again it greatly depends on your year. But yes, generally the tach splices in line from the distributor to the negative side of the coil. The tach counts how many pulses are being generated by the magnetic trigger in your distributor within a given amount of time.

I see in your profile you have a '98. Is the tach in this same truck? If so there is probably a terminal in the harness of the vehicles computer that would give a Tach signal. Thats where I would of spliced into for the aftermarket tach.

if my truck didn't have a tach stock I would of done one of those 5" Autometer monster tachs... I love those things, I put them in all my projects that didnt come with a tach.

[Hammer Head]

Does it just bounce around? If it's a cheap tach they do that. Liquid filled tachs & gauges mellow out the needle if that's the cause. If not it may have something to do w/ the capasitors being too big or too small.. possibly.

[Lastwagen]

well it is a cheap tach and when I get some $$$ I am going to upgrade. It used to work just fine but now it has been acting up. It bounces all over the place

[Mikz86TA]

The tach should have a ground,power and a feed wire(illumination too maybe). But anyway, it is common for GM to have a coil with 3 wires. A 12 ignition, a ground and a tach output. This is commonly white on the older ones, but i will check on my 2003 tomorrow. Most newer ODB2 vehicles have only a tappable tach in the ECM harness. First what year is your truck? I will look it up on mt DEI DirectTechs. Some kids Ive seen just put it anyhere they think is the right wire. Thats probably why it is reading erratically. Also the tach signal is AC current and not DC like most vehicle wires. Also Silverado is right. Some aftermarket tachs need a filter for certain makes and model vehicles. Has it done this since you owned it? Fill me in and Ill get you the right wires and locations to tap. TTYL, Michel

[Mikz86TA]

OK---I saw your profile and see you have a '98 S10 4.3 5sp. - nice truck =) Anyway, you will need to get the Tach feed wire at the PCM(powertrainControlModule) This is the black or silver box located on the passenger side under the hood. It will have 2-3 plugs facing the windshield. There is a million wires coming out,so its real hard to miss =). The plugs have corresponding plug end colors. This might be hard to see with them plugged in. You can disconnect the battery unplug the harness to see the color if nessesary. Most PCM plugs have a single 8mm bolt holding them in place in the center of the plug. Its Ok to unbolt and unplug as long as the battery is disconnected. The plug you are seeking is RED (32pin) and the wire in the red plug is WHITE(pin #20). There will be mist likely a couplw white wires into the PCM itself but only one in the RED plug. If you need to verify, get a digital multimeter and set it on AC voltage. Attach negative meter lead to ground and positive lead to the tach wire. Strip a little insulation back to probe if needed. After you re-connect all harnesses and bolts. Re-connect the battery and start the truck. The meter should read less than 1volt AC and rise smoothly while you slowly increase the throttle. Some meters have a bar graph at the bottom like my Fluke. Some do not. Some will also blank for a second when a certain voltage is reached and resets to next level. Depends on meter. Now you have the correct tach wire. Next verify a good source of 12V DC ignition for the tach IGN. wire. Under column the 10ga(large) PINK wire is direct key ignition. Fuse the tach close to the connection. DO NOT TAP FUSES IN FUSEBOX. Thats ghetto and dumb. Then find a good ground. To test the ground, find a 12 source for your pos. probe on your meter. Probe the outer metal on your cig. lighter with the neg. meter lead and note the reading. Next probe you potential grounding point with your neg. meter lead. If voltage is the same, you have a good ground. If its more than 1V less-find another spot. Bare metal works in the kick panel. Still check on the tach manufacturer for a filter you may need on your GM. Good luck and PM me if you need any more electrical help =) Michel

[04silveradomykk]

Mikz86ta, Damn dude. way to do the research. Thats pretty cool.

[Lastwagen]

hey thanks, I'll give it a try! My truck is in the shop for an on going problem (p0300). When I get it back I'll check that out and post the results

[Mikz86TA]

Ive had many experiences and been a certified tech for many years. Its no problem helping out a fellow commrade =) Any Time!
Mikz86ta, Damn dude. way to do the research. Thats pretty cool.