Factory tach

[Sena80k20]

Hi I just purchased a factory tach cluster from LMC for my ‘80 k20 custom deluxe which came with factory warning lights. Now I did go thru my gauge harness and wired in a factory plug out of the same year Silverado. All my gauges work except my tach which has its own separate harness. My question is do I have to change the module in the distributor to make it work? Yes it is hooked up correctly and I do have all necessary “power” up at the gauge plug. Seems like my module isn’t giving my tach the pulse I need. Any help will be greatly appreciated thank you

 

[bucket]

No, you don't need a different module (not saying yours ain't got something wrong with it though). Just hook the signal wire to the "tach" terminal that is right next to the "ign" terminal on the distributor.

Iirc, the factory GM tach signal used a condenser or something inline. Is that how the LMC tach is set up as well?

 

[nvrenuf]

Yeah, there’s a resistor or something built into the main wire.

 

[Gander]

I had to buy a replacement circuit board for my factory tach because of a failed resistor. Got a new board from Cajun Tach Shop with a potentiometer in the place of that resistor allowing me to dial in the tach with a little screwdriver.

When you say a resistor in line are you talking about the tach signal filter? When I installed my factory tach I didn’t have one. LMC’s tach wiring didn’t look to me to include a resistor in the wiring. The only aftermarket parts I used when building the tach cluster was the new tach board from Cajun Tach Shop and a printed circuit and tach pigtail from LMC. My tach worked fine without the tach signal filter EXCEPT in extremely cold weather where the needle would flutter and read inaccurately for the first minute or so after the first cold start of the day. I came across a mid-80s Z28 in a junkyard that had a tach. Still had the tach filter on the firewall. I pulled that and added it to mine and no more flutter. Another tip for you is that I eventually came across a 1987 Firebird in a junkyard with a 305TBI and a factory tach. I got the wiring off of that from the ignition control module on the distributor to the coil. That piece also has the tach signal wire on it but in the case of the Firebird had both ends of a connector instead of that one small end the wiring on a truck has. Probably has a tach filter on the firewall too but I didn’t think to look since I already had one. Anyway, just a couple tips for you.

 

[ChuckN]

My LMC tach kit did not come with anything but the brown wire to plug into the tach slot on the distributor. No filter or resistor that I can see, and as far as I can tell, it either doesn’t need it or there was some kind or work around internally for it because as is, mine has been flawless.

 

[Gander]

My LMC tach kit did not come with anything but the brown wire to plug into the tach slot on the distributor. No filter or resistor that I can see, and as far as I can tell, it either doesn’t need it or there was some kind or work around internally for it because as is, mine has been flawless.

Yeah, the filter isn’t required, mine worked fine without it except for a little flutter on extremely cold starts. All it does is clean up the signal going to the tach. If you ever want to add one just wait until you come across an 80’s Firebird or Camaro being parted out and grab the filter out of it. It’s bolted to the firewall and that mount is also its ground. I’ve also seen reproductions for sale on eBay but they’re a little too proud of them IMHO. $30-$50 for about $1 worth of caps/resistors/wire in a little can just seems ridiculous. There’s someone on eBay right now trying to sell an OEM one in the box for $200. Good luck.

 

[bucket]

Only some of the early-mid 80's F-cars got the tach filter. My '85+ TPI and TBI cars don't have them.

 

[Gander]

Only some of the early-mid 80's F-cars got the tach filter. My '85+ TPI and TBI cars don't have them.

Come to think of it, I don’t think it was on the 87 TBI Firebird I got the coil to ICM wiring from. I think if I’d have seen it I’d have gotten it just to have a spare. I cut that tach wire after both ends of the connector and that’s where the filter would have been. I know the Camaro Z28 I got the filter off of was mid-80s with a L69 305HO and it had the tach filter on the firewall. If I remember correctly it was a 1985. I also got the dual snorkel air cleaner off of it to try and use the base on my TBI but ended up selling the whole thing.

 

[bucket]

Come to think of it, I don’t think it was on the 87 TBI Firebird I got the coil to ICM wiring from. I think if I’d have seen it I’d have gotten it just to have a spare. I cut that tach wire after both ends of the connector and that’s where the filter would have been. I know the Camaro Z28 I got the filter off of was mid-80s with a L69 305HO and it had the tach filter on the firewall. If I remember correctly it was a 1985. I also got the dual snorkel air cleaner off of it to try and use the base on my TBI but ended up selling the whole thing.

Years ago, I had an '86 LG4 Z28 for parts but I can't remember if it had a tach filter. I don't *think* it did. I've got the entire underhood harness in a box somewhere, maybe I'll remember to look.

I know I've pulled tach filters from them before, but they were '83 or so. I also don't understand why the carb cars often had them, but the FI cars did not. It's the same ignition system and tach.