Installing a factory tach cluster in a 1987 V10 350TBI. Do I need a tach signal filter?

[Gander]

I’ve got a factory tach cluster out of a 1985 manual C20. Guy I got it from found it in a junkyard and didn’t think to get the wiring harness with it. I’ve got a new printed circuit and a wiring harness ordered which I’m already wondering about since LMC has the printed circuit listed as 81-87 but the wiring harness they have listed as 81-86 with a note saying “1987 MODELS WITH TBI WILL NEED TO MODIFY THE TACH FEED WIRE CONNECTOR.”. I’m looking to swap my Speedo and gear selector indicator into this cluster and install this in my 1987 V10.

Anyway, when looking at that wiring harness LMC is selling I don’t see a tach filter. You know, the little can like thing that cleans up the signal? Is it that it’s not required? Or should I spend the $25 or so to order one? Anyone else who’s made this swap I’d really appreciate their advice on this, I’ve read the threads on here about factory tachs but there’s no thread on installing one in a 1987, first year for TBI, and all that.

Also, is there a way to test this tach without having to install the whole shebang in the truck? Maybe just plug it up with the harness before installing any of it just to see if it works? Or is there another simple way to test it? I’d really hate to go through all the trouble of doing this swap just to find out it all needs to come back out so the tach can get shipped off and fixed.

Finally, LMC has a note in their description of the wiring harness they’re selling that says “1987 MODELS WITH TBI WILL NEED TO MODIFY THE TACH FEED WIRE CONNECTOR.”. Any idea what that modification would be? I mean, do I need a different connector on that feed wire?

 

[Sgt Gus]

I've heard that LMC can walk you through what you need. Try giving them a call.
Keep us posted. Some may chim in. I'm interested in eventually adding one in from then also and making clock gas gauge.

 

[bucket]

The modification is just due to the '87+ ignition system. In '87, the large, coil-in-cap HEI distributor went away for many GM models. They have a remote mount coil. LMC's tach harness should plug right into a large cap distributor... so you will have to cut the end off of the tach signal wire in order to wire it up to your '87 coil. Your coil harness will have a white wire coming off of the coil that leads to nowhere- that is the wire that you hook the tach signal to.

As for the factory original tach, it's very likely that it doesn't work.

As for the tach filter, I 'think' that it's only purpose is to smooth out the signal, if needed.

 

[Turbo4whl]

Medium duty trucks of the same era could have a tach that will work also. Diesel engines could have a 0-4000 RPM tach instead of the 0-5000 RPM. Diesels with a tach needed a signal generator mounted to the injector pump line on #1 cylinder. I'm glad @bucket knew the wire color you need. Three wires to the tach, ignition on, ground and the signal wire. This signal wire goes to the negative side of the coil.

 

[Gander]

The modification is just due to the '87+ ignition system. In '87, the large, coil-in-cap HEI distributor went away for many GM models. They have a remote mount coil. LMC's tach harness should plug right into a large cap distributor... so you will have to cut the end off of the tach signal wire in order to wire it up to your '87 coil. Your coil harness will have a white wire coming off of the coil that leads to nowhere- that is the wire that you hook the tach signal to.

As for the factory original tach, it's very likely that it doesn't work.

As for the tach filter, I 'think' that it's only purpose is to smooth out the signal, if needed.

So here’s an update. Got the wiring harness and printed circuit in from LMC. Got the printed circuit on the cluster back and all good there. The wiring harness was easy. All I had to do to connect to the white wire off the coil was remove the plastic socket on the harness, exposing the metal blade, and then insert that into the connector on the coil. A previous owner had already drilled out the dimple on the firewall for the pass through and the rubber grommet went right in. The other end of the wiring harness has a ground wire I connected at the kick panel, and then there were multiple connectors of different types that I could use for power. I used the one that plugged into the fuse panel in one of the empty spots and to clean things up removed the others from the harness.

With that done, I plugged into the tach with it loose on the floorboard and started the truck up. Turned the key and the tach zeroed out. Started the truck and the tach came to life BUT it reads about 750-1k high. Needs a new circuit board. Good news is that there’s a guy in Louisiana that sells replacement boards for $60. Ordered one, it’ll be in the mail by the end of this week, when it gets here I’ll update again.

Cajun Tach Shop | Tachometer Repair | United States

Cajun Tach Shop: GM, Pontiac, Oldsmobile Tachometer Repair, New Tachometer Circuit Boards, Easy Tach Fix, Lifetime Warranty

www.cajuntachshop.com

 

[Gander]

All right, here’s the update.

I got the new circuit board in the mail Christmas Eve. Already had the ‘85 tach cluster torn down, new printed circuit installed, small gas gauge in, had the bulbs, holders cleaned up, everything ready to go.

First things first, when I got the tach board in I installed into the tach on Christmas Eve and plugged it in just to make absolutely sure it worked. It worked BUT was still reading a bit high. Good news is on the tach boards you get from Cajun Tach Shop, it has a potentiometer installed so easy to dial it in. Just a heads-up because even if they’ve calibrated it in their shop you may still need to adjust it a bit after it arrives and before you put the whole damn cluster together just to find out you have to disassemble the whole thing again.

Next thing is, and I had no idea about this beforehand, but the 85 and 87 Speedos were different on the back. Both had provision for the VSS, but the orientation was a bit different. Also, the ‘85 tach cluster housing didn’t allow for the installation of the VSS, the ‘85 Speedo had never had one. Luckily, I had a tiny self-tapping screw to use to install the VSS on the ‘85 Speedo and with a little bit of cutting/filing I was able to fit the Speedo into the cluster back. I don’t think the ‘87 speedo would have gone in unless I cut basically the whole back of the cluster out. The hold down for the speedo cable was different too. The ‘85 cluster had it screwed to the back of the cluster, on the ‘87 that was a different design and screwed to the back of the Speedo itself. Luckily both speedos work fine and are 1000 miles of each other on the odo and look identical other than the ‘85 having a bump stop at 85mph and the odometer is a slightly different font.

Anyway, enough about that. Good news is I got it all back together, and in the truck. All lights now work and they’re brighter than they’ve been since I’ve owned it. All gauges work and read accurately now. Before, I’d have to tap the plastic in front of the temp gauge to get it to read correctly, voltmeter flickered a lot, fuel gauge was off regardless of which tank it was running off of. New printed circuit solved all of those issues.

So, long story short, it’s in, works and looks great. Just know that if you go this route and try to use factory parts there’s going to definitely be some work making it all fit.

You must be registered for see images attach

 

[Gander]

Here’s a better pic of the cluster finished up.

 

[Gander]

And here’s what it looked like when I bought the truck last Sept.

 

[bucket]

I'm gonna have to spend some money at that place, lol. I was pleased to see that they also offer the circuit board I need for my '92 Camaro.

 

[Gander]

Pic with the printed circuit still on is what the back of the tach cluster I have looks like. Notice how you can see the opening for the VSS but it’s partially blocked. I had to cut/file enough away to get VSS to clear and had to file enough of the VSS connector away to keep it from interfering with the speedo cable clip.

The other pic is the back of my ‘87 cluster with the 87 speedo set inside . You can see how much more of an opening it needed and what’s more when I tried to set it in to the tach cluster back it looked like I’d have to remove even more from the side just to be able to get it centered and just wasn’t sure how strong it would be if I took too much off. Anyway, something to consider for folks thinking about putting together a tach cluster with factory parts

 

[AuroraGirl]

Medium duty trucks of the same era could have a tach that will work also. Diesel engines could have a 0-4000 RPM tach instead of the 0-5000 RPM. Diesels with a tach needed a signal generator mounted to the injector pump line on #1 cylinder. I'm glad @bucket knew the wire color you need. Three wires to the tach, ignition on, ground and the signal wire. This signal wire goes to the negative side of the coil.

i wish more GMs would use the built in tach wire on the alternator "P" terminal, only available on CS130 and newer 4 pin alternators

 

[bucket]

i wish more GMs would use the built in tach wire on the alternator "P" terminal, only available on CS130 and newer 4 pin alternators

Why? Then you've got pulley variables to deal with.

 

[AuroraGirl]

Why? Then you've got pulley variables to deal with

amended statement:
boo that idea, i feel bad if someones trying to use it and has any modifications from stock

https://www.electricalrebuilders.org/eren/archive/ere_2008/ere_01_08_analyzing_the_alternator_tach_signal.pdf

 

[Camar068]

Next thing is, and I had no idea about this beforehand, but the 85 and 87 Speedos were different on the back. Both had provision for the VSS, but the orientation was a bit different.

Sorry Gander, little hijack.

Is this true for say an 86 k10 speedo? I'm using cable currently off the transfer case. Any benefits to using the vss signal on my LS swap? Calibration? If comments take this away from yours, we'll put a stop to it and I'll put up another post.

Thanks Dude

 

[Gander]

I don’t know, I tried to look at eBay ads and stuff like that and I saw some speedos one way and others the other way. Carb trucks only need the VSS for cruise control. I’m guessing the 85 I got the cluster out of didn’t have cruise so I guess that’s why the cluster back wasn’t built to accommodate it. My understanding is that a TBI truck like mine needs VSS not only for cruise but because the VSS sends speed data to the ECM. I don’t know anything about LS swaps but if it’s fuel injected the ECM is going to need to know the speed the vehicle is going.

I think 1989 was the last year for the cable driven squarebody Speedo with the VSS on the back of the Speedo. In 1990 they went to an electric Speedo.

As far as calibration, if you buy a tach circuit board from Cajun Tach Shop, the board he sells has a potentiometer built in. You can dial it in yourself with a flathead jewelers screwdriver. That’s what I did.