It's been a long time since I've been on and now I need help. I finally finished restoring my 67 rsss rag. I dropped in a new zz4, runs great, except my factory tach doesn't work. I checked and double checked my connections and all is ok. Just to make sure, I undid the wires and connected them to an aftermarket tach. Did I screw up my tach at some point during resto? I did remove the speedo/tach assembly and replaces the lenses during the resto, or is there something about the new GM HEI and the factory tach? Plse help. If it is the tach, I see they now sell repo tachs and would like some opinions please.
In case you were wondering, my car WAS Sierra Fawn, deluxe GOLD interior, 350 w/ 2sp. Now its a "typical" bolera red with red interior, and with a 4 speed muncie. I've owned the car for 28 years, unfortunatley, I threw out the original engine back in 1985 (who knew), so, because it will never be a true matching #, I decided to paint it for the first time. I don't know if I did the right thing making all the changes but my interior was tired and I couldn't stand GOLD, so that's why I changed the colors

I may be mistaken but some tachs need a filter in the line. I have a stock GM HEI system and bought an aftermarket tach. The instructions told me that if the tach did not work properly to install the filter. The tach worked fine so I didn't need the filter.
Just a thought.....

never heard of a filter before, I'll check it out. Thanks. Incidently, i forgot to mention the tach does go up to a maximum of 1300rpm and then quits

Do a search on TC. This has been discussed before. I believe there is some good info on this.

if it does move some, or if the needle jumps around, a filter will do it.

thank you all, I'll look into a filter. Does GM make this part

thank you all, I'll look into a filter. Does GM make this part
Viper,
Check with AutoZone.......if they dont have one then let me know. I'll dig around out in my shop and if I can find mine that came with the tach I'll send it to you...... :beers:

thank you Supv 26

my 67 RS has the factory tach and console gauges , my tach and clock will work perfectly in the summertime but neither will work in the winter , have owned the car five year's I got it in the summer on 99 and that winter when they stopped working I checked everything out and could not find a problem , I just gave up on them , but then that next spring they started back working , have done the same thing every year since , I thought I was crazy and all that I have told about it thought so as well ,

A bunch of manufacturers make a "tach adapter." I understand that it may be necessary with an original style tach and HEI. Check out Accel and Mallory. I found quite a bit of technical info there on their web site. I know Classic Industries sells a couple and their tech guy there actually recommended them - as it says with the repro tachs "NOT for HEI." Hope this helps!!

Just curious before you spend money, did you check to make sure that you have the proper grounds? You know, motor to frame and frame to body? Just something to check.

yes all grounds are good. I tried wiring all to an aftermarket tach out of the car and it worked. So it leads me to believe that a filter?, don't know what it costs yet, is probably the cheapest way to go. I'd hate to sned it out to someone, eg tachman, just to get a NFF, no fault found!

yes all grounds are good. I tried wiring all to an aftermarket tach out of the car and it worked. So it leads me to believe that a filter?, don't know what it costs yet, is probably the cheapest way to go. I'd hate to sned it out to someone, eg tachman, just to get a NFF, no fault found!

Did you find one yet?? I located mine that came with my tach. If you cannot find one then send me you address and I'll send you mine. All it is, is a small piece of wire with a "swelled" spot in it. I really dont see how it works but it came with my tach and the instructions said it was the filter. It just hooks into the tach wire slot on the distributor cap and then you hook the tach wire into it. Let me know where to send it and I'll get it to you.....
:beers:

I posted my other tach post under electrical...but here is what my take is after great experimentation and research.

Did your aftermarket tach work when it was hooked up. The circuitry of the OEM is different for aftermarket and OEM on yours. I know....my 69 did the very same thing. Your low reading indicates to me that the pulse width out of the GM HEI module is the problem.

Even if you put a filter on that doesn't guaranty it will correct the problem. That is a bandaid so to speak. These filters are not all the same "electrically" and certainly don't have the same dynamic electrical responses to whatever the modules are putting out. That's the problem....what the module is putting out! This is due to the "overseas" zealots trying to reverse engineer the module (most modules you find at Poop Boys or Auto Zomb which are usually cheap ...cheap...cheap and/or no-name HEI copycats) without real knowledge of the output wave use and/or caring really.

The real underlying problem is the output of the module from the HEI. The tach signal can be distorted either by peak size (causing "bouncing readings at idle and then smooths out around 3000rpms) or funky square wave of various sizes- signals that do all kinds of things to your tach readings, to very low fat retangle waves that don't do hardly anything. It's the modules design and output that is the problem.

Many of the inexpensive HEIs and/or replacement modules will cause these problems to happen, as well as are NOT designed for performance use period. In my experiments with several modules the cheapy and older type will severly reduce voltage output at around 4500rpms too! So, it's not only the distributor...it's the module and coil too that play a part in total performance and certainly tach compatibility.

MOst "performance" HEIs have it all....great coil/module output to 6500 rpms and complete tach compatibility. Plus, if you have a performance coil, don't just put a standard module in...you can eat it quickly.

The modules that I know work are the Accel and Echlin ones, but I won't guaranty that this is the exacting problem since I haven't looked at the output on my scope, so proceed wherever you go at your own risk. Yes, I know they are a tad expensive but when you wind that motor up to 5500 rpms like a song and power under foot, it's worth it.......and you can also read your tach! From Summit $40

ps....The GM HEIs usually need a vacuum advance limiter applied because they provide tooooooooo much VA under light and moderate throttle conditions resulting in pinging.

http://static.summitracing.com/global/images/prod/large/acc-35361.jpg