Another Tach question..............

[citruspilot]

Ok, so I get my new $59.95 tach rebuild kit from MidAmerica yesterday and I put it in last night. Easy right! :cool Well, the actual install wasnt all that hard but now the tach is reading different RPM's than the engine is really turning. It seems to get the idle speeds right but if you rev up the engine the tach goes way up into the yellow and red arch with ease. And thats impossible. And while driving the car at freeway speeds, like 65 for instance, the tach will show 4,700 RPM's. At 60 it will indicate about 4,300 RPM's. Now I know I have a 3:53 gear and all but I dont understand the tach problem. I put in this new rebuild kit that I was led to believe would fix "just about all of your tach problems". Any ideas???

Jeff

 

[citruspilot]

Hello?????????

:confused

 

[mike weyman]

is it the identical circuit??could it be flipped backwards?my circuit had the copper track burnt, i soldered a wire to the points,i'll find out if it worked soon.mike

 

[citruspilot]

Mike, nope. It only goes in one way as I remember. There were three posts that lined up so its only a one way install. Its really odd driving and seeing 5,000 RPM's at 70 MPH. But whats even worse is the car probably has 3:53 gear because it really does have a high RPM........maybe 3,000 RPM's at 70 MPH and not 5,000! My guess is the little metering motor, the thing that drives the needle, is probably bad. :confused Has anybody else had this problem after they installed the tach rebuild kits, ie erroneous tach readings???

Hey, I sold my house today. First looker made an offer and I only had to come off my price $1,900! Thats incredible! Wait, maybe I wasnt asking enough???


Jeff

 

[Jack]

Have you tried to reset or "zero" the tach needle? Do this with all tach wires connected. Gently pull the needle off. Crank & run the car for a minute or two. Switch ignition off. Then turn switch to "RUN" but do not crank motor. Then switch off. Then replace needle so it rests at zero rpm. G'luck.
JACK:gap

 

[citruspilot]

Jack, thanks for the tip. The tach does seem to read normal at idle but removing the needle and letting the "metering motor" settle into its proper location without the needle weight will work and not change the idle indications?? (I call it a "metering motor" for lack of knowing what its really called.)

Seems like an easy fix..........I'll give it a try........

Jeff

PS: You know, now that I think of it, I have seen other Vettes in my age group that the needle rests at about 700/800/900 with the key in the off position. I guess those need to be reset too.........

 

[Jack]

Jack, thanks for the tip. The tach does seem to read normal at idle but removing the needle and letting the "metering motor" settle into its proper location without the needle weight will work and not change the idle indications?? (I call it a "metering motor" for lack of knowing what its really called.)

Seems like an easy fix..........I'll give it a try........

Jeff

PS: You know, now that I think of it, I have seen other Vettes in my age group that the needle rests at about 700/800/900 with the key in the off position. I guess those need to be reset too.........

You're right about what you've seen ... but you cannot tell simply because they FIRST rest high ... so those others may not need resetting. Every elec tach I've seen ... OE and Aftermarket ... will rest at 500-1000 when FIRST switched "Off". BUT, when they're switched back to "Run" (but NOT cranked) and then returned to "Off" they should rest at zero. It has nothing to do with weight of needle. Go ahead with the zeroing procedure ... it'll still rest above zero when first switched off ... but should drop to zero as described above. Do go ahead with the zeroing procedure because you cannot tell if the needle is holding the "metering motor's" shaft from truly resting at zero ... EVEN IF IT RESTS AT ZERO AFTER THE CRANK-OFF-ON-OFF sequence. Suggest you also hook-up a "tuneup" tach-dwell meter and compare its tach readings at various rpm to what your dash tach reads.

Again, gently pull off needle. Then run the motor for a minute or two. Then switch off. Then switch on but do not crank. Then switch off. Then replace needle so it rests at zero.

BTW, I converted my 71s OE mech tach to OE elec from about a 75 model ... I had to needle-off zero it just as I've described.

Both autometer tach in our race car (and others) and OE tach in my GMC pickup do the same thing ... when first switched off they rest high ... then switch on (without cranking) and then return to off they'll all go to zero.

After the zero procedure is properly done, if the tach still reads high/low more than about +/- 5 percent (when motor running) then you'll probably want to have the tach properly serviced by a pro.
JACK:gap