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84 digital tachometer troubles

T

topcat_5150

Guest
Hello, I have an 84 Z51 Vette. I have owned this car about 9 months. After 3 days of driving it, the trans quit. It has been in the shop with various problems for about 8 months and 27 days. It was a basket case when I bought it. In some ways it still is. I find it humorous now because, The day I finally got to drive it a few weeks ago, I was able to get about 10 miles and someone hit me. It is now in the shop again getting the front repaired. One day I will get to put an interior and my stereo in it and drive it on nice Sunday afternoons. My friends tell me to get rid of it, It is an omen. I almost believe them, but I still enjoyed driving it the few days I did. I will get to drive it again.

My question is about the digital tach. usually it works. Sometimes when I start the car, it is not on. After running a while, it comes on and works fine. Is there something common that I can check.

Thanks.. ...Tim
 

BigRed

Well-known member
Joined
May 23, 2001
Messages
948
Location
Edmonton
Corvette
1985 Bright Red Rocket
I found this site for you I know theres anothers posted around here somewhere but I couldnt find it. Anyway theres alot of people who can hlep you with this. Some of the things ive red are

Clean/polish all Connectors
Check for Shorts
Have someone use a Code Reader on your car
Make sure all your connections to the dash are tight and not hanging off

Just some things heres the link
http://www.corvettemagazine.com/1999/dec99/dash/dashp1.asp
 
E

Eagle85C4

Guest
Behind the drivers side head on the engine you will find the "tach filter" attached to the head. Basically it is a capacitor used to keep voltage spikes and noise from getting to the dash and causing faulty readings and the possibility of ruining the tach circuits.
If you remove the tach wire and trace it back it will come to a connector that goes to the filter. It is just placed in line, remove the connectors at both ends of the filter and plug them together effectivly removing the filter all together.
I had the same symptoms, would not work until it was warmed up, and was very angry with the person I bought the rebuilt dash from until I was told about the filter. Once removed the tach works beautifully.
Actually I often wondered why they would mount a temperature sensitive device right to the head on the engine. It can only lead to problems after a while.
 

vigman

Motor head!!!!
Joined
Feb 13, 2001
Messages
3,471
Location
Valencia, CA,USA
Corvette
88 Convert ( SOLD ) /1973 coupe 4 speed/1964 Vert!
Old Caps

Yes the dreaded tach filter is the problem IF

The tach read's O for a while , then pops to life intermittantly.

Bypass the tach filter for now...but I would get another one the filter is an Resistor/capacitor 2 stage network and prevents noise ( read spikes & hash ) from getting to the input of the digital dash.

The filter is not 84 specific and you can nab one from an 84-90 See if the dealer has one
( a new part is best here ) or it's wrecking yard time.

Mike

BTW KEEP THE CAR, In the long run it will treat you good.. but after YEARS of neglect you will have a LOT of things to do..Par for the course!

And Welcome to the CAC
 

CorvetteArchives

Well-known member
Joined
Apr 19, 2001
Messages
547
Location
Carbon County, PA
Corvette
1984 Z51 Silver/Gray & as Much SS & CFibre we find
If you cant find the tach filter.....we have the diagram posted at our site so you can make a new one....its under"repairs"
 
E

Eagle85C4

Guest
Well Mikey,

My 85 tach did exactly the same thing as the one who started this thread. When you started the car ....no tach. Somewhere along the way to work...it "warmed up" and the tach would work fine. I originally blamed it on a faulty dash, but from a thread you had a while back you mentioned the filter. I removed the filter and the tach works perfectly. Now I have a diagram and will build a filter and mount it on the firewall and run the ground back to the rear of the head. Thanks man...
 
T

topcat_5150

Guest
Thanks so much! The next time I get home I will check the filter out.


Thanks again. ..Tim
 
E

Eagle85C4

Guest
I found my old filter and curiosity got the best of me and I disassembled it. What I found was that it was not a failure of any internal component, but water had gotten into one end of it and the resulting corrosion caused it not to make contact back to the tach.
Too late to put this one back together but with parts available off my bench I am going to build another and put it back in place.
 
E

Eagle85C4

Guest
Made a new filter and it works well. No problems and now have the safety of the filter in place. Anybody want instructions let me know...
 
T

topcat_5150

Guest
I looked that the web site corvette 84 posted. The filter looks like it is 2 resistors and 2 capacitors. What type of case did you put your homade one into??
 
E

Eagle85C4

Guest
Well I did some redesigning...I used 1 watt resistors instead of 1/2 and for the .1 mf cap I increased the working voltage from 100 to 200 vdc and for the 100 pf I used a 150 pf at 400 vdc. Using a diode curve tracer on my scope I could not see much of a difference from the original components.
I found a piece of plastic wire mold, installed the circuit board and filled it with epoxy. Of course I used the leads from the original and made a new ground wire with a new connector. Placed the filter on the firewall, connected the ground wire to the post where it was originally from on the rear of the drivers side head. Then put the filter in line. Fired the ole gal up and it works fine....
 
E

Eagle85C4

Guest
Do you still have the old one? I would say send me the old one and 20.00 and I will get you one out the same day. Turn on your AIM and send me IM at rb2153
 

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