1990 Tach Problem
Thread Starter
6th Gear
Joined: Nov 2015
Posts: 6
Likes: 1
1990 Tach Problem
I have a problem with my tachometer that I can't find a post to match. I'm hoping someone else has had this problem and knows the fix. When you turn the key on, the tach needle moves to about 900 rpms, then when you start the engine the tach goes up to full peg then down to the bottom peg. I have read about the tach filter and the panel being bad. I've read about high readings but my problem seems to be different. Help please.
Pro
Joined: Jun 2008
Posts: 674
Likes: 235
From: Louisville Kentucky
2023 C4 of the Year Finalist - Unmodified
2022 C3 of the Year Finalist - Unmodified
2021 C4 of the Year - Unmodified Finalist
2018 C4 of Year Finalist
2015 C4 of the Year Finalist
Advanced
Joined: Mar 2006
Posts: 70
Likes: 0
I have a problem with my tachometer that I can't find a post to match. I'm hoping someone else has had this problem and knows the fix. When you turn the key on, the tach needle moves to about 900 rpms, then when you start the engine the tach goes up to full peg then down to the bottom peg. I have read about the tach filter and the panel being bad. I've read about high readings but my problem seems to be different. Help please.
I hit a gnarly pot hole on Thanksgiving day and my tach went berzerk just like yours. Did You come into contact with anything that caused the a severe jolt to the car? I also noticed that my secondary cooling fans come on with the key turned in the on position , engine off. The fans aren't supposed to turn on until the engine heats up - mine came on before I actually started the car. Noticed anything else weird happening besides the tach? Manuals say the first place to look for a short/unwanted ground is at the distributor since the ignition module /distributor send the electrical impulses to the tach. What ever I find out I'll let you know if you're interested. Hope this helps.
c4tonezone
Team Owner
Joined: Sep 2003
Posts: 25,384
Likes: 2,736
I have a problem with my tachometer that I can't find a post to match. I'm hoping someone else has had this problem and knows the fix. When you turn the key on, the tach needle moves to about 900 rpms, then when you start the engine the tach goes up to full peg then down to the bottom peg. I have read about the tach filter and the panel being bad. I've read about high readings but my problem seems to be different. Help please.
Thread Starter
6th Gear
Joined: Nov 2015
Posts: 6
Likes: 1
Thanks for the responses Guys. I'm going to expand on my issue. I just bought this car in this condition. It only has 58K miles. It has sat for years. When I got it the speedo, trip display did not read correctly as well as the tach problem. The oil pressure,volts and temp gauges all worked ok. So I pulled the gauge panel, checked connections and then I plugged the circuit board on and off several times to hopefully clean the connections. When I put it back in the car, everything started working great except the Tach. I still have the same exact condition as I explained above. I have a dwell/tach/volt meter from the good ol days. If I pull the tach wire off the distributor and connect my meter the tach source there works fine but that still doesn't tell me if the problem is the filter or panel.
Race Director
Joined: Mar 2015
Posts: 15,926
Likes: 579
does it have the digital climate control? look on youtube to see how to use it to trouble shoot. looks llike they are telling you it shows rpm. I learned something new!!!!!!
from what I get from this if the climate shows right it is your panel. if not it is your filter.
from what I get from this if the climate shows right it is your panel. if not it is your filter.
Last edited by antfarmer2; Nov 30, 2015 at 08:56 AM.
Team Owner
Joined: Sep 2003
Posts: 25,384
Likes: 2,736
Thanks for the responses Guys. I'm going to expand on my issue. I just bought this car in this condition. It only has 58K miles. It has sat for years. When I got it the speedo, trip display did not read correctly as well as the tach problem. The oil pressure,volts and temp gauges all worked ok. So I pulled the gauge panel, checked connections and then I plugged the circuit board on and off several times to hopefully clean the connections. When I put it back in the car, everything started working great except the Tach. I still have the same exact condition as I explained above. I have a dwell/tach/volt meter from the good ol days. If I pull the tach wire off the distributor and connect my meter the tach source there works fine but that still doesn't tell me if the problem is the filter or panel.
Last edited by WVZR-1; Nov 30, 2015 at 11:41 AM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Thread Starter
6th Gear
Joined: Nov 2015
Posts: 6
Likes: 1
Hey Guys. Ok if I use the climate control display test 06, with the car running i'm getting a display of 33, 32, at idle and when I rev it up I get readings in the 80's then back to the 30's at idle. That seems to tell me that panel is getting bad signals just like the gauge cluster. Does that sound correct to you?. If so maybe it is before the cluster, like the tach filter.????
Team Owner
Joined: Sep 2003
Posts: 25,384
Likes: 2,736
Hey Guys. Ok if I use the climate control display test 06, with the car running i'm getting a display of 33, 32, at idle and when I rev it up I get readings in the 80's then back to the 30's at idle. That seems to tell me that panel is getting bad signals just like the gauge cluster. Does that sound correct to you?. If so maybe it is before the cluster, like the tach filter.????
You do realize the digital display requires a /25.
Last edited by WVZR-1; Nov 30, 2015 at 11:42 AM.
Team Owner
Joined: Sep 2003
Posts: 25,384
Likes: 2,736
C68 is the AUTO AC RPO (Regular Production Option) for the car, the display needs to be / (divided by) 25 to arrive at the RPM. This is a newer laptop and I didn't realize I didn't have a numeric functional keyboard for the correct divide sign (÷) which I've now figured out how to use from a character mapping thing.
Thread Starter
6th Gear
Joined: Nov 2015
Posts: 6
Likes: 1
Update on Problem. I ordered and installed a new tach filter. No change. Still get tach needle to go to the upper peg as soon as you crank the car up then back to the bottom peg. A note, in the course of doing some other work where the car would crank but not start, I noticed the tach needle responding and climbing slightly as the engine turned over as though the tach is seeing a signal from the distributor.
Race Director
Joined: Apr 2002
Posts: 10,036
Likes: 346
From: Anaheim CA
The ignition module generates tach pulses any time the engine is rotating, whether cranking or running.
Thread Starter
6th Gear
Joined: Nov 2015
Posts: 6
Likes: 1
Update. Tach problem fixed. I had read online threads that explained how to fix an inaccurate tach. Well I tried it before I was forced to ship my gauge cluster off for repairs. I wired / installed a 1 meg. pot. to the connections as shown online and the tach started working. Then I calibrated it to match my hand held Tach/dwell meter and its working now. I could not find the correct size resister today so I just taped over the pot adjuster and ty-rapped it up under the dash. Don't think it really needs to be replaced and If it ever needs to be re-calibrated i'm ready to go.
