Erratic tach

My tach is very squirrley--it's not smooth at all and jumps around pretty wildly. Cruising at say 50 it's sorta steady but if you "blip" the accelerator it will jump 'way up.

I know that it is activated by the coil (right?) and the coil connections are good.

What is causing this erratic tach?

Thanks!---Jack

I hope you find out the why, as I have same, and so do many others, apparently. I have been unable to find out what about the unit makes it do this, other than: "It's a cheapo Chinese POS." Well, in my case that is true: I have the Chniese look-alike. The fellows at North Hollywood Speedo say they will gut the thing you have, and replace w/ genuine VDO workings, and then pack it in the Chinese case and send it back. ~$200.00. Keeping the faceplate and can gives you the same look on the dash. Since my car is brand new, JPS says he will cover this. The replacement is a work in progress, so we'll see how it turns out, and how long it takes. I sent Steele my tach on Sat. "Quick" here would probably be 3 weeks.

PS: John sent me a replacement tach afew weeks ago, another Chinese version that he said was "reworked" by NHS. That one was not jumpy at all, but acted as though it had had a lobotomy. First several days, nothing I did w/ engine got a reading over 4,000. Then comparing speed and revs in 4th gear, found out that the tach was reading one thing yesterday, and something else today. 3,000 RPMs showed anywhere between 60 and 80 MPH. My bet is both instruments are whacky, but I can live w/out looking at the speedo -- not so the tach.

I had this issue with a chevy so may not be anywhere near the same. I upgraded from a points ignition to electronic and my older tach would do exactly as you describe. Throttle blip would bury it. Mechanic told me I had two choices, put in an inline capacitor to filter the DC spike or replace my tach with a newer unit that was built for electronic ignition (capacitor built into the input). I wish I was technical enough to know why... Something about difference in dwell between points and electronic... I opted for a new tach because I liked the shift light it had built in and everything worked like a champ. Good luck!

FWIW - I experienced a similar problem and checked about every part and connection in the engine compartment.... finally traced the erratic tach to a corroded fuse/fuse contacts in the fuse block. Cleaned the contacts, replaced the fuse -- problem was eliminated, with many happy miles now to prove it.

If you have an std points system - replace the worn points. If you have a CD/Transistorized ignition (like Pertrix -sp?) then you need to add a rectifier diode inline with the signal wire (Radio Shack, part# 276-1104, IN4005 Rectifier Diode). Had this prob on old 914 when I added a DELTA CD system - part is less than $1.

Wolfgang--yes I did install the Pertronix solid state in my dist.

Pardon my stupidity but which wire coming out of the coil sends to the tach?---Jack

Here's the tech article link at the VDO site -

http://usa.siemensvdo.com/NR/rdonlyres/CE4E6791-EBC1-42D7-AA5B-6B1262A19A1A/0/ToReduceorEliminateErraticOperationinTachometersTroubleshootingGuide.pdf

Wolfgang,

Excellent input -- you are a true font of knowledge. Now, if we could just get you back on the road . . .

I wonder if this [link] is what JPS had done by NHS for the second replacement tach I was sent? Maybe. Or maybe they just added a capacitor. By the sluggish way the second tach works, sounds like a capacitor to me. I have asked JPS to tell me how the original out-of-the-box Chinese tach and the replacement one differ; might as well have asked the ayatolah(?) for a martini, as no reply was given. so just going on guesswork. Armed w/ this new info, I will ask again, and see what happens.

In case I did not mention, I have an "electronic points" style distributor. I wonder if this has something to do w/ how things work?

Wolfgang--great stuff---thanks! Is there a diagram available for this fix from Siemens?--Thanks!---Jack

SOCers, the fix with the diode worked on my Tub interupted the tach line 5" from tach, it straighen the jumping around, but also fixed a top end miss that I thought might of been an exhaust leak, running really good now. Cabanaboy ~cheapest fix yet .79 cents :)

Desi--which line is the tach line? Thanks---Jack

Jack, wire color varies, the wire your looking for is on the negative side of your diztributor, when you cut the wire at the tach the engine stays running, tach won't move. Cabanaboy ~ getting tach~e

Thanks Desi--I'll try your fix.---Jack

Check this link out; it tells the whole story:

https://www.speedsterowners.com/forum/readmsg.asp?t=4661

A year or two ago I paid North Hollywood Speedometer $160.00 to do the VDO conversion on my Chinese repro tach; it's working beautifully.

Ricardo---when was your car nade? Mine was about in March of this year. I wonder if we got the same batch of tachs?---Jack

It's a 2004 model; probably the same batch of bad tachs; just get it retrofitted and forget about it; I did.