Tachometer Delay on engine start. 84 Targa
 

1984 911 Targa 3.2

From a cold start my tachometer doesn't start to move until the engine has been running for about 25 seconds. After that it works perfectly.

If I go for a drive, turn the engine off and then turn it back on the tach works instantaneously.

So is that normal or is there something I need to fix?

definitely not normal. something is causing the needle to stick after the car has been sitting for an extended time.

Or you have a bad solder joint or a failing component that is only working after it warms up.

Is there a capacitor in the tach that may be having issues charging up?

This unit has had issues before and you fixed it about 2 years ago. You noticed it had prior damage from someone trying to fix it before.

I'll keep an eye on it to see if I see a pattern. Likely I'll have to send it back to you this winter to have you take another look at it.

If I had it apart, then the capacitor that typically fails would have been replaced. I would guess it is a cold solder joint, but there are a couple of other components that fail on these boards. I have had 2 or 3 84-89 tachs this year with bad solder joints and/or failed traces on the board.

Send it back when it's convenient and I'll get it fixed for you.

Give the tach glass a little tap the next time it is not working. Does it spring to life? Mine had that problem.....

If mine sits in the sun during summer, after starting the tach doesn't work until the cabin cools down. My mechanic told me I'd have to get it rebuilt to get it resolved.

Mine had this symptom, which then turned into a combination of intermittently going dead or bouncing up and down, until ultimately failing completely. It's easy to remove and send in for repair. Palo Alto Speedometer or Hollywood Speedometer are the go-to for these. I made the mistake of trying a local shop, after having very good luck with Palo Alto on a number of cars, and it took them three tries and they still couldn't fix it. Learned my lesson, Palo Alto for me.

Maybe, but your problem might be simpler/cheaper than a rebuild. Check out this thread : http://forums.pelicanparts.com/porsche-911-technical-forum/75414-sticking-tach-needle-whats-fix.html

Mine doesn't start working until about half hour of driving (times varies), no matter the tempeture outside. does it start to work due to the car warming up?

What could it be?

Mine doesn't start working until about half hour of driving (times varies), no matter the tempeture outside. does it start to work due to the car warming up?

What could it be?

I did another test on mine.

Standard start up, tach has a 6 sec delay. Turn off and start up and there is no delay.

Turn on ignition but dont start car, wait a few seconds, then start car, tack works immediately.

I guess its time to break out the volt meter and see what signal its getting and when.

Still sounds like a component is failing or a cold solder joint. Once you apply power to the tach the parts start to warm up, and it works.

cold solder joint...so what can be done?

I've been seeing a lot more tachs with this issue. Especially the 84-89s. The tach needs to come apart and the driver board needs to be tested. You can just go over all the solder joints but I like to test the board first so I know what components or solder joints are bad.

would a solder point issue be obvious?... if not how can i test the board? This something a layman can do?

Sometimes they are obvious but most of the time not. I test them with a combination of things, but for starters, you need a tach tester or a signal generator. You need to be able to operate the tach on your workbench while you are troubleshooting it. I use a 134 electronics spray to quickly cool off sections of the board to narrow down where the issue is, then use a multimeter to test continuity between parts. Depending on the year/model of the tach there are typical components and sections of the board that are problematic.

If you decide to do it, you should start by replacing the 47uf capacitor because if it's not bad already, it will be soon.

Thanks!

Before you pull it apart to see if this is the issue, check to make sure your tach has an acrylic lens. They only used these for a few years in the mid to late 70s before going back to glass. It's easy to tell the difference by tapping on the lens.

what difference would the materiel of the lens make in troublshooting?