TheSamba.com :: View topic

Fri Jul 18, 2008 5:24 pm

Folks:

Our beloved P.O. installed a tach in our 85 weekender. It's the stock VDO tach. It appears he installed it correctly - one wire to the neg. side of the coil, and the other one to wherever the hell it's supposed to go. The tach works smoothly and seems fine...

Here's the issue - at 4000 RPM in 4th gear, we make c. 50 MPH. The engine is still willing to pull hard at this point - she's far from topped out. Our mechanic (not a VW guy, but our VW specialist of four years can't be bothered to return phone calls, so I thought we might use our regular guy) reckons the tach is reading crazy high. According to his ear, our tach-indicated 4000RPM is closer to 2000. This from a guy who's been a professional mechanic for longer than the 22 years I've been going to see him. He knows of what he speaks.

His opinion is supported by the indicated shift points on our speedo - the shift from first to second, second to third, and third to fourth all take place (according to the Never Over Four Grand rule) well below the lil' orange dots on the speedo that indicate where the tachless Vanagon owner should be looking to shift.

I figure a tach is pretty strightforward - two wires and it either friggin' works or it doesn't. Can they be miscalibrated? Can they read two pulses for each actual pulse?

Is there something we should know?

Thanks for your input, folks. I reckoned if anyone knew, it'd be you lot.

Fri Jul 18, 2008 5:37 pm

If your mechanic wants to scientific about it, he should use a digital timing light. It has a tach function, which you can compare to the tach, and even use it to time your engine! It does sound like it is set a bit low, those tachs had a switch to set up for the number of cylinders. Do a Google search, it may reveal the secret settings.

Fri Jul 18, 2008 8:39 pm

Dark Alley Dan wrote: at 4000 RPM in 4th gear, we make c. 50 MPH
18 mph/1000 rpm with a four speed and stock final drive. 50 mph with stock tires should turn a little shy of 2800 rpm.

Fri Jul 18, 2008 10:00 pm

Dogpilot wrote: If your mechanic wants to scientific about it, he should use a digital timing light. It has a tach function, which you can compare to the tach, and even use it to time your engine! It does sound like it is set a bit low, those tachs had a switch to set up for the number of cylinders. Do a Google search, it may reveal the secret settings.

I'd check that as well. The Volvo 240 had this issue. The car came with both a 4 and 6 cyl engine, and both used a Bosch tach. But the tachs were calibrated to the engine, so while they were identical visibly, they were marked in back because they weren't swappable.

The symptoms you describe exactly match the results of swapping tachs.

Best,

Lanval

Sun Jul 20, 2008 1:24 pm

The Vanagon VDO stock tach has a potentiometer screw on the back, that you can use adjust the needle settings if you are over or underreading the RPMs.

There is no switch for 4/6/8 cylinders. That's only on aftermarket tachs.

It's very awkward to adjust the factory pot screw with the instrument cluster in place, but it is possible. You might still have a factory sticker covering the tiny access hole for the pot screw.

If you bottom out that screw and you're still overreading, then you can change out one of the resistors in the tach. Caution... this is somewhat major electronic surgery, and should not be attempted if you can't solder well.

The diesel VW people are way ahead of the curve on reverse-engineering the VW tachs.

This guy replaced a 13K resistor on a diesel tach that was way overreading, after bottoming out the stock pot screw and still overreading. He put in an adjustable resistor and dialled it to about 8.2K ohms, and it was bang on the money.

http://www.vwdiesel.net/phpBB/viewtopic.php?t=12589

On the VW gas tachs, that resistor was usually 15K (not 13K like the diesel tachs).

http://www.vwdiesel.net/phpBB/viewtopic.php?t=3741

Either way, after you've bottomed out the factory pot screw, and you need to replace that resistor, remember this formula: The lower the resistance value of the resistor soldered into that "R6" spot on regcheeseman's schematic, = the lower the RPMS that will be indicated on the gauge face.

Mon Jul 21, 2008 7:41 am

Many thanks, Joe. You're the first person I've spoken with who knew anything about adjusting these instruments, and that includes the VW specialist shop I frequent, which gives me no shortage of heebie jeebies, let me tell you.

Sounds like a job for my mechanic and a timing light.

Cheers,