S4 proper rpm readings on tacho for idle?

3295 Views 15 Replies 5 Participants Last post by msiert, May 1, 2009

f512tr

Discussion Starter · Aug 10, 2007

The tacho on my 91 S4 appears to be showing about half of what I guess is the real rpm. From what I can tell it reads off the coil and at smooth idle shows on 500rpm. Would these cars normally be at 1000 rpm and if so any ideas how I can get the tacho to read properly? The car has 31k miles and I am sure has the original gauges but something is not right (unless 500rpm is the normal idle speed but it sure sounds faster than that!) Many thanks

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Gubi

· #2 · Aug 10, 2007

Should be a tick or two below 1000. I'd be more worried about it's accuracy off of idle, though. Verify the RPMs with a digital tachometer to the coil and then adjust the tach with the procedure below.

http://www.alfabb.com/bb/forums/showthread.php?t=2902

f512tr

Discussion Starter · #3 · Aug 10, 2007

Wow thanks very much, never knew this thread was around. Appreciate it very much

brownie93

· #4 · Aug 11, 2007

Gubi said:
Should be a tick or two below 1000. I'd be more worried about it's accuracy off of idle, though. Verify the RPMs with a digital tachometer to the coil and then adjust the tach with the procedure below.

http://www.alfabb.com/bb/forums/showthread.php?t=2902

f512tr said:
Wow thanks very much, never knew this thread was around. Appreciate it very much

yes, Thanks for the link Gubi

I've been wondering about the bouncing speedo on my S4

Gubi

· #5 · Aug 11, 2007

brownie93 said:
yes, Thanks for the link Gubi I've been wondering about the bouncing speedo on my S4

Don't thank me - John did the work.

Brownie: you have an automatic, right? There's a TSB specifically about fixing a bouncing speedometer needle on the auto S4s - it's 40.93.04 Do you have a cardisc?

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brownie93

· #6 · Aug 11, 2007

Gubi said:
Don't thank me - John did the work.

Brownie: you have an automatic, right? There's a TSB specifically about fixing a bouncing speedometer needle on the auto S4s - it's 40.93.04 Do you have a cardisc?

Yes I have the Cardiskand hubby printed it out for me

Thanks I'll have a look, it is strange when the needle bounces up to 120km an hour when I'm sitting at a light.
Joyce

Gubi

· #7 · Aug 11, 2007

That sounds like the problem in the TSB, yes.

AngloSpider

· #8 · Apr 30, 2009

I've just done a search and found this thread. I've got the car disk but not sure what a TSB is - how do I find it on the disk? Is it somewhere in the 450+ pages of the manual?

Thanks

Gubi

· #9 · Apr 30, 2009

TSB is a technical service bulletin. If you have the old Cardisc (with the Windows application to view pages) TSBs are near the bottom of the list of chapters. In the new Cardisc it's a separate PDF file called "1977-95 Technical Service Bulletins" or somesuch.

msiert

· #10 · May 1, 2009 (Edited)

I found my tach off by 200 rpm in 3rd and 300 rpm off in 4th in general. The amount I'm off varies to what rpm and gear I'm in. For example in 4th at full throttle at 6,500 rpm I'm actually at 7,000 rpm (500 rpm's off) but at 3,500 rpm's crusing in 4th I'm off by 300 rpm?

Always made me wonder how one can adjust the tach..............what rpm do you adjust too..............idle, 3,500 rpms 6,500 rpms when it is off by different amounts at different rpms and if you set it at idle will it be correct when the throttle peddle is floored rapping through the rpm's?

Sometimes I think the stock tach is not able to keep up when you are screaming up to red line.

I have a programmable digital shift light that has a digital tach that I use on the track to give me the true reading and my shift point is determine by a flash of the shift light vs looking at the tach.

I have adjusted the speedo to be coorect as per John's thread but my tach does not have an adjustment screw. I think that was gutted when it was sent off to Palo in San Fran to work with the aftermarket ignition.

The speedo is dead on after the adjustment. It took several trys but it's nice to know how fast your going. It didn't stop me from getting a ticket last Summer though.

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Gubi

· #11 · May 1, 2009

Murray: barring any other hardware issues, generally it's off by a fixed percentage amount, not an absolute shift. If you do the math you're off by 7% at the high end and 8.5% at the low end, which are within the error bars of being the same.

I actually graphed mine out across the whole rev range and before cal it was reading pretty consistently 10% low at all RPMs. After calibration it's good enough for government work.

Yeah, your tach has been extensively modified. You sure that white thing on the board isn't a pot? There should be an adjustment somewhere.

msiert

· #12 · May 1, 2009 (Edited)

Gubi said:
Murray: barring any other hardware issues, generally it's off by a fixed percentage amount, not an absolute shift. If you do the math you're off by 7% at the high end and 8.5% at the low end, which are within the error bars of being the same.

I actually graphed mine out across the whole rev range and before cal it was reading pretty consistently 10% low at all RPMs. After calibration it's good enough for government work.

Yeah, your tach has been extensively modified. You sure that white thing on the board isn't a pot? There should be an adjustment somewhere.

Never thought of it that way but you right between high and low it's about the same %.

What do you mean by "a pot"?...................... I looked for the adjustment screw and there is none.

Here is another angle of the tach guts FWIW

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Gubi

· #13 · May 1, 2009

pot = potentiometer = variable resistor = adjustment screw

If PA Speedo rebuilt it, you might want to call them and ask how you adjust it.

msiert

· #14 · May 1, 2009

Gubi said:
pot = potentiometer = variable resistor = adjustment screw

If PA Speedo rebuilt it, you might want to call them and ask how you adjust it.

Good eye Tom after zoming in it does look like the "pot" has a screw on it? At least it looks like it has a slot in the silver thingy.

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Gubi

· #15 · May 1, 2009

Could be. See if you can find some markings on it: maybe we can ID the component. Or just call PA Speedo and see what they say.

msiert

· #16 · May 1, 2009

Gubi said:
Could be. See if you can find some markings on it: maybe we can ID the component. Or just call PA Speedo and see what they say.

I'll call and see what he says.

I think it's a standard board that he uses for OE tachs to read the tach output of the aftermarket ignitions systems.

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