Tach pegged

Pat Crellin · 03-24-2007, 10:18 AM

I've been hearing a squeaking squeal from the back of my engine.

Thinking it may be the fan belt, I loosened it, want to change it but can't get the a/c compressor to slide (earlier question).

Now the noise is gone but the tach is pegged, drops about 1000 rpms when I let off the gas.

Showing 14.4 volts at idle, alternator & VR is about 2 years old.

Bad Cable? Are 14.4 volts OK?

Pat Crellin · 03-24-2007, 02:40 PM

Anyone? I'll take bad advice at this point.

tt_targa · 03-24-2007, 02:51 PM

I think 14.4 V is high enough to affect tach reading.

If the alternator is always at 14.4, the regulator may be shorted.

Does it ever go down to 12V?

The alternator light will only come on with under voltage.

These comments are correct of SC and Carrera years.

Pat Crellin · 03-24-2007, 03:03 PM

I checked again and it's 14.2 - 14.3. Doesn't go down when I turn on the lights.

Think there's a relationship with the noise stopping and the tach pegging?

joecrum · 03-24-2007, 05:11 PM

Check your other gauges carefully. If you find other gauges are way off, it may be that a wire has come loose/broken at the gauge.

Just a guess since a friendly mouse chewed through a single wire on my speedo and half the gauges were off.

Good luck,

Pat Crellin · 03-24-2007, 06:16 PM

Everything else electrical seems ok.

Here's what happened. I've had a noise, squeal/chirping noise coming from the back of the engine. Thought it was a belt, maybe a bearing. Was going to change the fan belt but couldn't get the a/c compressor to slide to get the a/c belt off. Ran out of time & put it back togeather.

Pulled the coil wire off the dist. to crank the engine to adjust the belt.
Satisfied I took off & noticed the noise was gone. Then noticed my Tach was pegged.

When I turn the key the tach pegs, red lights come on. Start the engine, the red lights go off & the tach remains pegged at 7000. If I run the rpms up the tack will drop to about 6000 and then back to 7000 as the rpms drop.

Looked at the back of the tach, everything seems in order.

Where does the tach get the rpms from the engine? Is there a connection I may have knocked loose?

rick-l · 03-24-2007, 07:12 PM

Quote:

Originally posted by Pat Crellin

Pulled the coil wire off the dist. to crank the engine to adjust the belt.
Satisfied I took off & noticed the noise was gone. Then noticed my Tach was pegged.

Does this car have a CDI unit?

joecrum · 03-24-2007, 07:21 PM

Pat,

The gurus will be around before too long to help you sort this out.
Meanwhile, you might check the other gauge connections too.

rick-l · 03-24-2007, 07:22 PM

Early_S_Man's how to fix it

Note the big red warning.

Pat Crellin · 03-25-2007, 04:20 AM

I was suspecting it had something to do with my pulling the coil wire to crank the engine.
I don't know what a CDI is, I do have a 3.2. What would be the fix?

rick-l · 03-25-2007, 08:40 AM

Oops

I didn't think an 84 had a DME. I think 84 was the first year? Your tach problem may be coincidental although it isn't a good idea to disconnect the coil wire and try to start the car.

Pat Crellin · 03-25-2007, 08:49 AM

Change the DME?

rick-l · 03-25-2007, 08:56 AM

Quote:

Originally posted by Pat Crellin
Change the DME?

No

In the DME there is a seperate driver for the tach that is pretty well isoated from the coil driver. If the car runs the DME is probably fine.

I am searching for LorenFB post on testing the tach with a mill bastard file.

Pat Crellin · 03-25-2007, 03:26 PM

That's what I was thinking, a mill bastard file.
What's a mill bastard file?

rick-l · 03-25-2007, 05:03 PM

Quote:

Originally posted by Pat Crellin

What's a mill bastard file?

It has a combination of mill cut teeth and course cut teeth, what you would buy at home depot.

What is important here is when you ground it, and slide a wire connected to the input of the tach along the teeth, it forms a crude function generator and the tach needle should deflect (assuming it unpegs after you disconnect it from the DME).

Pat Crellin · 03-25-2007, 07:08 PM

Could you explain this like I'm a 3 year old?
When I turn the key off the needle does go back to zero.

rick-l · 03-25-2007, 08:36 PM

I haven't been much help.

I did a search and found very little on an 84-89 VDO tach.

The tach works on an AC signal from the DME. To isolate the tach or DME fault you would have to unplug the DME signal from the back of the tach and see if it drops to zero. If it does then you could verify the tach operation using the file trick.

I may have some more information on another computer I will have access to tomorrow.

In any case your tach does not sound very healthy but you need to isolate the fault to one or the other.

Pat Crellin · 03-26-2007, 05:02 AM

So is this a way to test the tach? Is the plug for the DME the wide one on the bottom of the tach?

rick-l · 03-26-2007, 03:48 PM

Tachometer problems seem to have been few and far between on this board. Myself included I have no inkling of what is inside.

That being said if you isolate the problem to the tach (disconnect just the pin from the DME and it stays pegged) take it apart and snap a few pictures there will be electronics experts falling over themselves to figure it out.

I wonder if it has a SAK215 IC in it?

Pat Crellin · 03-26-2007, 07:26 PM

I was thinking it has a SAK215 IC in it.
What's a SAK215 IC?
I think I'll call Rick at speedometer.com tomorrow & see what he thinks.
I'll let you know.