Erratic tach dial during cruising

biosurfer1 · 07-29-2005, 07:23 AM

I am posting this for a friend. He has had a problem with his tach as of latly, he said he will be cruising at around 3000rpm on the dial and the tach dial will flucuate to 7,000 rpm's and behave very erratically. when he goes to idle, it will show the correct rpm, but when he takes off again, it will start up again erratically. it doesnt do this all the time, just occasionly.

Any ideas as to what is wrong? Is the tach going out?

Thanks everyone!

jdowty3 · jdowty3's Garage

this is a typical symptom of a voltage regulator going bad

Yellowbird RS · 07-29-2005, 08:08 AM

also check the points if you have it.

Early_S_Man · 07-29-2005, 08:22 AM

We need to know what year car and engine???

Tell him to monitor system Voltage and report the level at idle, 2000 rpm, and 4000 rpm ... also what the reading is when the tach is crazy????

coldstart · 07-29-2005, 08:38 AM

Excuse me for barging in on the thread, but the voltage regulator answer scared me. My tach will sometimes go dead for periods (5 minutes) and then come alive again! Is this a fuse/bad connection problem or the voltage regulator?

Early_S_Man · 07-29-2005, 08:47 AM

coldstart,

Do you have a digital multimeter? Special test cord from cig. lighter plug to multimeter is easy to fabricate ...

Hard to say without Voltage readings, both when tach is normal, and when it is dead!

It could be strictly an internal problem with a bad solder joint on the printed circuit board inside the tach, or it could be an intermittent ball bond inside a transistor [or other semiconductor] that is failing.

If it is an internal tach electronic problem, here is a potential solution with relatively low cost ...

$1.49 LM2917 IC does tachometer, MFI Speed Switch, rev limiter, or shift light ...

arcsine · 07-29-2005, 08:53 AM

I had the exact same issues and the problem was solved with a new voltage regulator.

dlrnzo · dlrnzo's Garage

I had a similar problem with and '83sc. It was a bum alternator. In addition to the wacky tach, my lights would surge and eventually fried my battery.
FD

bkreigsr · 07-29-2005, 09:40 AM

Had the exact same problem with tach in my '73 mfi. Happened about a year after I did the generator & regulator. Palo Alto serviced the tach and replaced the brains (?) control module for about $250. Have not had any issues since. I could have bought a replacement at a swap meet, but wanted to keep the correct date stamps. good luck

jdowty3 · jdowty3's Garage

Sorry coldstart, I didn't mean to scare you. Your symptoms sound more like an internal or external connection problem.

Biosurfer, as many have posted, the erratic tach is often a symptom of a problem with the VR, alternator, or both. My 83 exibited the tach bouncing and head lights surging brighter when the VR was on the outs. It's a problem you should diagnose quickly to avoid possible burned out components and melted wiring that can occur with overcharging problems.

biosurfer1 · 07-29-2005, 10:27 AM

JDOWTY3, i think you hit it on the head, because he also said his headlights would slightly brighten and dim, so i'll have him check the VR first....btw it is also an 83 sc

BlkBird · 07-29-2005, 11:24 AM

Sounds like my problem - voltage regulator went sour. It starts putting out 18+V intermitantly and the tach goes nuts. Plus the stereo would shut down as the power protection circuit cut in.

New voltage regulator and it all works fine.

HardDrive · 07-29-2005, 01:19 PM

Had the same issue, regulator fixed it for me. 82 SC.

biosurfer1 · 07-30-2005, 02:32 PM

so we got a voltage regulator to hopefully fix the tach, and now the problem is, where does it go?

we thought it would be in the engine electronic compartment, but cant seem to find it? can someone take a picture of where it goes or point me in the right direction?

its for a 83 sc...thanks!

Early_S_Man · 02:40 PM

You have to remove the alternator and fan housing ... reg goes on the back side [front of car] of the alternator.

wswilburn · 07-30-2005, 05:21 PM

I know this is not biosurfer1's problem, as he is seeing huge tach fluctuations, but I'll put this in in case someone does a search. I was seeing small tach fluctuations, typically around 3000 rpm and it turned out to be vibration from the front wheel bearings. Of course, there was also bearing noise, so I new they had to be repalced, but I was surprised that the tach problem went away with the new bearings. I had assumed it was a separate problem and was on my list to fix!