Factory Tach Problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

palallin · 09-04-2002, 09:58 AM

As I noted on another thread, I finally got my factory tach in last night. However, it's not reading right. The idle speed seems to be real close @ about 800. But, even on the highway, where I know I'm turning about 3000, the highest the tach reads is 1500.

I should point out that, the more I've drinen it, the better it's worked, and that I've only driven it 10 or 12 miles.

Am I dealing with a sticky gauge that just needs to loosen up (or be cleaned)? Does it need to be/Can it be adjusted? Or is there a more serious problem?

I also should note that I'm running an HEI; the tach is hooked up to the right terminal (where my previous SunTach was).

TIA.

HotRod929 · 09-04-2002, 10:14 AM

Could be a bad tach...I have no clue though.

HotRod

HYPERMAN · 09-04-2002, 11:33 AM

I have discovered that mine is off quite a bit as well.

About 200 RPM higher at idle. and read 2200 at 1600..

I wonder if the adjuster at the back will bring this in.

Glad to see that you got it in and working palallin.

How it will be more accurate.

Peter

PS...make sure the tach and cluster is grounding good. The blinking wrong lights in you eariler post got me thinking of that.

palallin · 09-04-2002, 02:05 PM

Adjuster? Hmmm. I'll have to check (thanks, Peter!). Does anyone know which way to adjust it (I'm guessing it's a screw?).

200 rpm isn't enough to worry about to me; 1500 is a bit much

I wonder if I can get to the screw without pulling the dash

I'm afraid to disconnect it

Thanks for the ground tip: I'm not really sure how it grounds. I do know that two of the paths on the circuit board are bad, so I have to round up one sometime: my tester showed me why neither the temp nor the amp gauge work (I may be able to wire in a temp fix for now).

It really amazes me how well some of this old stuff does work after so many years.

RainDog · 09-04-2002, 04:50 PM

From everything I've heard, the factory tachs tend to be pretty inaccurate, even when new. Never used one myself though, so that's second hand.

RainDog

Dandeluxe · 09-04-2002, 04:57 PM

bad ground !!!!

:p

palallin · 09-04-2002, 05:13 PM

Quote:

Originally posted by Dandeluxe
bad ground

But how/where does it ground? Even the Service Manual wiring diagrams are unhelpful there.

Protrucks · 09-04-2002, 05:32 PM

Is it a used tack? If so make sure someone didn't slip you one out of a C-30-50 Diesel motor. In not sure how one out of a Diesel would read but I put one in out of a 6cyl truck and the tack reads to high. Way to high. Diesel might be way to low???

Gene · 09-04-2002, 09:51 PM

I may be wrong, but I think there is also a 6 cylinder tach, and maybe that is what you have? Just a guess....

**Downtown** · 09-04-2002, 10:38 PM

Yes you can adjust the tach without taking it out. Reach up behind the tach and try to find a sticker. Once removed there will be a hole to put a screwdriver through. A short (maybe about 3") regular sized phillips works well.

palallin · 09-04-2002, 11:05 PM

Good to hear it's possible. According to the housing, it's a V-8, which I assume also precludes the possibility that it's a diesel. Of course, the tach itself may have been changed . . . .

I will try the adjustment ASAP.

too many · 09-05-2002, 12:02 AM

I dont think the tach could be out of a diesel. Im no D. mechanic, but I DONT think they have a distributor. I think those tachs were mechanically driven?

Oh, I think the tach grounds thru the srews when mounting it in the dash?

ANY WIZARDS on this?

Rod · 09-05-2002, 12:14 AM

They can be sticky also. I have taken a few apart and cleaned them up and they have worked well.

09-04-2002, 09:58 AM	#1
palallin Registered User Join Date: Jul 2002 Location: St. James, MO Posts: 1,238	Factory Tach Problem As I noted on another thread, I finally got my factory tach in last night. However, it's not reading right. The idle speed seems to be real close @ about 800. But, even on the highway, where I know I'm turning about 3000, the highest the tach reads is 1500. I should point out that, the more I've drinen it, the better it's worked, and that I've only driven it 10 or 12 miles. Am I dealing with a sticky gauge that just needs to loosen up (or be cleaned)? Does it need to be/Can it be adjusted? Or is there a more serious problem? I also should note that I'm running an HEI; the tach is hooked up to the right terminal (where my previous SunTach was). TIA. __________________ '69 Longstep K-10: 327/SM465/T-221/Closed Knuckle Dana44/12-bolt.

09-04-2002, 10:14 AM	#2
HotRod929 Registered User Join Date: Mar 2002 Location: Bondurant, IA but in Worcester, MA for school Posts: 767	Could be a bad tach...I have no clue though. HotRod __________________ 1970 Chevy C-20 350 V-8 Turbo 350 w/ shift kit Dana 60 w/ 4.10 gears AND Powr-Loc POSI New Additions: Summit Headers 135amp alternator Proform HEI Edelbrock 1406 600cfm carb Sunpro Tach Powr-Loc POSI Edelbrock Performer RPM Intake MSD 6A Ignition Box AIM: HotRod929 Visit My Site: http://1970chevy.netfirms.com/

09-04-2002, 11:33 AM	#3
HYPERMAN Registered User Join Date: Mar 2000 Location: Moncton, New Brunswick Posts: 239	TACH I have discovered that mine is off quite a bit as well. About 200 RPM higher at idle. and read 2200 at 1600.. I wonder if the adjuster at the back will bring this in. Glad to see that you got it in and working palallin. How it will be more accurate. Peter PS...make sure the tach and cluster is grounding good. The blinking wrong lights in you eariler post got me thinking of that. Last edited by HYPERMAN; 09-04-2002 at 11:42 AM.

09-04-2002, 02:05 PM	#4
palallin Registered User Join Date: Jul 2002 Location: St. James, MO Posts: 1,238	Adjuster? Hmmm. I'll have to check (thanks, Peter!). Does anyone know which way to adjust it (I'm guessing it's a screw?). 200 rpm isn't enough to worry about to me; 1500 is a bit much I wonder if I can get to the screw without pulling the dash I'm afraid to disconnect it Thanks for the ground tip: I'm not really sure how it grounds. I do know that two of the paths on the circuit board are bad, so I have to round up one sometime: my tester showed me why neither the temp nor the amp gauge work (I may be able to wire in a temp fix for now). It really amazes me how well some of this old stuff does work after so many years. __________________ '69 Longstep K-10: 327/SM465/T-221/Closed Knuckle Dana44/12-bolt.

09-04-2002, 04:50 PM	#5
RainDog Registered User Join Date: Mar 2001 Location: Burien, WA, USA Posts: 391	From everything I've heard, the factory tachs tend to be pretty inaccurate, even when new. Never used one myself though, so that's second hand. RainDog __________________ "Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin slitting throats."

09-04-2002, 04:57 PM	#6
Dandeluxe Senior Member Join Date: Nov 2001 Location: st.louis mo. Posts: 717	tach ???? bad ground !!!! :p __________________ 1969 cst swb 350/400 th . DANNY C . ["]FRIENDS DONT LET FRIENDS DRIVE F@RD$!! 1969 cst swb corvette hugger orange with 400 hp sbc I started out with nothing and i held on to it

09-04-2002, 05:32 PM	#8
Protrucks Registered User Join Date: Jun 2001 Location: Kalamazoo MI USA Posts: 161	Is it a used tack? If so make sure someone didn't slip you one out of a C-30-50 Diesel motor. In not sure how one out of a Diesel would read but I put one in out of a 6cyl truck and the tack reads to high. Way to high. Diesel might be way to low??? __________________ 1967 Chevy 1/2 SWB (under construction) 1968 GMC (propane-under construction) 1969 GMC Flatbed (propane) 1970 3/4 LWB (propane) 1970 Stepside 402 (gasoline) 1971 GMC Burb

09-04-2002, 09:51 PM	#9
Gene Registered User Join Date: Aug 2002 Location: Muskogee, Oklahoma Posts: 2,914	I may be wrong, but I think there is also a 6 cylinder tach, and maybe that is what you have? Just a guess.... __________________ 1971 SWB 4X4 Build BOARD MEMBER #5448 BOOMER SOONER!!!!

09-04-2002, 10:38 PM	#10
Downtown Senior Member Join Date: Dec 1999 Location: New Palestine, IN Posts: 471	Yes you can adjust the tach without taking it out. Reach up behind the tach and try to find a sticker. Once removed there will be a hole to put a screwdriver through. A short (maybe about 3") regular sized phillips works well. __________________ 72 Super Cheyenne Longbed 350 49,000 original miles and My dad ordered it new 66 Chevy Caprice 2 dr 77 Caprice Estate Wagon New Palestine, IN

09-04-2002, 11:05 PM	#11
palallin Registered User Join Date: Jul 2002 Location: St. James, MO Posts: 1,238	Good to hear it's possible. According to the housing, it's a V-8, which I assume also precludes the possibility that it's a diesel. Of course, the tach itself may have been changed . . . . I will try the adjustment ASAP. __________________ '69 Longstep K-10: 327/SM465/T-221/Closed Knuckle Dana44/12-bolt.

09-05-2002, 12:02 AM	#12
too many Registered User Join Date: Jul 2001 Location: nw IL Posts: 211	Just a thought? I dont think the tach could be out of a diesel. Im no D. mechanic, but I DONT think they have a distributor. I think those tachs were mechanically driven? Oh, I think the tach grounds thru the srews when mounting it in the dash? ANY WIZARDS on this?

09-05-2002, 12:14 AM	#13
Rod Join Date: Dec 1999 Location: Eastern - Manitoba Canada, Winnipeg Posts: 4,369	They can be sticky also. I have taken a few apart and cleaned them up and they have worked well. __________________ Senior active founding member. 70 3/4 ton GMC Suburban 4x4 250/4 spd. 71 1/2 ton Suburban Chevy 4x4, 350/350 72 Chev 1 ton tow truck 402BB w/Holmes 440 wrecker on propane 2005 FLSTSCI Harley Springer 85 FXSB Harley Lowrider 72 Triumph 650 Bonneville 5spd "Poor people don't have hobbies" Quote from wife.

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