OK. So I installed a Cavalier digital dash last year and it worked fine with the 88 2.5 DIS system on the engine. It read lower than actual but that was b/c of the difference b/w 4 and 6 cylinders. I figured with the V8 the tach would read higher than actual...and it does up to about 2k and then the needle drops to zero. let off the throttle and the tack goes back to normal as the rpm drop below around 2k. Anyone know why and have a solution?

Bump

Are you running the tach line directly from the dist to C500?

Have you checked what you get with a fiero or other analog tach?

quote Originally posted by phonedawgz: Are you running the tach line directly from the dist to C500? Have you checked what you get with a fiero or other analog tach?

1. yes I am. Do you know if this is ok?

2. That's the thing, the gauges are all digital except for the tach, it's analog! Don't know why...must have been cheaper during manufacturing. This tach in the cavalier dash worked fine with the 2.5 DIS system though. I have read that there is some noise in the distributor tach signal on these 4.9 distributor types that can set off the tach display. I looked but didn't find any definitive type of sigal filter or whatnot. Anyone out there know?

[This message has been edited by Reallybig (edited 09-04-2012).]

1. - Yes that is ok

2 - The ICM used in the 4.9 distributor is the same (or at least the replacement is the same) as the SBC

BUICK CENTURY (1980 - 1985)
BUICK ELECTRA (1981 - 1986)
BUICK ESTATE WAGON (1981 - 1989)
BUICK LESABRE (1980 - 1990)
BUICK REGAL (1980 - 1987)
BUICK RIVIERA (1980 - 1985)
BUICK SKYHAWK 1980
BUICK SKYLARK (1981 - 1982)
CADILLAC ALLANTE (1987 - 1992)
CADILLAC BROUGHAM (1987 - 1990)
CADILLAC COMMERCIAL CHASSIS (1981 - 1992)
CADILLAC DEVILLE (1981 - 1995)
CADILLAC ELDORADO (1980 - 1993)
CADILLAC FLEETWOOD (1981 - 1992)
CADILLAC SEVILLE (1980 - 1993)
CHECKER MARATHON (1981 - 1982)
CHEVROLET ASTRO 1985
CHEVROLET BEL AIR 1981
CHEVROLET C10 PICKUP (1983 - 1986)
CHEVROLET C10 SUBURBAN (1983 - 1986)
CHEVROLET C20 PICKUP (1983 - 1986)
CHEVROLET C20 SUBURBAN (1983 - 1986)
CHEVROLET C30 PICKUP 1983
CHEVROLET CAMARO (1981 - 1986)
CHEVROLET CAPRICE (1981 - 1990)
CHEVROLET CELEBRITY 1982
CHEVROLET CHEVETTE (1981 - 1982)
CHEVROLET CITATION (1981 - 1982)
CHEVROLET CORVETTE (1981 - 1991)
CHEVROLET EL CAMINO (1980 - 1986)
CHEVROLET G10 (1983 - 1986)
CHEVROLET G20 (1983 - 1986)
CHEVROLET G30 (1983 - 1986)
CHEVROLET IMPALA (1981 - 1985)
CHEVROLET K10 PICKUP (1983 - 1986)
CHEVROLET K10 SUBURBAN (1983 - 1986)
CHEVROLET K20 PICKUP (1983 - 1986)
CHEVROLET K20 SUBURBAN (1983 - 1986)
CHEVROLET K5 BLAZER (1983 - 1986)
CHEVROLET MALIBU (1981 - 1983)
CHEVROLET MONTE CARLO (1980 - 1986)
GMC C1500 PICKUP (1983 - 1986)
GMC C1500 SUBURBAN (1983 - 1986)
GMC C2500 PICKUP (1983 - 1986)
GMC C2500 SUBURBAN (1983 - 1986)
GMC CABALLERO (1980 - 1986)
GMC G1500 (1983 - 1986)
GMC G2500 (1983 - 1986)
GMC G3500 (1983 - 1986)
GMC JIMMY (1983 - 1986)
GMC K1500 PICKUP (1983 - 1986)
GMC K1500 SUBURBAN (1983 - 1986)
GMC K2500 PICKUP (1983 - 1986)
GMC SAFARI 1985
OLDSMOBILE 98 (1981 - 1985)
OLDSMOBILE CUSTOM CRUISER (1986 - 1990)
OLDSMOBILE CUTLASS (1980 - 1981)
OLDSMOBILE CUTLASS CALAIS (1982 - 1984)
OLDSMOBILE CUTLASS CIERA (1982 - 1985)
OLDSMOBILE CUTLASS CRUISER 1981
OLDSMOBILE CUTLASS SALON (1985 - 1987)
OLDSMOBILE CUTLASS SUPREME (1987 - 1988)
OLDSMOBILE DELTA 88 (1980 - 1985)
OLDSMOBILE OMEGA (1981 - 1982)
OLDSMOBILE STARFIRE 1980
OLDSMOBILE TORONADO (1981 - 1985)
PONTIAC 6000 1982
PONTIAC BONNEVILLE (1980 - 1986)
PONTIAC CATALINA (1980 - 1981)
PONTIAC FIREBIRD (1980 - 1986)
PONTIAC GRAND LEMANS (1980 - 1981)
PONTIAC GRAND PRIX (1980 - 1987)
PONTIAC LEMANS 1981
PONTIAC PARISIENNE (1983 - 1986)
PONTIAC PHOENIX (1981 - 1982)
PONTIAC SAFARI (1987 - 1989)
PONTIAC SUNBIRD 1980
PONTIAC T1000 (1981 - 1982)

So I don't think your problem is one with a 'weird' ICM that was used in only the 4.9.

I would try swapping the ICM to see if that changes this problem. If that does not, then the tach is the next suspect.

http://www.rockauto.com/cat...025738&parttype=7172

[This message has been edited by phonedawgz (edited 09-05-2012).]

Thanks. I'll give it a try.

I had a problem with my tach on the f-body cluster - turned out that the tach itself was not compatiable with the HEI signal coming off the distributor.

Now, I do not remember what I did to fix the issue (seem to recall it was a simple fix once I figured out what was happening) - I did post the solution on here, but where I have no idea. I will see if I can find the info.

Ok, found the post:

quote Originally posted by Mickey_Moose: Ok, this may apply to other dash swaps out there with people running an engine swap using a HEI ignition system (ie: 4.9, carb SBC, etc). Sympton: tach bounces, or work fine till the higher rpms. Why: the modern tachs are driven off either the ECM or a DIS system and supplies the tach with a nice clean signal - the older systems such as the HEI have quite a bit of noise on the signal and the 'modern' tach can not deal with this noise. This circuit should eliminate any noise and send a cleaner square wave to you tach input. Edit: fixed picture I kind of took the lazy way out by hacking up an old Fiero tach to get the filtering circuit out of it. I also have no ideal if the component values are the same as the above diagram (never cheacked). Sorry, I didn't have a working digital camera to take pictures as I cut up the board, but you should be able to get an ideal of where I cut it from the pictures. Front view, the large round hole in the lower right is the input to the tach, the resistor pack (white chip) is part of the one that get removed when doing the V8 tach mod. Rear view, shows a better ideal of the cut in relation to the traces. The brown wire is ground and the upper yellow wire will feed your tach. The lower yellow one is the input from the distributor. Anyways, there you have it. Tim

[This message has been edited by Mickey_Moose (edited 09-06-2012).]

quote Originally posted by Mickey_Moose: I had a problem with my tach on the f-body cluster - turned out that the tach itself was not compatiable with the HEI signal coming off the distributor. Now, I do not remember what I did to fix the issue (seem to recall it was a simple fix once I figured out what was happening) - I did post the solution on here, but where I have no idea. I will see if I can find the info. Ok, found the post:

Awesome Mickey, I'll have to get some parts and give it a try!