Hey guys,

My 1990 Vette's tach seems to be running high. The scanner still shows a reading of 825 rpm. The mechanic replaced the CCM and the prom to fix another problem and now my tach seems to be messing up.
Any comments that are serious would be greatly appreciated. The problem with the tach was not present prior to the mechanics' fix.

Thanks

 

Not even a helpful guess?

 

I am having the same problem on my 91. I haven't had a chance to look at it to indepth, but I have determined with a hand held tach that mine is exactly twice what the engine is running. I am an Electronics technician by trade and am planning on disecting mine but my truck blew its fuel pump and its one thing at a time and I don't like driving my vert every day rain or shine. I am betting good money that it's a burnt up component in the tach circuit board on the back of the dash cluster. I'll let you know what I find if that is any help at all.

 

My '90 had the same problem. The cause was the tach filter. It is mounted on the drivers side of the transmission on one of the upper mounting bolts. It is a small round piece that looks similar to a condenser for an older distributor, its got one wire going in and one coming out. It is inline between the tach output from the distributor to the wiring harness going to the tach in the dash. Mine had frayed and exposed strands on one side of it. I tried to get a replacement from the dealership but the part had been discontinued with no replacement sources. I have since seen a part available from ecklers but never tried it out (no longer have that car.) I also used a scan tool and was getting a good RPM reading from the ECM, or at least closer to what it should be. With the bad filter I was getting 6000+ RPM shifts, which isn't really believable with the L98. I ended up disconnecting the filter and using an aftermarket tach wired into the tach output from the distributor. I was kinda worried about just bypassing the filter and running straight into the electronic dash for fear of burning out the dash. I was told that the dash tach is an analog air guage readout but without knowing (plus the air guage part seemed strange as there is no air input) I just used the aftermarket tach. With that in place I was getting the same RPM readout that I was getting from the scan tool.

I would guess the filter is some sort of resistor or a choke, but with a bad one I wasn't sure of what value to use to replace it.

 

Armed with that knowledge, I'll check the filter first. I would bet its a band pass filter and I think I have seen one in one of the mags.
Thanks!

I relocated to Arkansas (Home State) after Katrina and just got another membership to MGCCC are you a member? I lived north of I-10 D'Iberville area. I sure miss the ocean.

 

Never joined, got a buddy, Rain, who is a member. I work as an ET at Stennis, but Im out on the ship for at least 1/2 the year, main reason for not joining.

I don't miss the ocean ... 'cos Im on it 1/2 the year and when Im home I've got the bayou in the back yard.

I took that part to an auto repair place in Biloxi that I was told was pretty good with Corvette electronics (dash repairs, ect) the tech was supposed to give me a call with some info on the part but never did. Can't remember the name of the place off the top of my head.