FOURSPEEDVETTE

Does anyone have a picture of where the tach filter is located on a 1985 L98? My tach just started becoming very erratic. Engine runs fine at idle and at speed. Tach is jumping all over the place. All the other gauges are perfect. From what I've read, the filter should be the problem.

Joe C

don't have a pic, but the tach filter is mounted to the rear of the LH cylinder head - looks like a big capacitor - white wire on both ends. before you replace it, clean the electrical connections.

FOURSPEEDVETTE

Do I need to remove the valve cover to get at it? I think I can see it but getting it out looks like a pain in the butt. Are new ones still available?

Joe C

as for the VC, can't really say. I recently pulled the heads on my 85 for a valve job. I pulled the bracket w/ the filter off with the head. availability - factory OEM, i'd say no (i'm thinking the GM # is 14020025), but replacements are - try willcox, corvette central, etc.. replacement filters may require some wiring creativity. only NOS will have factory connectors. replacements have pigtails.

http://www.ebay.com/itm/1975-1991-Corvette-Tachometer-Filter-/351542448374?hash=item51d99010f6&vxp=mtr

bill mcdonald

https://www.corvetteforum.com/forums...1990-vert.html

DanielRicany

Best way to remove it is to remove the windshield wiper motor. It's bolted to the back side of the drivers side cylinder head. It is connected to the distributor via a wire, and has another wire connected to it that goes to the ECM/Digital Dash or wherever.

FOURSPEEDVETTE

Well I just replaced the tach filter on my 85 in about 45 minutes!. It works fine now. Instead of removing the valve cover, air pump or windsheild wiper motor, I just snipped the wires off of the hidden and hard to get at old filter and left it in place. No one will ever see it anyway. I grounded and bolted the new tach filter down by using one bolt on one side of the thermostat housing. There is a lot of thread showing on that bolt so I just added a new nut and washer on top of it. I then soldered the two wires and shrink tubed them. I ran the two wires through black wire tubing back to the distibuter area where the old part was. It looks like it should have been done this way from the factory! Very easy!

357L98

Much less trouble the way you did it. That thing is really hard to get to in it's original location.

Cruisinfanatic

Exactly what I did. Sniped the wires and mounted new one elsewhere. Didn't solve my problem unfortunately. Get an immediate reading (2700) on tach for a second when key is turned on, but nothing after starting. Lights fine, just says zero

FOURSPEEDVETTE

Is the new filter bolted down on something metal from the frame or engine block? Sounds like yours may not be grounded or the attachment clip on the distributor is not connected completely.

Cruisinfanatic

putting new one in didn't change what it did before

Joe C

pretty sure that metal bracket has nothing to do with grounding. not saying someone doesn't need to mount it securely though -

FOURSPEEDVETTE

Wrong. The metal bracket MUST be attached to the frame or a metal component. Simple automotive electronics. It completes the electronic circuit.

383vett

This is correct. The bracket is simply a bracket, not a ground. Mine's been flopping around the bellhousing area for years unsecured and the tach has never skipped a beat.

Joe C

sorry, I stand corrected -

FOURSPEEDVETTE

True. The stock tach can still work without the filter. But if the circuit gets a spike, the stock digital tach may not operate. When the filter goes bad in a stock digital (early C4) display, the gauge will go nuts if the bad filter is left in place. Part of the instructions that come with the new replacement filter specify that it MUST be bolted down to ground it so it can operate (filter) as it's is designed to do. As I said it's just simple electronics. I won't take the chance of burning out the gauge cluster over a simple $30.00 part replacement.

FOURSPEEDVETTE

Exactly.

383vett

I see the schematic. I don't understand how the unit is grounded when the bracket clamps around the nonconductive outer surface of the filter. I may very well be wrong but nonmetal surfaces don't ground and mine works after 30+ years.

Joe C

that what I was originally thinking. looking at he diagram, it must be internally making a conductive path to the mounting bracket. I initially thought it was nothing but a capacitor until I looked at the wiring schematic. now, it looks like a sort of band pass filter.

FOURSPEEDVETTE

The metal mounting bracket clamps around the metal jacket of the filter. One end of the filter is metal and the other end is plastic or hard rubber. Both ends have one wire coming out to complete the circuit. The ground is conducted internally. If you look at the ones being supplied now from the after market you will see how its made. The new one looks just like the orginal one, only its silver instead of black. Same exact size. If you still have the original filter working with the original early C4 digital tach and it's not grounded by its original bracket, it may have been grounded somewhere behind the dash. Or it's operating without the filter. Just watch for electrical power spikes. That can damage your digital dash from what I've read from quite a few sources.