Opihi59

Well folks, it's been a long time since I posted, but have been lurking around the forum keeping quiet.

Here's the problem: Two days ago my tachometer quit working. The needle will jump to zero when I turn the ign switch to "on," but it didn't seem to move at all with any increase in engine revs. I noticed today that it does seem to move a tiny smidge, like up to 300 RPM range (three hundred, not three thousand) with revving. Nothing else appears to be wrong, and this problem has never happened before. There are no modifications to this vehicle, it is stock 94 with LT1, manual, tachometer is analog, not digital. Infrequent driver with about 30K miles on it or thereabouts.

Here's what I have done. I read thru my Factory Service manual to see the info there which suggested connecting a special diagnostic electronic tool to it which of course I do not have. The remedy was to either replace Tach filter, or to replace tachometer assy. Not all that helpful. It did say to check the ground which is at G108. The schematics tell me that the tach filter and G108 are to the passenger front of the engine. I checked the ground, cleaned, it is fine. No change. The tach filter itself looks fine, as in no charcoal, no broken wires, fraying or other obvious electronic disasters. I looked up on my Corvette parts CD and it gave me a PN AC-Delco 12088559 which is also GM 12088559, but in Eckler's catalog the parts listing only shows Tach filters up to 93 and that one has PN 44837 with the annotation that it replaces GM 1208859. Ecklers also has an offer to rebuild your tach/speedo gauges if you send it in to them along with $400. I'd rather not spring on that option at present.

I have done several searchs on the Forum over the last 2 days and read thru the relevant links on the FAQ, and understand that the 90/91 series tachs are notorious for a problem with the tach circuit and there are very helpful photo writeups detailing how to add Radio Shack sourced resistors in series across the pins to correct the problem with those year groups. But I've got a 94. The Tach problem which was addressed seemed to be associated with the tach needle jumping wildly, or not being calibrated to actually be a reliable instrument. I do not know that the symptoms being addressed match my problem. I did find 2 threads which were short and didn't have any resolution or followup on the problem one was a 94 the other a 96.

So now comes the question. Does this sound as if this is a tach filter issue, or a tach circuit board issue? Or of course am I barking up the wrong tree entirely. Is the tach filter listed with Ecklers for the 93 the same for a 94?

I appreciate your thoughtful consideration in reviewing this problem I am having, and offering me some suggestions on this. I prefer not to drive without a tach.

Aloha

ccrazor

no help but a bump to the top..

Opihi59

No new developments. Today it decided to hang out at ZERO and not move at all at any RPM. I'm still trolling for thoughts on this.

CCrazor, thanks for your earlier bump.

larry93lt1

No Idea, but will be willing to stand next to you and scratch our collective foreheads.

Coach class round trip ticket and a couch are all that would be needed.

Oh yeah some Kona coffee in the AM also.

Opihi59

I would likely spring for some Kopi Luwak if someone came up with a solution. I can't really tear this apart at present, it is my only ride at the moment while my truck is out on loan and the kids have my Jeep for school.

I hate being a parts swapper and throw various new parts at it until something finally works, but I may just have to spring for a new Tach filter from whereever I can find it, and I have an very generous offer for a loner 93 instrument cluster to swap in if that isn't the solution. At any rate, this weekend I'll start pulling stuff off, cleaning and putting back on, and see if I can't order the Filter tomorrow.

BlackbirdZ07

I bought the Eckler's tach filter for my '92 a while back but I never used it. I'd be happy to ship it to you. If it fixes the problem, pay me something for it. If not, send it back.

I'd probably try to determine whether it'll work for your '94 first though. The issue could be the different optispark. Maybe it's just a simple matter like a different connection to the tach filter, but maybe the different optispark requires a different tach filter.

0ChandlerMotorsports

Possible, but doubtful.

BlackbirdZ07

Read carefully. I didn't say his problem could be the optispark. I was simply wondering why the tach filter Eckler's sells is specified for 1992-93, not 1994-96. I was simply guessing that the reason may be because the optisparks are different.

Opihi59

I have a new Tach filter inbound, I'll put it in likely next weekend and let everyone know how it comes out. I've already unwrapped the wiring etc so it's just a matter of getting the part.

If that fails, I have a line on a loaner cluster up the road that I'll try out next.

I know the topic may put a lot of people off. "Tachometer Filter" may sound like some fictitious part, such as Timken Muffler Bearings, or brake-light fluid. It actually is a legitimate part, so I promise I'm not pulling anyone's leg. It's not like someone is getting bogus advice on where to buy Metric Fallopian tubes, etc.

At any rate, I will post back up how it all turns out.

HlhnEast

Please do post up what you find out, my tach does almost exactly what your talking about on my '85. Mine will almost always jump at cranking but goes back to zero. It worked for a short period the last time I drove it so it seems to point to a weak connection/solder joint to me. The atari dash was recently rebuilt according to the PO but I think if the function works on the bench the rebuilder does nothing with it.

Good luck!

Opihi59

So today was the day to install the new Tachometer filter. It didn't fix the problem. I suppose now I need to look at the instrument cluster itself. Either used one off Ebay, or send mine in to get "rebuilt."

Photo of the tachometer filter. Location on the 94 is right front of engine, above and to right of Optispark, just adjacent to the power steering pump fluid reservoir.

Stu 91 drop top

I see you have already read tach reading high in this forum. The problem there is a resistor that over time degrades and makes the tach read higher. if there is not enough resistance the tach will be low. I would check the resistance just like the other forum. There may be a poor or shorted conection.

http://forums.corvetteforum.com/c4-g...ch-repair.html

Thsi is a link to the camaro forum. may be of some help.
http://www.thirdgen.org/techboard/el...meter-fix.html

Opihi59

Thanks Stu, yes, I read thru all the 90/91 threads. I haven't pulled out my Instrument Cluster yet as I need to be able to drive this. I may very well be able to check resistances but would imagine the board will be significantly different. I will report back to the forum as I go along. Thank you for the Camaro forum link, I'll dig into that.

Opihi59

Today JamesLT1 dropped by with the Instrument cluster (IC) from his 93. Nice to meet you James, my first meet with a forum member.
From looking on ebay at ICs, there was a cut off between 93 and 94, implying that the 93 would not work for the 94. One difference is that the 94 has White numbers, the 93 has Orange.
I put it in, and fired up the motor--the tach works, so that tells me my IC has a bad tach in it. I did find one other difference between the 93 and 94s, the 93 had a "shift" light while the 94 does not. The shift light stays ON when the 93 IC is plugged into my 94. (EDIT-I have since found out that the pin feeding this light is for the "door open" warning light on my 94 cluster. With my door closed, the "shift" light goes out, and comes back on with the door open) My current thought is to put it all back together with the 93 in while I figure out what to do with my IC, either get one off ebay, or send in to Wilcox, or Ecklers and get it refurbished. Not too excited about dropping 400 bones plus the shipping. I will likely pull mine apart and see if there is something obvious on the circuit board that I can fix. My guess is there won't be anything I will be able to find wrong with it. I would certainly love to hear from any wire-heads on the forum now that James and I have narrowed down my problem.
I will submit some photos in a bit when I put the dash back together and download the camera/load to photobucket. I hope you all have a peaceful and relaxing day with your families this labor day.

Opihi59

So now the photos. There are a number of well written and photo documented writeups available for this task, I'll just show a few photos. Disconnect your neg terminal at the battery first. After you drop the carpeted panel above the pedals (multiple small screws, pretty obvious) you will see this:



Remove the heavy gauge corrugated sheet metal, 4 bolts, you will get to this where you must remove the 2 15mm head bolts that hold the steering column in place. You only need to drop the column down enough to be able to clear the bezel and the instrument cluster when they come out. You can support the steeering wheel on your lap at this point while doing this.



It is a simple task to remove 4 phillips head screws that hold in the bezel, and then 4 hex head sheet metal screws that hold in the instrument cluster itself. The cluster presses in to its connector, there is not a wire bundle that pulls out that you need to disconnect. Connector can be seen to the left rear once the cluster is pulled out.



Comparison photos--White letter 94 cluster on top, and Orange letter 93 cluster below. Photos front, and rear with the same arrangement, 94 on top.





I am looking into repair on my original unit now, willing to listen to any advice from the forum.

Opihi59

Figured I'd update with new information and some more photos. Everyone reading thru my thread should make sure they are familiar with PopEvans' thread: http://forums.corvetteforum.com/c4-g...ch-repair.html In post #7 of that thread, Jim85IROC tested resistance across pin 4 and 10 of the resistor pack and got 69.6 kOhms, I got 16.X kOhms which is way too low. The low resistance in a board Jim had across those 2 pins produced the same problem as I am having--tach reads Zero and won't move. In post #14 of that same thread, Stu 91 drop top shows the pin out schematic of that resistor pack, and a table below with some referenced readings across selected pins. I didn't check my resistance across those points other than 4--10 so I am afraid I don't have anything to add to his findings. Of course, I had to take apart the actual instrument cluster to do this which is a simple task so long as you have torx head screwdrivers.

If you snap the black plastic cover from the back of the cluster, you get this



The circuit board proper is held to the rest of the assembly by pin plugs into the individual instruments, carefully disengage these pin plugs and disconnect the lead to the LCD board and you get this--it is the front side of the board you saw above.



The tachometer circuit can be identified easily enough on the board. I have identified the two 14 pin components (IC chip and black-on-white resistor pack) with an O ring since I'm not smart enough to photoshop on a circle like the rest of you seem to do so easily.







I have explored options to replace those 2 components, or at least the resistor pack entirely. I can't readily find the components in Hawaii. Ebay has used Instrument clusters for $200 in general, various dealers like Eckler's, MA motorworks, etc can repair your cluster if you ship it in and cost runs between $325 to $400. I decided to send just the board itself to an electronics company that will replace my components/test for $50 plus shipping of course. That unit went into the mail yesterday and I will fill everyone in once it is back, reassembled and installed. In the meantime, I'm using the loaner 93 cluster, but haven't driven the Vette more than 15 miles (got freon re-charged) and am using my truck instead.
Hopefully this will fix my problem. I'll let everyone know. Thanks to the above mentioned forum members who have PM'd me recently on this issue.

Opihi59

So here is an update for anyone interested. My Circuit board came back today, with the Tach circuit chips replaced. If you compare to the earlier photos, you can see what has been replaced.







The good news of course is that I reassembled the Instrument cluster, reinstalled and the tachometer now works correctly. I am not certain as to how much help this will be to the forum in general, but it was a lot simpler and less expensive than sending in my entire Instrument cluster to the "usual places" and spending 8X as much for this repair.

If you have any questions on this, please feel free to chime in, or to PM me. No, I am not an expert, I am just an aging dude that bought a 94 Vette new and it is aging as well. I appreciate all the tips and tidbits I learn from the forum, and feel obligated to pass on whatever I am able.

ccrazor

Glad to hear you are up and back to normal.. Few years ago I looked at a 94 that had this problem and passed for lack of a fix.. That's why I jumped in with my bump.. Curiosity.. LOL

Opihi59

Thank you for the interest and support. There are many threads and posts and you have to shuck a lotta oysters to find a pearl. Hopefully my topic is "searchable" and will be of benefit to someone in the future. This really wasn't a difficult repair, just took some time to work thru, a few false starts, a lotta digging thru the FSM, etc. I did meet a great bunch of people though in doing this so it is all worthwhile. And the best thing is it didn't set my wallet back a ton.
Now you gotta go back and find that 94 you passed up.........
If anyone has this issue, let me know and I can connect you to a source for repair.

Stu 91 drop top

Fantastic. half the problem is knowing where to look. This will definetly help the next guy.
Stu.