Hello all! Bought my first Corvette this weekend, a 91 black/black w/ the ZF6. So far I love it! I also have a 97 BMW M3, a lifted Jeep YJ, a lifted 95 Yukon and a daily driven 93 BMW 325i. Although I'm new to a vette, I do all my own mechanic work. I'm trying to fix a few things on the C4 before I start driving it regularly and I've run into some questions.

1) My tach doesn't do what most others do; it goes all the way past 6k when I crank it up, then drops to zero after that. My first guess after some searching is the tach filter. Agree? Should I just bypass it? *EDIT* Fixed! Read here: http://forums.corvetteforum.com/c4-t...ch-repair.html

2) My next question is the PO has installed a cheesy aftermarket temp gauge and I want the OEM one to work instead. Is there anyway to test the cluster gauge before I buy a new sensor and hook it up? Some chevy's IIRC seem to be 0-5V to operate the temp gauge, is that the case for a 91 corvette? *EDIT* Fixed! Found my green temp wire near the starter and a new temp sensor solved the problem!

3) Anywhere to download the FSM for free? and Owners manual? I don't have either yet.

4) What is that sensor that the red arrow is pointing to in the pic below?

 

That is the oil pressure sender for the dash gauge.

 

Ok cool thanks. I couldn't see what kind of line it was connected to. Kind of a weird place for a oil pressure sensor.. that gauge works fine on my car.

 

I found the temp sender wire. There was a green wire above the starter that I couldn't tell where it went. Opened the loom up and it was just a cut wire. I grounded it and my temp gauge went to max.. so this tells me that the gauge works off resistance; open circuit or infinite resistance is <100* (cool), and some resistance, not sure exactly how much yet, is warm, or >100* depending on how many ohms the sender gives, which will depend on temp of coolant. Picking up a sender tomorrow..

Along with 4 rear axle shaft u-joints. I think they might be bad. maybe.. hard to tell.

 

oh i dont know, looks like you could get a few more miles out of them

seriously though how do people let things get like that and still be driving it.. i would think strongly about changing all 6 though if it were me..

 

I'll probably get the drive shaft joints too at some point. They looked good though & had no play that I could tell, and plus I'm ready to drive it! I want to get the cooling system flushed & 100% operational and hoses and fluids changed first then I'll knock other things out as i go.

 

I don't believe that 91s have a tach filter, so that won't work for your fix. It looks like you have a lot of missing/damaged wires going to your distributor which is probably causing your tach issues.

The sender for the dash gauge is (or at least was) in your pass side head, between the 6 and 8 plugs.

 

There's a little black box that looks like a filter. Can someone confirm if these have tach filters or don't?? Not sure what would make my tach do what it does? What missing wires? The tach wire is just unplugged and the hot wire has a cut in the jacket.

 

The '91 does have a tach filter and it's in a module of sorts and likely has the part # 12088538 on it. It's different from the early filter and was later changed to the same part numbered piece for the LT1 cars. You modified the LT1 harness and module to fit. It's not been available for several years either separately or as the harness for the LT1 cars. That part # was 12088559. In the '91 car it could have been found in the engine harness near the bulk-head connector near the left valve cover. I've never had one of those cars so I can't say I've had a "hands-on"!

Correct all of your wiring at the distributor and connect the tach wire. Now with the key on does the tach park at 0 ? What does the tach do or maybe not do now?

If the tach parks at 0 and just does some unusual and random jumps or seems to have a consistent high error I don't believe your tach issue is the filter. I believe it's in the board of the cluster. There are many threads regarding that. The tach filter I don't believe has ever been known to correct a fluctuation error or a higher than expected value.

This was the latest part #'d harness that you modified. The filter is the "box" - does the '91 resemble the box? I believe so but it should bear the # 12088538 regardless of it's appearance.

 

I've read differences of opinion regarding tach filters on a 91. Some say it has one others say it doesn't. I can tell you I couldn't locate one on my 91 and that was looking for it when it was in the condition of the below picture. Not saying it doesn't have one, but if it does it doesn't look like tach filters I've seen before.

 

The OP has located what he calls a "black box" and that somewhat represents the filter as I displayed it and as it's been explained to me. If it has the part # on it maybe the OP can enlighten you with it's location on his. Who knows maybe someone/PO bought and attempted to use the replacement on the car. I don't know but the OP says there's a "box"!

The '91 FSM displays a filter also. If you followed the tach lead into the engine harness before you get to the bulk-head connector I'd think you're sure to find it. It could easily be taped into the harness also and just not readily visible.

 

If you follow the red wire coming off the coil in my first pic, down to the electrical tape, its right there. You just baaaaarely can't see it. I'll take a pic tonight. There was no slack in the harness so I couldn't pull it out easily. The tach rests at zero til you fire the engine up. Then it immediately goes to 6k+ for about 2 seconds. Then back to zero where it doesn't move again until you crank the engine again.

 

TA, I think I can almost see yours taped to the wire loom in the same location as mine. Hard to really see on my phone but looks like a little back box with electrical tape around it near the firewall where the driver side valve cover would be in your pic. Follow your coil wires and look about 8" down.

 

Yep, I think you're correct. I went out to the garage and sure enough I have a little square box taped in the same spot. Don't know how I missed it (and with the friggin engine out no less). Guess I expected it to be hanging out in the open.

 

http://forums.corvetteforum.com/1563898781-post20.html
http://forums.corvetteforum.com/c4-g...ch-repair.html

 

So yall think my tach issue is likely the tach head itself? Any (semi) easy way to be sure? I wouldn't say it has a consistent high error.. more like doesn't work. Its at zero while you drive. Tach just goes to rev limit upon initial crank & start then back down to zilch.. I did unveil the tach filter but I didn't really see a good way to get to it unless your engine is out! I didn't get too far with the temp gauge today either. The coolant sensor I picked up as the single spade terminal and it read about 1.8k ohms at room temp. Hope it's the right one! Parts store said it was..

This is about the only 2 fingers I can get on the tach filter. Even after I took the tape off.
That's the tan-ish color rear driver side valve cover you see in the bottom left side of the pic.


I did however, get some new u-joints in the passenger side axle shaft. What a PITA!! Not looking forward to doing the other side..

 

I believe you could use a scanner at the ALDL and if the scanner is registering correct RPMs then I believe it could be assumed that it's the board in the cluster.

 

Not a bad idea. What scanner do you recommend? I've already got a Snap-On OBDII & ABS scanner. Wonder if I could just get an OBDI to OBDII adapter cable? My scanner has the capability to give live freeze frame data but not sure about OBDI readability..

 

What S/O scanner? I'd investigate anyway if it can be adapted to OBD-1 it would certainly be of value. Enormous value!

 

It's a blue point YA3160. I'll see if I can find out if it'll work.