I just purchased a 92 convertible that had been sitting for over a year and it has 2 problems that I need help to fix.

1. When sitting still with the ignition on or engine running the speedometer actually displays numbers -- jumping around with different numbers -- while sitting still! Now once I start moving the speedometer seems to work accurately. Ideas?

2. I'm putting the retro fit top with the glass rear window with defroster. What do I need to tie the window into the wiring harness?

This is my first outreach on the forum and I do appreciate any and all advice. Thanks

 

My suggestion.....buy the FSM (factory service manual) off eBay or other sites. It will help you with this problem and will be a valuable asset to you in the future.

 

It can be had for $100 or less and will save you at least that much the first repair you do yourself. Even my mechanic that charges $60 an hour anything I can do without taking it to him saves me money.

 

What are the numbers? Is it possible that the previous owner left a jumper in the ALDL?

 

I have ordered the manuals from ebay, the numbers on the speedo are random -- constantly jumping around with no discernable pattern,. ALDL?

 

ALDL is the diagnostic link for getting fault codes to display on the speedometer. This link will tell you more than you want to know about the codes.
http://forums.corvetteforum.com/c4-t...n-96-cars.html

 

The verts had the power lead for a rear widow defroster as that came with the factory hardtop. If a vert was ordered with the factory hardtop, the car aslo got a ground lead for the other side. The power lead will be near the passenger side shoulder belt mounting tower. Carefully pull the carpet back along the plastic trim piece on the body side near the tower. IIRC there is a seam for the two pieces of carpet that overlap. Look for a heavy purple wire with a connector. Coupes and hardtop-equipped verts also had a defogger relay under the Rh side of the instrument panel From the passenger seat looking forward, it would be the left-most connection.

The fuse block on the passenger side door opening would have a 35A circuit breaker in the second position on the right side of the block.

If the vert didn't have a hardtop ordered, there will be no defogger ground wire. It would normally be on the driver's side of the car in the same relative location to the purple hot wire. But you can always make a ground lead; just get a matching connector for the leads on the new window and use a ring terminal to screw the wire to a good metal ground.

Verts that got the factory hardtop also had a different HVAC control unit that operated the defogger, I believe it was the same button for the outside heated mirrors. You may have to find the correct control panel from a hardtop-equipped vert and install it. I don't think your existing control panel will work.

Because the CCM controls the operation of the HVAC system and the defogger, it may not recognize an ON command at the switch. Note that any testing of the circuits have to be done with the engine running. If you have the FSM, refer to Vol 2, page 8A-61-0

 

Thanks for the advice guys. I will look into it and get back with the results.

 

I switched my '92 to a glass window top several years ago. IIRC, all the wiring was there for the defroster, I just plugged it in. There is no need to change the control head on the dash, the rear window defroster comes on with the button for the mirror defoggers.

As for the OP's second question, there have been a couple of other threads for erratic speedo's lately, don't recall how or if they were resolved, but an easy first step is to check grounds and connectors relating to the dash. The FSM will likely help with this issue.

 

When I couldn't find the right ECM for my 92 I put in one that fits but different part number. Car drove fine but the speedometer would move by itself at idle. When I finally found the right part everything was perfect.
Maybe the previous owner did the same.

 

Thanks Steve,
Were the wires just tucked back under the carpet at the site the gentlemen mentioned above? Did you have to install the relay? I hope your right and my car is equipped the same.

 

The factory hardtop was also a dealer-installed option so I would think that the factory would have made it as easy as possible for a dealer to do an install. I remember seeing a GM hardtop kit some years ago that included everything including a circuit breaker, the relay and the ground pigtail. Have you checked to see if the circuit breaker is in place on the fuse panel on the side of the dash by the door opening?

The relay is easy to see once you remove the hush panel above the passenger side footwell. There would be a group of three on the console side and the defogger relay is the one closest to the trans tunnel.

If you have the owner's manual, there is a section that describes how to operate the hardtop defogger based on either the standard C60 system or the H68 Electronic Control system.

 

Just went out and looked in the car- the two leads that plug into the grid on the glass are both black, and the wiring diagram shows the power lead to the window is purple (on the passenger side). Also checked the instructions that came with the retrofit kit. The kit I used came with extensions for the factory harness, and c4cruiser is right that there is no ground wire provided in that location. But, the power lead is definitely there, I did not have to run any wiring, and I didn't have to install a relay. I just drilled a small hole in the bulkhead and used a sheet metal screw to attach the ground wire. The purple wire is behind the seat in back of the carpet. There is nothing else in there that you could mistake for it.

If you are planning to do this, I suggest you use a retrofit kit, even though they get way too much money for it. The glass window is too heavy to just hang on the fabric, and the kit comes with, among other bits and pieces, four brackets to attach to bows #4 and #5 to support the glass when the top is up. There are also a pair of springs that attach to the bows with stepped rivets to help the top stack correctly when lowering.

As stated, I did this project several years ago, and the results were great. Now I can easily identify any driver who rear-ends me (or maybe even see him/her coming and take evasive action!).

 

Thanks again to all for the help!
Thanks Steve for the specific information. I have ordered the retrofit kit from Ecklers as well as the glass window top, new seal kit and the wire cables that run down each side of the top. I will get back home Friday and check for the wires. The retro kit I ordered (yes -overpriced) specifically states that it doesn't contain any wiring.

 

If it's any consolation, when I did mine, the only kit available was from GM, now no longer available, and even wholesale, it was well over $200 but it did include a pair of VERY nice wires to connect the window grid.

 

from the sound of things I can fabricate the extension wire from power and the one for ground. Thanks again.

 

got the old top off with no problem --- cleaned up the old parts (removed old adhesive). Put the retro kit onto the top frame (directions were a little vague with the kit from Ecklers) and went searching for that purple wire. I looked all around the seat belt mount tower on the passenger side and found a large wire bundle -- all taped up with various wires -- found one loose wire, a black one with an orange stripe with a connector on the end. I tried to check the voltage by connecting a voltmeter into the connector and turning on the mirror defroster switch. The small light above the mirror defrost switch would come on for about 2 seconds then go out. During those 2 seconds I showed no voltage at the black and orange wire. Any ideas and help would be appreciated.

UPDATE: The mirror defroster will not stay on unless the engine is running! Next, I went to the FSM to look at the wring diagrams for the rear defogger. For the convertible the wire for plug in by the passenger seat belt tower is suppose to be purple. For the coupe the wire is black with orange strip. My car is a 92 convertible and guess what color the wire is? black with orange --- must have been out of vert wiring harnesses that day. BTW: I did have good voltage with the engine running and the mirror switch on --- there is also a good place for a ground on the driver side by the hatch actuator. Hopefully tomorrow I can get the wires run and installation of the new top started. I will update on completion.

 

did you resolve the speedometer issue? if so - how?

 

No I haven't. I've been taking care of several other issues like installing a new convertible top with a glass rear window and the retro kit required for that. I have two issues still pending. The speedo numbers just jump all around with no discernable pattern while sitting still but works accurately once the car starts moving and the other problem is that whenever I first start the car I have a LTPMS light illuminate in the advisory panel, as soon as I roll a short distance with a little speed the light goes out and stays out until I shut the car off. As soon as I turn the switch back on it is there again. The top went on very well with no major issues, it is actually a very forgiving job and easy to correct minor problems.

 

please post the method to repair speedo problem if you resolve it. mine does same when parked. its only done it twice but it looks like its a sign of things to come. thanks , chuck