New C3 Owner Needs Help
Thread Starter
Cruising
Joined: Aug 2010
Posts: 13
Likes: 0
From: Portland OR
New C3 Owner Needs Help
Recently purchased my first Corvette, a 74 Coupe. Two issues: after operating fine, the tachometer stopped working; stays at zero during acceleration. Why? Secondly, at approximately the same time both door courtesy lights and the back "dome?" light remain on after doors are shut. Headlight switch doesn't seem to turn them off either. Only way to get them off is to disconnect the battery, which is a pain.
Remedies are welcome. Thank you in advance.
Fox
Remedies are welcome. Thank you in advance.
Fox
Burning Brakes
Joined: Oct 2010
Posts: 1,160
Likes: 23
From: Prescott AZ
I agree, check the tach cable. As far as the interior lights, I don't know if the 74 has a delay timer that keeps the interior lights on for a bit after the doors are closed but I know my sons 82 dose. The timer failed on his and kept the lights from shutting off. Didn't realize it until the next day when the battery was dead. Just a thought.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
No delay timer on the '74. One of your door jamb switches is likely the problem. What happens when you open each door [separately] and manually push the jamb switch buttons in? The results may give you a clue as to the source of the problem. Before you start working on any of those switches, pull a battery cable off. Use an ohmmeter to test any wiring or switch continuity with no power on the system...so you can't create a short or blow a fuse.
P.S. Those switches are very specific to a particular location and for each year. The local parts stores can sell you a 'generic' switch, but it is very unlikely to fit properly. Find a good vendor source that has the correct switches, if you need to replace one.
P.S. Those switches are very specific to a particular location and for each year. The local parts stores can sell you a 'generic' switch, but it is very unlikely to fit properly. Find a good vendor source that has the correct switches, if you need to replace one.
Advanced
Joined: Jul 2007
Posts: 72
Likes: 2
Tachometer problem is most likely stripped tachometer drive gears inside the distributor - a very common issue. You can verify this by removing the cable from the distributor and hooking the cable to a drill (run the drill in reverse as I recall) - if the needle moves then you know the problem is the distributor. If it is the distributor, do not continue to drive the car as the metal fragments from the chewed up gears can do more damage inside the distributor and drive up the repair cost. To do the repair correctly the distributor must be removed, disassembled and the damaged parts replaced. If only the gears and the nylon button need replacing the parts cost is about $100.00 and if you have someone do the rebuild about another $100.00 in labor. Caution: It is tempting to not pull the distributor and to just remove and replace the driven gear which can be removed with the distributor on the car. If the driven gear is damaged then it is quite likely that the drive gear (which is the main distributor shaft) is also damaged. If you just replace the driven gear it will get quickly chewed up again by the damaged drive gear and hence you have just wasted time and money.
Drifting
Joined: Oct 2010
Posts: 1,972
Likes: 6
From: Chester Virginia
My 76 has a contact switch on the car frame that when the door closes, it pushes the plunger and shuts the interior lights off. Once, when I put the door panel back on I forgot the screw in the attachment clip that is right there and it would not depress the plunger. Easily remedied by putting the screw in. Check that too if yours is like that.
Safety Car
Joined: Jun 2004
Posts: 4,652
Likes: 80
From: Lakeland Florida
On your tach issue Robert.Sperry has good advice. The 1974 Vette should have a mechanical tach drive. I think this changed to an electronic version in 1975. The mechanical tach drive has a short 1/2 to 1inch long cross gear that is mated to the distributor drive gear.
The cable is then inserted into the square hole in the cross gear, and that is what turns the cable to the tach.
If the cross gear is bad, then the teeth/cogs on the drive gear may also be bad and is chewing up or damaging the cross gear.
The previous owner could have installed a new cross gear ( approx. $50) but this will last only a couple of days before it is damaged again.
The main drive gear may have to be replaced as well.
kdf
The cable is then inserted into the square hole in the cross gear, and that is what turns the cable to the tach.
If the cross gear is bad, then the teeth/cogs on the drive gear may also be bad and is chewing up or damaging the cross gear.
The previous owner could have installed a new cross gear ( approx. $50) but this will last only a couple of days before it is damaged again.
The main drive gear may have to be replaced as well.
kdf
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Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Driver's door front switch should be a single pole and the other 3 switches should be double pole (2 rear for the "Door Ajar").
Single is $1.99 and doubles are $2.99 ea. at Autozone. Cheap enough, look close enough to original and work well. You can use the double in place of the single if wanted, no biggie.
Single is $1.99 and doubles are $2.99 ea. at Autozone. Cheap enough, look close enough to original and work well. You can use the double in place of the single if wanted, no biggie.
Le Mans Master
Joined: Aug 2006
Posts: 7,419
Likes: 1,555
From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
On your 74 the switch at the front of the door grounds 3 wires on the drivers side:
the courtesy lights
the alarm and the
key lift in buzzer.
On the right side GM used the same switch but that one makes a ground for the courtesy lights and the alarm.
I could only find the correct switches and connectors from the Corvette vendors.
the courtesy lights
the alarm and the
key lift in buzzer.
On the right side GM used the same switch but that one makes a ground for the courtesy lights and the alarm.
I could only find the correct switches and connectors from the Corvette vendors.