Shifter interlock cable

[Richard L.]

I'm the new owner of a 1971 small block coupe with an automatic. The previous owner had the drivetrain rebuilt and the shifter interlock cable has been cut/disconnected from the trans. The transmission (TH 400) shifts fine, though with a shift kit it is a little abrupt. My question is, what does this cable do or actuate? Does it harm the trans if its not connected? Should I replace the cable and connect it properly? Will it make any difference in the way the trans shifts?
I hope somebody can help since I know nothing about automatics, but have been told that the TH 400 is a fine unit.
thanks in advance!
Ric

[Paul D.]

Re: Shifter interlock cable

That cable should run from the trans to the steering column where it passes through the firewall. It's purpose is to "lock" the steering wheel when in park. I thought it also affected the operation of the key lock. If the neutral safety switch is column mounted, it will affect that also. It will have no effect on the operation of the trans. I would imagine that the cable is "frozen". Sometimes a loose or missing ground causes interlock and shifter cables to sieze up. Make sure the lever on your cloumn is secured so as to avoid locking your steering wheel while going down the road. If it were mine, I would try to get it sorted out. Chip.

[Terry M.]

Re: Shifter interlock cable

TH 400 IS a fine trans. They don't need a shift kit -- as you are finding out. It will (with or without the shift kit) eventually wear out, but it will be bullet proof behind your small block. In 1969 one could order a TH400 with ANY of the nine engines offered in Corvette. Some of those engines had twice the horsepower and torque of your 1971, but to be fair there were some "minor" () differences in those TH 400 and yours. In 1971 one could order an LS6 454/425 with a TH400. Is that good enough for you?

Your M40 will shift just fine without that interlock cable. The interlock cable connects the shift linkage to the ignition lock and prevented removing the ignition key unless your M40 (M40=TH400. M40 is the option code. U69 is the AM/FM radio, C60 is Air conditioning. You will learn all these if you hang out with the C3 guys here.) is in Park. If that interlock cable got kinked it will not work well or at all. Simply disconnecting it is a common Bubba cure for this. Replacements are available if you want to make that system work. It is a relatively simple R&R, but access to the underside of the car is a real advantage. The task will become much more difficult if you have to put the car on jack stands and lay on your back.

[Terry M.]

Re: Shifter interlock cable

That cable should run from the trans to the steering column where it passes through the firewall. It's purpose is to "lock" the steering wheel when in park. I thought it also affected the operation of the key lock. If the neutral safety switch is column mounted, it will affect that also. It will have no effect on the operation of the trans. I would imagine that the cable is "frozen". Sometimes a loose or missing ground causes interlock and shifter cables to sieze up. Make sure the lever on your cloumn is secured so as to avoid locking your steering wheel while going down the road. If it were mine, I would try to get it sorted out. Chip.

Chip that interlock is strictly mechanical. There is no electricity involved; and it has nothing to do with the neutral safety switch. Richard's neutral safety switch is in the center console by the shifter.

[Jim S.]

Re: Shifter interlock cable

Here is everything (and probably more) that you ever wanted to know about the GM Steering Column/Transmission Interlock System.

http://jimshea.corvettefaq.com/?p=928

The system basically provided anti-theft and safety functions for both manual and automatic transmission C3 Corvettes 1969 through the 1976 model years:
1. Prevented locking the steering wheel while the car was still moving forward (i.e. the manual transmission must first be shifted into REVERSE; the automatic transmission must be in PARK.)
2. Locked both the steering wheel and the steering shifter when the ignition key was rotated to the OFF-LOCK position.

Note, one last tip. There are five positions of your ignition key when fitted in the lock cylinder. Those positions from full CW are as follows:
START, RUN, OFF, OFF-LOCK, and ACCESSORY. With the interlock system connected, you can always turn the engine OFF. But you cannot lock the steering wheel until you can continue rotating your ignition key to the OFF-LOCK position.

Note that this complicated system was abandoned on the Corvette starting in 1977. That year, a small lever next to the ignition key and lock cylinder was added to the steering column. You had to trip the lever in order to rotate your ignition key to the OFF-LOCK position.

Jim

[Paul D.]

Re: Shifter interlock cable

Terry, thanks for the info. I wasn't sure about the safety switch as my 71 is a 4spd and the clutch safety is trans mounted. The reason I mentioned checking grounds, when I was working in a dealership in the 80's, we had some trouble with late 70's Camaros' shifter cables siezing up. Someone at Chevrolet suggested checking for loose or missing grounds when that problem presented it self. Usually, we could find a loose or missing ground strap, and if not we would add one from the trans to the body. Never would see those cars again. It must have been a common problem, as I remember the parts guys started advising customers about it when selling the cables. Had more than 1 cable returned to the parts department as "...defective...only worked for a few days!" Chip.

[Jim T.]

Re: Shifter interlock cable

I'm the new owner of a 1971 small block coupe with an automatic. The previous owner had the drivetrain rebuilt and the shifter interlock cable has been cut/disconnected from the trans. The transmission (TH 400) shifts fine, though with a shift kit it is a little abrupt. My question is, what does this cable do or actuate? Does it harm the trans if its not connected? Should I replace the cable and connect it properly? Will it make any difference in the way the trans shifts?
I hope somebody can help since I know nothing about automatics, but have been told that the TH 400 is a fine unit.
thanks in advance!
Ric

Agree with Terry M. (3966) that the turbo 400 is a fine automatic. I have used my original for over 41 years and accumulated over 160+ miles. My son bought me a B&M shift kit for it about 25+ years ago, never installed the shift kit and no intentions to do so. At a WOT shift from first to second my turbo 400 has in the past lost traction with its firm shift. One of the best things I have changed on my 70 was to purchase and install a GM replacement 400 transmission fluid pan. The GM replacement is made from thicker metal and attributes to getting a no leak/no seeping of transmission fluid when the retainer bolts are properly torqued at 12 ft lbs.

[Jim S.]

Re: Shifter interlock cable

Paul,
Two things.
1. I might be wrong but I always thought that the manual transmission C3s used the clutch pedal safety switch in place of a neutral start switch. The backup light switch was attached to reverse gear linkage on the manual transmission. Automatic trans cars had the combination neutral start/back up light switch built into the consule shifter mechanism.
2. I think that you are saying that a defective ground (somewhere else in the car) could cause the shifter cable to become a grounding path and the cable could fail.

PS: I hate it when these threads start to take dual paths.

Jim

[Richard L.]

Re: Shifter interlock cable

Thank you all for the informed advice! I'll go ahead and get a replacement cable and install it. Since I have a lift the job looks to be easy. I agree with Jim Trekell, no need for a shift kit--the shifts are almost too positive/abrupt!
And, by the way, even though I'm an old member (7989), this was my first post. So, again, thanks to all. I imagine that as I begin to sort out this car, I'll be back periodically for more helpful advice.
have a great weekend!

[Paul D.]

Re: Shifter interlock cable

Jim,
Your are exactly right, my mistake. My car has been apart far too long! Thank goodness for the AIM. If I try to work from my memory, I doubt anything will work on the car. Chip.

[Reba W.]

Re: Shifter interlock cable

Welcome to the DB, Richard, there are lots of very knowledgeable folks here. Another bit of adivce, if you ever have the car judged in NCRS, the interlock function will be checked during operations.

[Terry M.]

Re: Shifter interlock cable

Richard,

After you purchase the interlock cable, but before you install it, you might consider lubricating the internal cable as best you can. I suggest a graphite fluid like is used on locks as the best way to get the lube to travel from one end to the other. Hang the cable vertically and allow gravity to work for a while. If you don't have the patience for that; white lube working the cable back and forth often, would be my next suggestion.

Welcome aboard.

[Mike E.]

Re: Shifter interlock cable

Trying to remember whether you go by Ric or Rick---you've had some pretty great cars in the past--good to see you posting!!!!!

[Richard L.]

Re: Shifter interlock cable

Mike! It's great to hear you. I'm back in the Corvette fold full time--aside from the '71 I still have my two 67s. Sold the Ferraris, though still have a nice 911 and a 280SL. And I do go by Ric.