Electrical Kickdown Switch on a 1967 400

Q: Electrical Kickdown Switch on a 1967 400

This car is original, but the engine/trans have been out before, so who knows what small parts are missing.

The passing gear on the transmission has never worked. When cruising about 35 mph, if you floor it, it just slowly(relatively) accelerates. It seems pretty obvious that it is not downshifting. The trans is rebuilt and has no problems shifting normally otherwise.

I found an electrical switch on the back of the trans that I was told was the kick down switch. I followed the wiring harness bulkhead on the the firewall, and have absolutely no idea where it went from there. Does the gas pedal actuate a switch that tells the trans to downshift?

Any thoughts/answers would be greatly appreciated.

A: Yes, the Turbo 400 has an electrical kickdown switch. I’m not sure where it is supposed to be located on a 1967 bird, but it will either be at the carb throttle (rare) or under the gas pedal. It should be set up so the switch activates at around 3/4 throttle or so.

Here is text from a reply by Jim Hand on the classicalpontiac web page (www.classicalpontiac.com) which explains kickdown/modulator funciton on TH400.

Folks, Here is some additional information on the Detent circuits (kickdown switch), and modulator function on a T 400.

The vacuum modulator is the prime controller of shift points when vacuum is available for a signal. However, as there is not adequate vacuum available at full throttle, regardless of what gear is being used, the modulator has no control over full throttle shifts.

Here is a summary of operation: The detent system (kickdown system or passing gear) on a T-400 has several functions. In addition to activating the kickdown gear (may be 3 to 2, 3 to 1 or 2 to 1), it also overrides the vacuum modulator circuit. Since most engines at wide open throttle (WOT) under load have little or no vacuum, the modulator has no reference point, and adjusting it will make no difference in shift quality or shift rpm under WOT load. The detent system replaces the modulator as controller of shift points at WOT, and it must be operating correctly before attempting to adjust the governor for higher rpm automatic shifts. The detent system also modifies accumulator pressure to increase oil pressure to the clutches for increased durability, so it is vital that it be functional in all cases.

All this means that if the kickdown switch is incorrectly adjusted to function at part throttle, the transmission will act as if the throttle is wide open in respect to the next gear change whenever the switch is activated. As quickly as the detent switch is off (by letting off on the throttle), the shift pattern will return to control of the modulator, because there will then be adequate vacuum for the modulator to function. It also means that if a T-400 is run without a correctly operating detent system, the clutches will suffer accelerated wear because the oil pressure is less at full throttle shifts. Additionally, the WOT shift rpm points will be lower without a correctly operating detent, or kickdown system. The detent switch should be adjusted towards the full throttle end of the accelerator pedal range.

A: Yes, the 1967 Firebirds with the Turbo 400 transmissions use a switch activated passing gear, (kickdown). The switch is a plunger style unit which is mounted to the intake manifold near the left rear of the carb. The bracket is secured to the manifold via the left rear carb bolt which holds the carb on the intake manifold. The switch is then secured to the bracket via it’s threaded neck, through a large hole in the bracket. The switch gets activated by an extension bracket which attaches the pedal linkage to the carb. There are two wires connected to the switch, one is black, the other is black with a green stripe.

The GM part number for the switch is #9785545 (same…. 65-66 P/8 M-40, 1967, 1968, and 1969 F/8 M-40).

I believe Classic Industries sells a reproduction of the extension bracket which mounts to the carb.

I haven’t seen anyone with a reproduction of the bracket which mounts the switch to the intake manifold. However, I went through many a junk yard looking for this switch before finding a guy who located an NOS switch for me. I “may” have a manifold bracket laying in my junk parts bin, I’ll check when I get home. If I have one, it is yours.

Let me know if you need pictures of this setup. You WON’T find them in ANY Pontiac, Firebird, or GM service manual, and there is no reference in any Firebird wiring diagrams that I have run across. I’ll be happy to take a few pictures and digitize them for you.

A: See also Engine F.A.Q. – Carburetor for more Kickdown Switch Information.

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