1973 Corvette Coupe Delay Relay

[Thomas R.]

Noticed two holes in firewall (Driver side) and my 73 Assembly Instruction Manual shows a Delay Relay.

Not sure what option on the vehicle this Delay Relay is for. Appreciate any perspectives. Thanks

[Joe L.]

Re: 1973 Corvette Coupe Delay Relay

Noticed two holes in firewall (Driver side) and my 73 Assembly Instruction Manual shows a Delay Relay.

Not sure what option on the vehicle this Delay Relay is for. Appreciate any perspectives. Thanks

Thomas------


I believe that relay is the transmission controlled spark delay relay. It was GM #6270698 for your 1973. Many owners disabled this system which had a miserable effect on driveability. That's probably why yours is missing.

[Thomas R.]

Re: 1973 Corvette Coupe Delay Relay

Joe, In the 1973 Chevrolet Service Manual on page 6T 13 (Emissions Control System); a Time Relay (TR) is identified and to summarize the description, the TR is an electrically operated on-off type switch. When the coil is energized it begins to heat the bi-metal strip to open the normally closed relay points in about 20 seconds after the ignition switch is turned on.If the vehicle does not start with in the 20 seconds it denies vacuum advance until the relay has cooled and then can be retried. It says two self tapping screws attach the relay to the vertical wall of the cowl to right of the center line.

My 73 is an L-82 (built March of 73) and I wonder if, this time relay was not part of the original installation but, on another engine option. I did not see any sign of it having been installed before (I did not see any wiring near it). I've owned the car since new and do not remember it ever being there.

I would appreciate anymore thoughts you might have.

[Joe L.]

Re: 1973 Corvette Coupe Delay Relay

Joe, In the 1973 Chevrolet Service Manual on page 6T 13 (Emissions Control System); a Time Relay (TR) is identified and to summarize the description, the TR is an electrically operated on-off type switch. When the coil is energized it begins to heat the bi-metal strip to open the normally closed relay points in about 20 seconds after the ignition switch is turned on.If the vehicle does not start with in the 20 seconds it denies vacuum advance until the relay has cooled and then can be retried. It says two self tapping screws attach the relay to the vertical wall of the cowl to right of the center line.

My 73 is an L-82 (built March of 73) and I wonder if, this time relay was not part of the original installation but, on another engine option. I did not see any sign of it having been installed before (I did not see any wiring near it). I've owned the car since new and do not remember it ever being there.

I would appreciate anymore thoughts you might have.

Thomas-----

Transmission controlled spark is part of the emission control system and interruption of distributor vacuum advance is the primary working principle of the system. I do not think that the installation of this system varied with engine option and the information I have indicates it was used with all transmissions. So, I do not know why your car was not originally so-equipped. The system was in use from 1971-74.

[Edward J.]

Re: 1973 Corvette Coupe Delay Relay

Thomas-----

Transmission controlled spark is part of the emission control system and interruption of distributor vacuum advance is the primary working principle of the system. I do not think that the installation of this system varied with engine option and the information I have indicates it was used with all transmissions. So, I do not know why your car was not originally so-equipped. The system was in use from 1971-74.

Thomas, My 72 L48/auto, has a delay relay, It works when the Auto transmission is shifted into Reverse, You can hear a slight RPM change when it engages. I am not total sure about 73 but I believe the 4 speed engages when put into 4th gear. It may not have the delay relay with the 4 speed.Maybe Joe will know.

[Jimmy P.]

Re: 1973 Corvette Coupe Delay Relay

Maybe I can shed some light on this. First off, Tom is your 73 an automatic? If so the time delay relay (6270698) isn't mounted on the firewall. The transmission controlled spark TCS is used with four speeds and M40 but the M40 cars use a variation of TCS. IF you take a look at pages 192-193 in the new 73-74 TIM&JG you'll find we discussed it in detail. I've included a footnote from the manual, a photo from the relay on my 73 L48 M20. Going back to the 73 AIM you reference above it only shows the relay/ delay 6270698 being used on M20 & M21 applications.
Edward, you are correct....the RPM's increase when the manual '73 is put into 4 th gear, and in reverse with the M40... same as '72. Does your 72 auto have this time relay on the firewall? Just curious...

[Edward J.]

Re: 1973 Corvette Coupe Delay Relay

Hi Jimmy, yes the same as 73, 20 Second delay. And the relay is inside the car. firewall somewhere.

[Thomas R.]

Re: 1973 Corvette Coupe Delay Relay

Joe and All you folks, Thanks for your input.

I have a Book "Corvette Service, Repair, Handbook All Models 1963 -1973 It discusses on page 159 The Time Delay relay and notes the 454 ci (LS4)system is identical to the 350 ci system except there is; no time delay relay circuit for the 454 engine.

My Vet is L-82 with Turbo 400 (M40) transmission.

The 73 Corvette Assembly Manual shows in section 12H-5:the Delay Relay placement is on the fire wall for M20 & M21 transmissions. No mention of the M 40 transmission. Do you think, the relay is not used with the M 40 transmission like the LS4?

If the Time Delay Relay is on all 350 ci engines and transmissions; where would the relay be located?

Again, your thoughts and all other folks would be appreciated.

[Jimmy P.]

Re: 1973 Corvette Coupe Delay Relay

Joe and All you folks, Thanks for your input.

I have a Book "Corvette Service, Repair, Handbook All Models 1963 -1973 It discusses on page 159 The Time Delay relay and notes the 454 ci (LS4)system is identical to the 350 ci system except there is; no time delay relay circuit for the 454 engine.

My Vet is L-82 with Turbo 400 (M40) transmission.

The 73 Corvette Assembly Manual shows in section 12H-5:the Delay Relay placement is on the fire wall for M20 & M21 transmissions. No mention of the M 40 transmission. Do you think, the relay is not used with the M 40 transmission like the LS4?

If the Time Delay Relay is on all 350 ci engines and transmissions; where would the relay be located?

Again, your thoughts and all other folks would be appreciated.

Tom,
Not sure why that Repair handbook would say there isn't a Time Delay Relay (620698) on a 73 LS4. Maybe it was talking about an LS4 with the M40 transmission. In that case it wouldn't have this particular relay mounted to the firewall in the engine bay. However, the 73 LS4's with manual transmission do have the relay in question mounted on the firewall. I've included a photo here showing it on a 73 LS4 with the M21.

[Joe L.]

Re: 1973 Corvette Coupe Delay Relay

Joe and All you folks, Thanks for your input.

I have a Book "Corvette Service, Repair, Handbook All Models 1963 -1973 It discusses on page 159 The Time Delay relay and notes the 454 ci (LS4)system is identical to the 350 ci system except there is; no time delay relay circuit for the 454 engine.

My Vet is L-82 with Turbo 400 (M40) transmission.

The 73 Corvette Assembly Manual shows in section 12H-5:the Delay Relay placement is on the fire wall for M20 & M21 transmissions. No mention of the M 40 transmission. Do you think, the relay is not used with the M 40 transmission like the LS4?

If the Time Delay Relay is on all 350 ci engines and transmissions; where would the relay be located?

Again, your thoughts and all other folks would be appreciated.

Thomas------


First of all let me say that I have no personal knowledge of this system on a 1973 Corvette. The information I am providing is sourced from GM information.

According to the information I have, the delay relay was used for all 1973 applications----all engines and all transmissions. However, keep in mind that the electrical system configuration shown in the AIM applies to the STANDARD configuration (i.e. base engine with 4 speed transmission). It could well be that the location of the delay relay varied with optional engines or transmissions.

[Thomas R.]

Re: 1973 Corvette Coupe Delay Relay

Joe, Jimmy & Ed,

My original question was what the two holes were for on the divers side firewall. That is answered; Time Delay Relay. Thanks for the input on that.

I've looked the 73 over and could not find that relay if, it is required for the L-82 with M40 transmission equipped, it must be somewhere that requires some type of access. At this point, I'm satisfied and thanks for all the assistance. Have a nice Thanksgiving

[Terry M.]

Re: 1973 Corvette Coupe Delay Relay

Joe, Jimmy & Ed,

My original question was what the two holes were for on the divers side firewall. That is answered; Time Delay Relay. Thanks for the input on that.

I've looked the 73 over and could not find that relay if, it is required for the L-82 with M40 transmission equipped, it must be somewhere that requires some type of access. At this point, I'm satisfied and thanks for all the assistance. Have a nice Thanksgiving

Just a question out of curiosity: Are those holes not sealed up some way? I would expect them to be sealed to keep engine gasses from the interior of the vehicle.

[Edward J.]

Re: 1973 Corvette Coupe Delay Relay

Tom maybe under the console?? I remember there being a relay there, but do not recall what it was for. Maybe the AIM will show where it is located.my 72 M40 does have one, I am sure the L82 /M40 has one.

[Thomas R.]

Re: 1973 Corvette Coupe Delay Relay

Terry & Ed, To Terry's thought on the holes; they are in the firewall but in the area that connects with the wiper tray which is open to the air in front of the windshield.

Ed I did look into the console area below the instrument cluster and found a relay for the power windows. I did more reading and in the 1973-1974 NCRS Technical Information Manual and Judging Guide 3rd Edition 2018 under "Transmission-Control Spark (TCS); a sentence A transmission-controlled spark (TCS) system is used with four-speed Corvettes both years while M40 cars use a variation ofTCS. The components include a time-delay relay, a temperature switch, a transmission switch, an idle-stop solenoid , or TCS solenoid. "The M40 applications, while using the same or similar parts, less the time delay relay and trans mission switch, is functionally a modification of the TCS. A note 11 is referenced. Looks like; the time-delay relay is not part of the M40 tramsmission TCS application.

Thanks you all for your perspectives and have a Happy Thanksgiving.