Smog Control Relay

[Kenneth T.]

Spent some time this weekend organizing some old parts and came across a relay for my '71 grp 4.075 Part # 3990842.

I have no idea what it is for or where it goes. I have searched my engine compartment and see nothing like it. Nice to have an NOS part, but tough to realize I don't know its use. I will post a picture for clarity.

Any help, thanks.

Ken

[Sal C.]

Re: Smog Control Relay

3990842 is a TCS reversing relay for a 1971. It is mounted in tandem with a 3990843 delay relay on the driver's side firewall.
The relay in your photo does not resemble typical factory production, however. It is either mis-boxed or some sort of factory superceded part. The original 3990842 was the "Littlefuse" style with the part number stamped on top.

[Kenneth T.]

Re: Smog Control Relay

Thanks
Sal, if I had a brain, I might be dangerous. I just check my stuff by the numbers and have the relay you elude to, Pic posted. The one hole on the mounting threw me off.

Ken

[Jack H.]

Re: Smog Control Relay

I have to respectfully disagree here, Sal. I've seen quite a few Corvettes as well as Chevy passenger cars from the '71 vintage. There were TWO factory qualified suppliers for both the CEC reversing and delay relay.

The primary vendor was Littlefuse (both relays) and the secondary vendor was UCC for the reversing relay and FASCO for the delay relay. I've seen all combinations on untouched original cars INCLUDING Corvette...

In my opinion, the relay depicted is a bona fide GM original part that WAS used in St. Louis on the assy line. Though it's packaged as a service replacement, about the only difference between it and the real deal factory original would lie in the date code marks.

Littlefuse stamped their relays with date codes (month and year in digit format) while both UCC and FASCO applied rubber ink stamps to their relays (alphanumeric sequence). The date coding aspect of these relays is NOT taught in the NCRS judging literature...

[Tom L.]

Re: Smog Control Relay

Ken, the first picture in this post looks suspiciously like the power window relay thay I needed back in june. The number in my AIM does not match the number you post, but my car is a '72. Wish you brought it that day!! It may have made my ride home easier!!

[Jack H.]

Re: Smog Control Relay

I'm going to correct myself on this one!

First, I do maintain there were two qualified vendors for these 1971 CEC relays Littlefuse for both and UCC for the reversing relay + Fasco for the delay relay.

But, what's shown in the picture is NOT a typical UCC reversing relay. The mounting tab and bottom side pin out aren't correct. So, the relay HAS been mis-boxed.

Here's a picture of a factory original, UCC alternate supplier, 3990842 reversing relay. The metal can is much larger than the equivalent part from Littlefuse. But, the mounting tab has two holes (just like Littlefuse)and the relay is rubber ink stamped on one end of the cover.

The ink stamp is hard to read due to age, but it says "3990842" on the top line, "UCC" on the next line, "TOP" in large letters next (passenger cars mounted their relays sideways relative to Corvette), and a two character date code last. This one reads 'M0' which I equate to December 1970...

[Kenneth T.]

Re: Smog Control Relay

I'm going to correct myself on this one!

First, I do maintain there were two qualified vendors for these 1971 CEC relays Littlefuse for both and UCC for the reversing relay + Fasco for the delay relay.

But, what's shown in the picture is NOT a typical UCC reversing relay. The mounting tab and bottom side pin out aren't correct. So, the relay HAS been mis-boxed.

Here's a picture of a factory original, UCC alternate supplier, 3990842 reversing relay. The metal can is much larger than the equivalent part from Littlefuse. But, the mounting tab has two holes (just like Littlefuse)and the relay is rubber ink stamped on one end of the cover.

The ink stamp is hard to read due to age, but it says "3990842" on the top line, "UCC" on the next line, "TOP" in large letters next (passenger cars mounted their relays sideways relative to Corvette), and a two character date code last. This one reads 'M0' which I equate to December 1970...

Jack,

I read your article in the restorer on the CEC systems, great stuff. This is not the relay that belongs in that box. I have parts that have been moved and shuffled and this may be the power window relay as Tom Larsen noted. Thanks.

Ken