Jcpstrat

I've gone through checking the vacuum to see why my headlights aren't opening and closing and the problem appears to be both Actuator Relay Valves. 12" to 13" of vacuum at 800 rpm at the manifold side (same reading I get at the manifold). About 2" on the other two ports of relay that go to either the front or back side of the vacuum diaphragms that move the linkage for the headlights. Both actuators seem to just be leaking vacuum significantly.

My question is can these be reconditioned or do they need to be replaced?

I don't know what the inside of the relays look like but if there are seals in there .... I guess they've dried up.

The headlights do go up and down when vacuum is manually applied to front and back side of the headlight diaphragms. John

Dave J

Let's see here, lots of vacuum to the middle relay port, and not much coming out of the top & bottom ports. Classic case of original relay leakage syndrome. Your relays are shot. Most of the vacuum is being lost around the seal of the relay's internal pistons. The fix is to simply replace them, or you can get them rebuilt. Not many places rebuild these, and from a 74 there is not much value to them.

If you want to make sure they are bad, connect a short piece of spare hose and block the bottom port. Try to blow through the relay. You shouldn't be able to if the seals are good.

Patro46

Willcox has an excellent vacuum headlight troubleshooting chart that's sure to isolate the issue. My 74 had 2 leaking relays and a leaking canister seal. Also, my vacuum hoses were in pretty rough dry rotted shape. I went ahead and went through the entire actuator assembly, cleaning, painting and a good lubrication. Works like brand new now!

Iceaxe

A new relay is $25, and takes 10 minutes to replace. Why waste time messing with a bad/questionable one?

Personally I keep a spare vacuum relay around because they are cheap and swapping in a new one is the fastest and easiest way to see if a relay is bad. Your mileage may vary.

Jcpstrat

Thanks everyone …. replacement was what I thought would be the answer but just want to get some experienced opinions.

Dave J … thanks for tip and I’ll try your tip on checking before I pull trigger on ordering.

Patro46 … yep Wilcox’s info was great but didn’t actually show how to test relays unless I missed that.

Iceaxe … I’ve had this ’74 for 32 years … not bad that I never had to replace these before so I don’t mind that part and it was interesting learning how it all worked finally.

0Willcox Corvette

This is the first link I've posted from the new site.... You guys can't see this portion of the site yet.. but it's giving me a chance to see if things will be normal.

Here is the vacuum chart, and the info on testing the relay both the pic and the video from the website.

I'm not real happy with my first search results either..

Jcpstrat

That's a really good drawing of the headlight vacuum system. The only difference on my '74 is the two (2) brass fittings label as "1" in your drawing are both actually located between the bumper brackets on the Vacuum Tank, about 4" apart. Thanks

Dave J ... both relays failed the test tip you provided. Thks

C3Hawk

Like Dave mentioned above, the relays can be deceiving. Here's how I tested mine:
https://www.corvetteforum.com/forums...s-of-pics.html
They were the culprit; they would move but not hold vacuum.
Good luck!

Jcpstrat

Sweet write-up C3Hawk ... could have used that if it had popped up in my search!

LRB

Quick question, is there a test I can do with to check to ensure I have the vacuum lines hooked up correctly to the the headlight switch, without pulling the dash to confirm. I am 99% sure I am right, but want to confirm. I installed new actuators and relays and the light open, but slow and then after sitting over night, they did not want to open the next day. Just your thoughts would be appreciated.

VetteNURSE

Dave pm sent

Jcpstrat

2 new vacuum relays solved my headlight issues on my '74 ... thanks for info. & tips.

gungatim

I was gonna post that it is relay, not relay's, but I see they went to 2 relays for headlights in late '69 or '70.

what exactly was the reason for change to two relays? seems pretty redundant, you only need one right? it's not like you would ever want to raise just one and need to control them individually, or am I missing something?

just curious...