'75 Headlight Troubleshooting

[Stephen]

A couple of weeks ago I posted a message about how my headlights were slow to open and close. Several members were of great help and suggested that I check the Actuator Seal for holes or tears that would retard the opening. I did find that one of the seals was faultly and have replaced it. Although the system is improved both are still slow to open. So, I took vacuum readings to see if the proper vacuum was being received. Here is what I found:

-There is about 10 inches of vac coming from the reservior to the vac control switches next to the actuators. -There is about 8 inches of vac coming from the light switch control, and it appears to cut on and off correctly as the light switch is turned on and off.

Are these numbers numbers within specs?

Thanks for your help!

[Patrick H.]

Re: '75 Headlight Troubleshooting

Stephen,

It is tough to diagnose vacuum systems by using readings, as funny as that may sound. It is much better (easier) to test individual components, and see what is working, and what isn't (or is leaking).

Despite various comments recently, the one thing Dr Rebuild is good for is his vacuum system troubleshooting manual. It is part number 2777555, and is $3.50 in their just-off-the-presses catalog. You can order at 203-366-1332, but he has a $20 minimum order. I used this book last year and found several leaks in my system that I never knew existed. Now, everything works MUCH better than before. You can also fax an order to 203-576-0715 if you already have his catalog and need a few other items anyway.

Hope this helps.

Patrick Hulst

[Patrick H.]

Re: '75 Headlight Troubleshooting

Stephen,

It is tough to diagnose vacuum systems by using readings, as funny as that may sound. It is much better (easier) to test individual components, and see what is working, and what isn't (or is leaking).

Despite various comments recently, the one thing Dr Rebuild is good for is his vacuum system troubleshooting manual. It is part number 2777555, and is $3.50 in their just-off-the-presses catalog. You can order at 203-366-1332, but he has a $20 minimum order. I used this book last year and found several leaks in my system that I never knew existed. Now, everything works MUCH better than before. You can also fax an order to 203-576-0715 if you already have his catalog and need a few other items anyway.

Hope this helps.

Patrick Hulst

[Jack H.]

Re: '75 Headlight Troubleshooting

I agree with Pat's analysis, but there's something to work with here. Troubleshooting guides tell you to start at the vac head, collect that reading as an absolute and work progressively back through the system being suspect of any series branch where vac falls by 2 psi or more.

Sounds like there's a pretty healthy drop (2 inches!) in the path you're tracing....

[Jack H.]

Re: '75 Headlight Troubleshooting

I agree with Pat's analysis, but there's something to work with here. Troubleshooting guides tell you to start at the vac head, collect that reading as an absolute and work progressively back through the system being suspect of any series branch where vac falls by 2 psi or more.

Sounds like there's a pretty healthy drop (2 inches!) in the path you're tracing....

[Patrick H.]

Re: '75 Headlight Troubleshooting Instructions

Jack,

I didn't mention this, as the 8 and 10 mmHg readings are definitely too low, and its hard to diagnose with the exception of knowing that something is definitely leaking.

Stephen, normal engine vacuum is approx 13-14mmHg for an L-82, and closer to 16 or above for an L-48. On my car, I have had all 13-14mmHg at the headlights, so in a working system there should not be a significant drop. If there is, then the component across which the vacuum drops is the likely candidate.

I suggest checking manifold vacuum, and then working your way "out" from the motor. When the vacuum goes down as you go "around" a part, then it is likely not functioning correctly. I would still consider checking each part as per the Dr Rebuild manual, however.

On my car, I replaced both of the headlight valves (the "trees" under the nose) as well as the windshield door valve behind the right fender. At $35 each it wasn't cheap, but my headlights sure work fast!!!

Patrick Hulst

[Patrick H.]

Re: '75 Headlight Troubleshooting Instructions

Jack,

I didn't mention this, as the 8 and 10 mmHg readings are definitely too low, and its hard to diagnose with the exception of knowing that something is definitely leaking.

Stephen, normal engine vacuum is approx 13-14mmHg for an L-82, and closer to 16 or above for an L-48. On my car, I have had all 13-14mmHg at the headlights, so in a working system there should not be a significant drop. If there is, then the component across which the vacuum drops is the likely candidate.

I suggest checking manifold vacuum, and then working your way "out" from the motor. When the vacuum goes down as you go "around" a part, then it is likely not functioning correctly. I would still consider checking each part as per the Dr Rebuild manual, however.

On my car, I replaced both of the headlight valves (the "trees" under the nose) as well as the windshield door valve behind the right fender. At $35 each it wasn't cheap, but my headlights sure work fast!!!

Patrick Hulst

[Jack H.]

Re: '75 Headlight Troubleshooting Instructions

PS, don't forget the $4 fix! If your system has been leaking (as it appears), ask yourself where the atmospheric leak source (and it's contaminates) have been going. Answer -- into, through the branches in your vac system and ultimately to the vac source (your intake mani).

The first element in the system is an in-line filter ($4 item) to guard against vac system 'crud' creeping into your intake and the cylinder head combustion chambers. This is a replacement part (I change mine every year or two) still available from AC as well as most of the major catalog houses.

If/when the filter screen gets plugged, guess what's likely to break free and suck into your intake manifold? Also, with a plugged screen that hasn't broken loose, you can see other vac system 'funnies' based on kinetic energy issues (headlights and/or wiper door 'dance' on start-up, accel, decel because parallel paths of vac system are in a 'race' condition to see who sees/gets vac pressure first).

So, advice is when you find and kill the gremlins (don't think there's only one problem and/or leak source), by all means replace the in-line vac filter!

[Jack H.]

Re: '75 Headlight Troubleshooting Instructions

PS, don't forget the $4 fix! If your system has been leaking (as it appears), ask yourself where the atmospheric leak source (and it's contaminates) have been going. Answer -- into, through the branches in your vac system and ultimately to the vac source (your intake mani).

The first element in the system is an in-line filter ($4 item) to guard against vac system 'crud' creeping into your intake and the cylinder head combustion chambers. This is a replacement part (I change mine every year or two) still available from AC as well as most of the major catalog houses.

If/when the filter screen gets plugged, guess what's likely to break free and suck into your intake manifold? Also, with a plugged screen that hasn't broken loose, you can see other vac system 'funnies' based on kinetic energy issues (headlights and/or wiper door 'dance' on start-up, accel, decel because parallel paths of vac system are in a 'race' condition to see who sees/gets vac pressure first).

So, advice is when you find and kill the gremlins (don't think there's only one problem and/or leak source), by all means replace the in-line vac filter!