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Smoky Mountain Region PCA
 
Join Date: Mar 2014
Location: Pikeville, Tennessee
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Test Cruise Servo 911SC

I'm trying to revive the cruise on my '82 911SC - when I got it, it did not have a cruise control module. The Bentley manual diagnostics at the socket all checked good, clutch switch is activating relay, etc., and I just got a used cruise module (not necessarily known-good, but I have a 90 day warranty) and plugged it in, but still no cruise.
Of course, I have no way of knowing if the module is good, but I also didn't know the status of the servo other than the Bentley check of terminals 3 & & is OK at 15.2 ohms (acceptable range 12-17).
I also read that the module puts out 6v when accelerating, and you can apply 6v directly across the servo terminals with the engine running and it should accelerate. I did this and nothing happened and the bench power supply indicated a .42 amp current. Someone else said test with 9v and I did and nothing happened and the current was .6 amps. Of course, the vacuum lines were attached and seemed secure and the engine was fast idling about 1800 rpm. Is that enough rpm for functional vacuum?
Are these valid tests and do I have a bad servo?
If bad, are there any further things to do to the servo to fix it, or just replace it?
I know 90+% of the time it's the cruise module, but I'd like a functioning servo before swapping out the module.
John

Last edited by jaustinmd; 01-28-2016 at 05:36 AM..
Old 01-28-2016, 05:29 AM
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Smoky Mountain Region PCA
 
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Hold the presses!

Well, this is embarassing! I just "thought" the vacuum lines were secure! The vent hose was secure, but the vacuum line was not. Let me secure it and do a little more testing!
John
Old 01-28-2016, 05:59 AM
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I really doubt you have a bad servo, I have yet to come across anyone on this forum
that actually had a bad servo which also had a good ohm reading, but I think with the ohm reading you have on yours you are AOK. Here's statement from a thread by a Pelican who built a "test bed" to electronically test a CC amp.

"One interesting observation is the output to the vacuum actuator is a series of what appears to be non-uniform pulses. The duty cycle of the pulses seem to determine the operation of the actuator."

Given that statement, I don't think the mere application of a solid DC voltage to the servo will tell you anything! If possible, you need to find a KNOWN GOOD CC amp to
verify things. Any possibility of any Pelicans in your area that could do a test swap with
you?

Here's the thread by the Pelican who did the testing, very detailed and interesting reading and even has circuit schematics! Quite an old thread and I don't think the author is still an active Pelican.
Understanding the VDO Cruise Control Amp
__________________
'80SC Widebody 3.6 transplant Anthracite "The Rocket"
Long gone but still miss them all:
'77 911 Targa, '72 BMW 3.0CS Coupe(finest car I ever had!)
'71 911T Coupe White, '70 911T Coupe Blue
'68 911 Coupe Orange, '68 911L Soft Window Targa
Old 01-28-2016, 06:02 AM
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Quote:
Originally Posted by uwanna View Post
I really doubt you have a bad servo, I have yet to come across anyone on this forum
that actually had a bad servo which also had a good ohm reading, but I think with the ohm reading you have on yours you are AOK. Here's statement from a thread by a Pelican who built a "test bed" to electronically test a CC amp.

"One interesting observation is the output to the vacuum actuator is a series of what appears to be non-uniform pulses. The duty cycle of the pulses seem to determine the operation of the actuator."

Given that statement, I don't think the mere application of a solid DC voltage to the servo will tell you anything! If possible, you need to find a KNOWN GOOD CC amp to
verify things. Any possibility of any Pelicans in your area that could do a test swap with
you?

Here's the thread by the Pelican who did the testing, very detailed and interesting reading and even has circuit schematics! Quite an old thread and I don't think the author is still an active Pelican.
Understanding the VDO Cruise Control Amp
Don't know if you saw the post above yours ... very embarassing! I just "thought" the vacuum lines were OK - they're not! The vacuum line isn't there/isn't connected! When I looked, the view of the servo's vacuum port was obstructed by the vent line. I swear I thought I saw the vacuum line behind it, but I was mistaken! I've now carefully explored the vacuum system and can't even find a plugged tee to run a new line to! The PO did not have the cruise module and when the engine was overhauled, I don't think the tech bothered to hook up the vacuum to the servo since the cruise was inop!
So ... is any vacuum line OK to tee into? There are 3 easily accessible lines on the left side of the engine (side the servo's on). There is one that runs from the throttle body area to a fitting beside an intake runner on the left rear corner of the engine, a red line running to the end of the vacuum advance on the distributor, and a black line running to the same thing, on the other side of the vacuum cannister, closer to the actual distributor. Any of these OK to tee into?
Thanks,
John
Old 01-28-2016, 06:40 AM
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Quote:
Originally Posted by jaustinmd View Post
So ... is any vacuum line OK to tee into? There are 3 easily accessible lines on the left side of the engine (side the servo's on). There is one that runs from the throttle body area to a fitting beside an intake runner on the left rear corner of the engine, a red line running to the end of the vacuum advance on the distributor, and a black line running to the same thing, on the other side of the vacuum cannister, closer to the actual distributor. Any of these OK to tee into?
Thanks,
John
If the source of the vacuum line is from the throttle body, T into it and you should be good to go! Give her a try!
__________________
'80SC Widebody 3.6 transplant Anthracite "The Rocket"
Long gone but still miss them all:
'77 911 Targa, '72 BMW 3.0CS Coupe(finest car I ever had!)
'71 911T Coupe White, '70 911T Coupe Blue
'68 911 Coupe Orange, '68 911L Soft Window Targa
Old 01-28-2016, 06:50 AM
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Quote:
Originally Posted by uwanna View Post
If the source of the vacuum line is from the throttle body, T into it and you should be good to go! Give her a try!
Thanks, I didn't want to mess up the vacuum advance in the distributor or something!
John
Old 01-28-2016, 06:55 AM
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This line (red arrow) looks to be a likely candidate for the tee. Is that the thermostatic valve (circled in red) the line is going to ?

John
Old 01-28-2016, 07:38 AM
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Quote:
Originally Posted by jaustinmd View Post
This line (red arrow) looks to be a likely candidate for the tee. Is that the thermostatic valve (circled in red) the line is going to ?

John
I would "T" to the leftmost vacuum line that goes to the Dizzy which will provide "below the throttle plate" vacuum. An additional line "Teed" to the servo will not change the vacuum to the dizzy.
__________________
'80SC Widebody 3.6 transplant Anthracite "The Rocket"
Long gone but still miss them all:
'77 911 Targa, '72 BMW 3.0CS Coupe(finest car I ever had!)
'71 911T Coupe White, '70 911T Coupe Blue
'68 911 Coupe Orange, '68 911L Soft Window Targa
Old 01-28-2016, 08:29 AM
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Hey, nice marmot.
 
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The thing in the red circle on the right is the Warm Up Regulator (WUR). In different years, there were different models of the WUR. On some the 'vac line' is really just a vent - pretty sure this is the case for '81-83. On other years/models, I believe that the WUR actually used the vacuum to modify the control pressure.

On my '83 the black vac line that comes off the back of the airbox has a 'tee' that goes to the decel valve on the right and the cruise servo on the left. Normally the retard/advance vac lines to the dizzy are blue/red (or a shade of those colors due to age). But sometimes people replace the colored ones with a black line, or even worse, get them reversed.

The retard line pulls a vacuum at idle and the advance line pulls a vacuum off idle.
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'83 911 SC 3.0 coupe (NA)

You can't buy happiness, but you can buy car parts which is kind of the same thing.

Last edited by tirwin; 01-28-2016 at 10:02 AM..
Old 01-28-2016, 09:18 AM
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Look for what Tim mentions and tee there!
__________________
'80SC Widebody 3.6 transplant Anthracite "The Rocket"
Long gone but still miss them all:
'77 911 Targa, '72 BMW 3.0CS Coupe(finest car I ever had!)
'71 911T Coupe White, '70 911T Coupe Blue
'68 911 Coupe Orange, '68 911L Soft Window Targa
Old 01-28-2016, 09:33 AM
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Smoky Mountain Region PCA
 
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Quote:
Originally Posted by tirwin View Post
The thing in the red circle on the right is the Warm Up Regulator (WUR). In different years, there were different models of the WUR. On some the 'vac line' is really just a vent - pretty sure this is the case for '81-83. On other years/models, I believe that the WUR actually used the vacuum to modify the control pressure.

On my '83 the black vac line that comes off the back of the airbox has a 'tee' that goes to the decel valve on the right and the cruise servo on the left. Normally the retard/advance vac lines to the dizzy are blue/red (or a shade of those colors due to age). But sometimes people replace the colored ones with a black line, or even worse, get them reversed.

The retard line pulls a vacuum at idle and the advance line pulls a vacuum off idle.
So the black line is the retard line, correct?(New red arrow!)

That line with the arrow is black on my dizzy, the other line is red and that is the advance line and the one to tee for servo vacuum, correct?
"measure twice, cut once!"
John

Last edited by jaustinmd; 01-28-2016 at 12:04 PM..
Old 01-28-2016, 11:42 AM
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Hey, nice marmot.
 
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No, that's one of the ones going to the dizzy. I've found the PET diagrams for the vac lines aren't extremely clear. There should be a black line that comes off the back side of the airbox. If you have a vac line going to the decel valve (right side of the airbox, kinda toward the back, gold color thing that looks like a flying saucer on it's side). If you have that (some people disconnect the decel valve) and trace it back behind the air box you should find where it connects to the airbox. At least on mine, there is a tee that goes over to the cruise servo.

I'm not where I could take a picture right now or I would.
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There are those who call me... Tim
'83 911 SC 3.0 coupe (NA)

You can't buy happiness, but you can buy car parts which is kind of the same thing.
Old 01-28-2016, 12:02 PM
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Quote:
Originally Posted by tirwin View Post
No, that's one of the ones going to the dizzy. I've found the PET diagrams for the vac lines aren't extremely clear. There should be a black line that comes off the back side of the airbox. If you have a vac line going to the decel valve (right side of the airbox, kinda toward the back, gold color thing that looks like a flying saucer on it's side). If you have that (some people disconnect the decel valve) and trace it back behind the air box you should find where it connects to the airbox. At least on mine, there is a tee that goes over to the cruise servo.

I'm not where I could take a picture right now or I would.
OK, I went and looked again and found this on the right side behind the airbox with a vacuum line sticking out of the top. I can barely touch the line, much less see anything as it goes behind the airbox. I do think I could tee in at the top of the decel valve - that should be OK shouldn't it? I felt behind the airbox from the left and the line seems to continue uninterrupted (no tee) to the airbox. Like I said, the cruise was inop when the PO had the engine rebuilt and I imagine they didn't bother plumbing to the servo - the vac line looks brand new, so I'm sure it was replaced.
Here's the pic:

Last edited by jaustinmd; 01-28-2016 at 12:31 PM..
Old 01-28-2016, 12:19 PM
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I think I've figured out a way to tee the line and make this easy! I'm going to detach the vacuum line from the decel valve and connect it with an in-line connector to a length of new vacuum hose. Then, I can reach around the left side of the airbox, pull the hose toward me (the extended length will keep me from losing the distal end) and when I get enough slack, I can comfortably tee it on the left side of the airbox, connect the cruise servo vacuum line to the tee, then pull the line back around to the decel valve, disconnect it from the length of hose, and connect it.
This way, I won't have to run several extra feet of servo hose ... I can get the tee closer to the left side of the engine and the servo!
Old 01-28-2016, 12:46 PM
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Hey, nice marmot.
 
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Looks like you found it. That sounds like it should be fine.
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'83 911 SC 3.0 coupe (NA)

You can't buy happiness, but you can buy car parts which is kind of the same thing.
Old 01-28-2016, 08:18 PM
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Quote:
Originally Posted by tirwin View Post
Looks like you found it. That sounds like it should be fine.
After locating the decel hose and verifying it was connected to the airbox, I tee'd into it and the servo was still inop! Put a vacuum gauge on the line and no vacuum! Zero! I don't know if the line is plugged or what, but since it was mentioned that the decel valve is disconnected sometimes anyway, I just plugged-off my tee and will leave that to the tech at the next maintenance visit!
Seeking a vacuum source, I checked the vacuum line going to the WUR (warm-up regulator) and it had a good vacuum. I tee'd into that line and connected to the servo. I started the engine, put 6v across sockets 3 and 7 of the cruise module connector, and the engine immediately went to full throttle! So, I'm convinced the servo is OK!
BTW, tee'ing into the WUR vacuum line didn't seem to adversely affect anything so I'll leave it there for now.
I thoroughly cleaned the pins and sockets and plugged the cruise module in, but the cruise is still inop. Now, a new problem has arisen - when I attempt to engage the cruise, the brake warning light comes on! I'm going to start a new thread since this is getting pretty far from the topic of testing the servo. That thread is here.

Last edited by jaustinmd; 01-30-2016 at 07:47 AM..
Old 01-30-2016, 06:56 AM
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