No timing belt warning light post HVAC pod replacement
05-14-2016, 10:59 AM
#1
Drifting
Thread Starter
No timing belt warning light post HVAC pod replacement
I've only driven about 150 miles since all leaking pods were replaced, but already there are a few pleasant surprises:
1. The transmission shifted fine during light driving for as long as I've owned the car. After the pod repairs the car shifted more smoothly and more consistently when driven hard. It's difficult to describe the overall feeling - but someone might mistake the before and after as the result of have the transmission serviced. It just feels like it should. Prior to replacing the the HVAC pod actuators the shifts when mashing the pedal were abrupt and harsh, and possibly a little delayed. Now the trans feels more responsive, shifts are firm and quick, and the abrupt harshness is gone.
2. And here is the really good part (I hope). No more timing belt warning light when mashing the kick down switch. Formerly, the first upshift would be abrupt and harsh and the light would come on. I always imagined this was due to a short moment of slack in the belt during the shift. Now with the more controlled shifting the light won't come on...and I've tried to make it come on.
Of course now it will come on the next time I drive the car.
I'll update if the light returns. (note: belt tension measured within spec). If it doesn't return it may be that a compromised vacuum system causes abrupt shifting during aggressive driving, and may trigger the timing belt light.
1. The transmission shifted fine during light driving for as long as I've owned the car. After the pod repairs the car shifted more smoothly and more consistently when driven hard. It's difficult to describe the overall feeling - but someone might mistake the before and after as the result of have the transmission serviced. It just feels like it should. Prior to replacing the the HVAC pod actuators the shifts when mashing the pedal were abrupt and harsh, and possibly a little delayed. Now the trans feels more responsive, shifts are firm and quick, and the abrupt harshness is gone.
2. And here is the really good part (I hope). No more timing belt warning light when mashing the kick down switch. Formerly, the first upshift would be abrupt and harsh and the light would come on. I always imagined this was due to a short moment of slack in the belt during the shift. Now with the more controlled shifting the light won't come on...and I've tried to make it come on.
Of course now it will come on the next time I drive the car.
I'll update if the light returns. (note: belt tension measured within spec). If it doesn't return it may be that a compromised vacuum system causes abrupt shifting during aggressive driving, and may trigger the timing belt light.
05-14-2016, 06:13 PM
#2
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Interesting.
Like you I also think that the vacuum problem (leak) you had with the HVAC probably affected the vacuum at the modulator. Now that there is no more leak at the HVAC, the vacuum is better and the modulator is doing it's job.
The timing belt probably didn't like the harsh shifting either!
Always nice when you fix something and you can feel the difference.
Keep up the good work.
Like you I also think that the vacuum problem (leak) you had with the HVAC probably affected the vacuum at the modulator. Now that there is no more leak at the HVAC, the vacuum is better and the modulator is doing it's job.
The timing belt probably didn't like the harsh shifting either!
Always nice when you fix something and you can feel the difference.
Keep up the good work.
05-14-2016, 06:55 PM
#3
Drifting
I'm pleased your car is running so well, but I'm struggling to see how HVAC vacuum could affect the vacuum fed to tranny. Latter is fed directly from intake, but the HVAC is a secondary vac system, isolated from the brake booster by one way check valve. Losing vac in HVAC should not affect vacuum supplied to tranny (or the brake servo for that matter).
I may be wrong, so will be interested to read more expert comments!
I may be wrong, so will be interested to read more expert comments!
05-14-2016, 11:19 PM
#4
Drifting
Thread Starter
Stratford - Yeah, I also understood there is a primary and secondary vacuum system. This puzzled me as well, so much so that I thought I'd not post about this until I had more miles without the light. If the light comes back again I'll update. If it doesn't come back....well, then I'll just not question it and be happy.
05-15-2016, 02:34 AM
#6
Rennlist Member
Meaningless or not, if indeed its behavior changed after fixing that HVAC vacuum system, that's interesting...
05-15-2016, 04:44 AM
#7
Drifting
Stratford - Yeah, I also understood there is a primary and secondary vacuum system. This puzzled me as well, so much so that I thought I'd not post about this until I had more miles without the light. If the light comes back again I'll update. If it doesn't come back....well, then I'll just not question it and be happy.
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05-15-2016, 06:50 AM
#8
Rennlist Member
Jon,
If the timing belt warning light is [was] coming on then something was wrong- no need for debate on this- just a question of what was triggering it and eliminating it. If anyone wants to debate that then try convincing a certain Dr Brown! I have had my transmission with the vacuum modulator disconnected [the nylon tube fell out of the connector], transmission shifts are then harsh [& very swift] but no timing belt warning light came on. I can perhaps understand that if the belt tension is at the low end of correct tension range then maybe the alarm could be initiated by a harsher shift but....? Also remember that the front mounted dampers on our cranks are also largely toast by now [as per the good doctor] and that may contribute to the problem.
When you tackle a job and behaviour changes [be it for better or worse] then quite rightly a smart owner should investigate why the "unexpected" change occurred if for nothing more than completeness- there is always a reason- even if it is the placebo effect!
If I understand your post correctly then presumably you pulled the dash panel to get at the vacuum actuators within the HVAC system. These actuators are driven from a single vacuum line taken from the passive vacuum reservoir. It is called "passive" because vacuum does not change with engine dynamics. However, it is a reservoir and if the vacuum drain is greater than the rate of replenishment the vacuum levels will go down and at some vacuum level [about 7 or 8 inches of mercury I seem to remember] the actuators cease working. Thus you should be confident that the vacuum reservoir is holding vacuum and if you have not done so already suggest you test this- a good motor should pull down to a vacuum level about 20 inches/ 500mm of mercury when idling at the correct speed and should be able to hold vacuum for an hour or so with little change.
As Adrian states, the passive vacuum reservoir is supplied from the nipple on the servo nozzle. The little blue check valve should hold vacuum and should be connected [in the correct orientation] at this point. Presumably you have checked this- some DIY hacks have been known to connect the reservoir via the "active" vacuum manifold [the rubber multi way connector behind the inlet manifold]. On the active system a deep vacuum is pulled when you come off the throttle and the items connected to it, such as the vacuum shift modulator in the gearbox, are designed to work with such.
Theoretically, you have not touched anything that would impact the shift modulator but maybe you routinely checked the tightness of the connections - simply gave them a quick "push" whilst you were in the general neighbourhood?
Either way if it is now running well let's hope it stays that way.
Rgds
Fred
If the timing belt warning light is [was] coming on then something was wrong- no need for debate on this- just a question of what was triggering it and eliminating it. If anyone wants to debate that then try convincing a certain Dr Brown! I have had my transmission with the vacuum modulator disconnected [the nylon tube fell out of the connector], transmission shifts are then harsh [& very swift] but no timing belt warning light came on. I can perhaps understand that if the belt tension is at the low end of correct tension range then maybe the alarm could be initiated by a harsher shift but....? Also remember that the front mounted dampers on our cranks are also largely toast by now [as per the good doctor] and that may contribute to the problem.
When you tackle a job and behaviour changes [be it for better or worse] then quite rightly a smart owner should investigate why the "unexpected" change occurred if for nothing more than completeness- there is always a reason- even if it is the placebo effect!
If I understand your post correctly then presumably you pulled the dash panel to get at the vacuum actuators within the HVAC system. These actuators are driven from a single vacuum line taken from the passive vacuum reservoir. It is called "passive" because vacuum does not change with engine dynamics. However, it is a reservoir and if the vacuum drain is greater than the rate of replenishment the vacuum levels will go down and at some vacuum level [about 7 or 8 inches of mercury I seem to remember] the actuators cease working. Thus you should be confident that the vacuum reservoir is holding vacuum and if you have not done so already suggest you test this- a good motor should pull down to a vacuum level about 20 inches/ 500mm of mercury when idling at the correct speed and should be able to hold vacuum for an hour or so with little change.
As Adrian states, the passive vacuum reservoir is supplied from the nipple on the servo nozzle. The little blue check valve should hold vacuum and should be connected [in the correct orientation] at this point. Presumably you have checked this- some DIY hacks have been known to connect the reservoir via the "active" vacuum manifold [the rubber multi way connector behind the inlet manifold]. On the active system a deep vacuum is pulled when you come off the throttle and the items connected to it, such as the vacuum shift modulator in the gearbox, are designed to work with such.
Theoretically, you have not touched anything that would impact the shift modulator but maybe you routinely checked the tightness of the connections - simply gave them a quick "push" whilst you were in the general neighbourhood?
Either way if it is now running well let's hope it stays that way.
Rgds
Fred
05-15-2016, 07:57 AM
#9
Drifting
Only on Rennlist could we frown over why a 928 is running better, and not be satisfied till reason is proved. I love this group - genuinely no irony intended!