Top C Posted June 13, 2021 Share Posted June 13, 2021 I've look in the forum here about HVAC and vacuum line problems but didn't see my problem discussed. I have 87 MJ 4.0 4 wheel drive with automatic transmission. I've moved the vacuum bottle from the front bumper to near the firewall and replaced the hoses. Everything is working okay except I've noticed when I accelerate the HVAC shifts from the vents to the defroster. Well I thought it was a broken line to HVAC but that's not the case. I've check the vacuum at idle and then at acceleration in park and there is no drop in vacuum. However when I put the vehicle in drive I notice the airflow starts to shift to the defroster. I haven't fully investigated why this is happening. Anybody had this problem? thanks Mike Link to comment Share on other sites More sharing options...
ghetdjc320 Posted June 13, 2021 Share Posted June 13, 2021 51 minutes ago, TOP said: I've look in the forum here about HVAC and vacuum line problems but didn't see my problem discussed. I have 87 MJ 4.0 4 wheel drive with automatic transmission. I've moved the vacuum bottle from the front bumper to near the firewall and replaced the hoses. Everything is working okay except I've noticed when I accelerate the HVAC shifts from the vents to the defroster. Well I thought it was a broken line to HVAC but that's not the case. I've check the vacuum at idle and then at acceleration in park and there is no drop in vacuum. However when I put the vehicle in drive I notice the airflow starts to shift to the defroster. I haven't fully investigated why this is happening. Anybody had this problem? thanks Mike You will likely just need to add a vacuum check valve on the vacuum line going through the firewall to the hvac panel. The vacuum motor that controls that hvac door is located on the drivers side at the plenum box. If the check valve doesn’t solve the problem, you can try using a vacuum pump (just a little minivan hand unit works fine) and test the vacuum motor to make sure it’s holding. You can backwards troubleshoot from there. It is also very common to have a vacuum leak in the hvac vacuum hose right near the firewall. Good thing to double check as well Link to comment Share on other sites More sharing options...
scaleless Posted June 13, 2021 Share Posted June 13, 2021 Isn't there supposed to be a check valve built into the Renix vacuum canister? Either it's that or a fairly significant vacuum leak. Engine vacuum naturally drops under load to the point where the AC can't hold its settings. Link to comment Share on other sites More sharing options...
ghetdjc320 Posted June 13, 2021 Share Posted June 13, 2021 3 minutes ago, scaleless said: Isn't there supposed to be a check valve built into the Renix vacuum canister? Either it's that or a fairly significant vacuum leak. Engine vacuum naturally drops under load to the point where the AC can't hold its settings. I don’t believe it’s built into the canister. Even so though, that hvac line should have its own check valve. That’s absolutely true though. An engine under load will produce almost no vacuum and the default position on that vacuum motor is defrost. Link to comment Share on other sites More sharing options...
cruiser54 Posted June 14, 2021 Share Posted June 14, 2021 There's still a leak somewhere. Link to comment Share on other sites More sharing options...
Top C Posted June 14, 2021 Author Share Posted June 14, 2021 Okay so today I look at the Vacuum Canister black line going through the firewall to the HVAC controls there is no check valve in line. So maybe whoever had this MJ before me didn't put the check valve back in. So before I go any further I have another question related to this. I have the football looking Canister which has two sections. The one section is handling the HVAC Vacuum. Then the other side of the Canister has an output line going to a device mounted the fire wall which I can't find any information on but it does have an arm on the opposite end with a cable connected to the accelerator control cable. I don't have any cruise controls on the steering wheel so I was wondering what the purpose of this device is? The main vacuum line that comes from the intake manifold to the Vacuum canister has two tee fitting in it the first tee in the line has a line that I believe goes to the 4 wheel drive harness the second tee splits the vacuum line for each side of the Vacuum Canister. So should there be a check valve on the main vacuum feed line or just the one section of the Canister handling the HVAC? I just want to make sure I put the Check valve in right place. Thanks Mike Link to comment Share on other sites More sharing options...
Top C Posted June 14, 2021 Author Share Posted June 14, 2021 Update my bust that unit on the firewall is part of the cruise control. So now I know what the functions of both half's of the Vacuum Canister are doing my remaining question is where should the Check Valve go, on the main vacuum line that feeds both sections of the Canister or just the output line from the section that controls the HVAC? Sorry any confusion on my part. Mike Link to comment Share on other sites More sharing options...
scaleless Posted June 14, 2021 Share Posted June 14, 2021 Renix never had an inline check valve. If you want to add one, though, it should go before the reservoir. Once you add one, put a vacuum guage on the reservoir side and see if it holds a vacuum with the engine off. If not, you have a leak that needs to be addressed. Even if you don't have a leak, test all the HVAC settings to see if you have a leak in or going to one or the vacuum actuators. Link to comment Share on other sites More sharing options...
Top C Posted June 15, 2021 Author Share Posted June 15, 2021 Thanks Link to comment Share on other sites More sharing options...
87MJTIM Posted June 15, 2021 Share Posted June 15, 2021 The barrel on the fire wall is the cruise control servo. That would pull on or back off the accelerator for cruise control. The turn signal stalk would have your cruise control commands. Look under the driver's side lower panel. If there is a yellow box there, then you have/had cruise control. Link to comment Share on other sites More sharing options...
ghetdjc320 Posted June 15, 2021 Share Posted June 15, 2021 I prefer just adding the check valve to the hvac line after the canister to eliminate other possible vacuum leak locations. Link to comment Share on other sites More sharing options...
Top C Posted June 16, 2021 Author Share Posted June 16, 2021 Update on the HVAC I followed the advice here and got a vacuum tester. What I found is on the two stage canister, one side has a leak in it. So just make sure that my finding was correct I disconnected the cruise control servo and plug the HVAC line in its place and disconnected the main vacuum line from the tee fitting that feeds both sides of the Canister and directly couple it the side with the HVAC line and wallah it worked! When I accelerate now the HVAC stays on vent side and there is no leak through the defroster vent. Just found a new old stock two stage canister for $13 dollars on Ebay site. Hope my findings along with the replies here will help the next person. Thanks everyone! Mike Link to comment Share on other sites More sharing options...
cruiser54 Posted June 16, 2021 Share Posted June 16, 2021 Nice job. Link to comment Share on other sites More sharing options...
scaleless Posted June 20, 2021 Share Posted June 20, 2021 On 6/13/2021 at 2:08 PM, ghetdjc320 said: I don’t believe it’s built into the canister. I just found a new small vacuum ball from Dorman that I had lost and it does have a check valve build into the manifold side of the ball. Link to comment Share on other sites More sharing options...
ghetdjc320 Posted June 20, 2021 Share Posted June 20, 2021 1 hour ago, scaleless said: I just found a new small vacuum ball from Dorman that I had lost and it does have a check valve build into the manifold side of the ball. Nice! Link to comment Share on other sites More sharing options...
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