Heater bypass control?

Got a member that needs a heater bypass control for a 1990 F350. Here's her description of it. If anyone has one you can post back here or you can PM her at gsgIDI THANKS!

Anyway the diesel is not at home today for me to get pictures. On the heater as best as I can remember. The plug that goes into the heater control has 3 wires control is kinda round looks like it has some type of screws that attaches the front of the control and sits on a small flat plate on the passengers side inner fender under the hood with a vaccuum hose going out of it. This is all from memory as the truck left this A.M. with my hubby. The truck is a 1990 F-350 CC Srw XlT Lariat with 7.3L NA and has the 8 ft bed with dual tanks. Thanks Doug for the help and I will get a picture of it tomorrow and send it to you via E-mail. Please thank Ken for me also and have a very Merry Christmas from my family to you and everybody at Oilburners. Gina

Doug, I don't think I quite get what she needs? My '88 Lincoln has this dumb thing that will keep the blower from working on the "floor" setting until coolant is hot enough, those like to fail so we fullsize passenger car guys bypass them right away. My truck has nothing like that tho. Now, I do have a vacuum-controlled heater core bypass valve, but that's not original to the truck, it's a Ranger part that I installed to help the A/C while I still had one - in the pic below tis immediately to the left of the add-on fuel filter:


So I think she's not talking about neither of those, her description tho very well matches the low-vacuum switch for the brake warning lights on the dash...

I was hoping someone here would be able to identify what part she needs. I had already gotten rid of my shell so I wasn't able to check it out. Basically she has no heat to the cab and was either told, or figured out this part was bad. If she has low vacuum, the heater doors won't work, correct? Would it also kill all heat to the cab?

like this?

http://www.dieselwarden.net/ford/coolbypass.html

I'm using that setup and it works for me.

Rick

GenLightening said:

I was hoping someone here would be able to identify what part she needs. I had already gotten rid of my shell so I wasn't able to check it out. Basically she has no heat to the cab and was either told, or figured out this part was bad. If she has low vacuum, the heater doors won't work, correct? Would it also kill all heat to the cab?

Click to expand...

Doug, yes, that's correct, to a point - I'm running around with a half-dead vacuum pump, but my HVAC stuff still works, it just takes a bit to build up vacuum. Her issue could be the diaphragm in the low-vacuum switch, I suspect if tis busted so it could creates a vacuum leak. BUT! on the other hand, her brakes are also vacuum-operated so they would know right away if they have a big vacuum leak as brake pedal would get real stiff to step on, and brakes will feel weak. I'd suggest that she pulls the hose and electric harness off the low-vacuum switch (leave the harness to just lay there) and also to plug the vacuum hose to eliminate any potential vacuum leaks - that should rule out the switch as a potential source of the problem.

Hi guys,
I checked the part at LMC and they list the heater bypass control for a 6.9 the vaccum hose on my 7.3 comes out straight the back of the controller is not the same the vaccum hose that comes out of a 6.9 it is at a 90 degree angle out at the back of the controller. I had to cut the tie wrap that was holding the plug to the heater bypass control as it smelled hot when I pulled up to the post office to get the mail part of the plug is melted so I assume the bypass heater control is shiot as well. My question will the 6.9 bypass heater controller work in my 7.3? I will need to get a replacement plug for this as well as mine is partially melted. M.L.S.C. the one on your Lincoln is pretty close to the one that I have on the truck. So am I correct in assuming that the vaccum that controls the heat and air cond getting to the cab is controlled by the bypass heater control? My air conditioning is not blowing cold at all warm air only. Thank you all for the help and advise on getting this fixed properly for me. I looik foreward to a warm cab in the near future. Thanks again . Gina

Gina, the blower bypass thingy in my Lincoln actually installs in one of the heater core hoses, it has two vacuum lines going to it and a two-wire harness - here's a picture of it:



My F350 has absolutely nothing like this tho, none at all. I have an XLT truck with cruise control, A/C, and power everything, I guess that would be about as loaded as it could be, and I don't think I'm missing any parts...

Now, look at this picture below, is the plug you're having issues with one of those I have circled in bright colors?

M.L.S.C.
You saved my bacon, it is the picture that is in RED. The plug is totally fried to the controller. Got hot I smelled from inside the cab outside of post office. The plug was attached by a tie wrap, it is sorta melted now. Thank you for posting the picture and reference colored markers. Now you know the controller and the plug that is giving me trouble. Thank you for the help. Gina
Anybody have a spare one lying around to sell. Gina

Gina, this is exactly the low-vacuum switch me and Doug were talking about a few posts ago - what it senses the amount of vacuum in your system (hence the vacuum line attached to it), if the vacuum drops dangerously low this switch triggers the red "BRAKE" warning light on your dashboard - your brakes have a vacuum booster to assist you when you get on the pedal, so if you lose vacuum the warning light should come on to notify you that the next time you need to stop you better put both feet on the brake pedal and lean on it as hard as you can.

This is the only thing this thingie does, it has no effect on your heater whatsoever. I know this because mine is completely removed - my vacuum pump is very weak so the switch would trigger the red dashboard light all the time, I just unplugged it from the harness and pulled the vacuum line off that vacuum tree on the firewall and capped it off there - so now the harness is just laying around in there behind the battery, and my heater still works great. My brakes are not affected by the low vacuum like yours would be because I use the Superduty system that runs off the power steering pump).

For now just unplug the harness from the vacuum switch, if you smell burnt and it melted something ain't right so just pull the plug out.

Hi M.LS.C.
I appreciate you taking the time to respond to my post. I am already ahead of you the plug is fried and it has been left disconnected from the low-vacuum switch now all I need to do is plug the T at the vaccum line. Thanks for the information, I really appreciate it as this is my first diesel and I am trying to learn as I go along. I really love my truck, my hubby hates it but since I am the one that drives it I just love it. It may look like a battered piece of junk as my boys call it, but it is all mine and paid for and I just love it.
Diesels Rock. Gina

1990 F-350 XLT Lariat CC SRW 8ft bed 7.3LIDI 4.10's

Gina, how are your brakes? Does the pedal feel hard to press? I'd suggest you either cap off the vacuum tree on the firewall, or just plug the hose with a bolt or something to get you by for a few days, as if the hose is left open that creates a vacuum leak that may affect the brake booster, and is sure to throw your HVAC vacuum doors out of whack somewhat... if you move the lever on the dashboard that changes where the air is coming out, does it switch immediately, or it takes it a bit to start blowing from the new place? A delayed action would certainly indicate low vacuum in the HVAC system, which could explain your blend door not operating properly and you not getting as much hot as you'd like to.

Hi M.L.S.C.
The question about the brakes. I learned to drive on a 1942 jeep manual steering and manual brakes about 40 years ago. The brakes are not too bad I do not have to stand on them to get the truck to stop, but I realize they are not the throw you threw the windshield people are used to today. I will put a bolt in the vacuum line tomorrow, I had to use the truck today to take a vehicle to my son who in on Recruiters Assistance for the Marine Corps till the 3rd of January. His right head gasket blew on his Mustang (mom to the rescue). My hubby and I had to take our Honda to my son to use to get home from work 30 mile commute one way while he is here at home.
The HVAC question it takes some time to get a change when moving from one setting to another on the dashboard. I looked today my heater hose is directly clamped to the engine with the other hose rigged up clamped behind the alternator and down to the bottom of the radiator (wierdest set up I have ever seen in hoses). The hose that is clamped is some kind of hose with high pressure fitting at the alternator end of the hose that is clamped to the back of the alternator. Anyway thanks again for the help, I guess when finances are better I may need to go to the local junkyard to pick up a few parts for the truck. My truck is a work in progress, but I love it so I'll keep plugging on it and learning from it as well. Thank you everybody for your help in getting my questions answeered. You all have a safe and Happy New Year from my home to yours. Gina

Gina, your truck may have a factory tow package - mine has a weird heater hoses routing two, one of the hoses goes over the alternator (through a hoop attached to the alternator bracket) then to the water pump, but the other hose when it comes from the heater core travels towards the front of the truck then a bit behind the low-vacuum switch it splits in two and one branch goes down to a port in the engine right above the exhaust manifold, but the other branch goes down to the bottom tank of the radiator. Is this something like you're seeing in your engine bay? If so that's normal setup for the tow-package trucks, and nothing really rigged up.

As far as brakes are concerned, these trucks actually have a pretty light pedal feel, it's when you jump on the brakes during an emergency situation and when the RABS kicks in when you realize that you could really use some more braking power at the front wheels. So yah, it does seem like you are experiencing a somewhat low-vacuum situation, if you have a vacuum gauge available (for instance from your son's Mustang, or another performance car project) you can stick it in that hose you pulled from the melted low-vacuum switch to see what numbers your vacuum pump is really pulling.

M.L.S.C.
Could you post a picture of your hose setup please. I also have a plug in in the bed of the truck to plug in trailer lights left side. If I am correct, this is a set up for towing trailers right. Yea my hose hookup sounds like yours, I'll check it out today and post you back. I need to drive to town to take the trash to the landfill 64 mile drive no local trash pickup in town. The county will pick it up at a cost of $90.00 per month but we can take it to the landfill for about $3.00 a trip excluding diesel fuel cost. Thank you for posting me back with answers to my questions as always. Mustang is in repair shop it had a head gasket problem car getting hot found out it needed to have the right head taken off and checked for cracks (aluminium) heads not iron. My Marine son's (Michael) car it happened right after he got home on recruiters assistance. He is using one of the others vehicles around the house and will be leaving Saturday. A nice 3 hour trip one way to San Antonio, TX. to the airport for his flight back to Florida.