Low Vacuum Pressure Switch

[Bo-Jangles]

Can anyone tell me how this switch works? It's the tuna can located on passenger wheel well that has a vacuum line plugged into it and a three wire electrical connector. My issue is that the brake warning light on my dash is always dimmly lit. It's not because of the parking brake itself as I've checked the switch and return spring for the lever. When I engage the parking brake the light becomes bright and when it's released it goes dim. I've unplugged the electrical connector from the tuna can and used my test light to determine one wire is power, one is ground and one dimmly lights the test light. With the connector unplugged I still get the dim light on my dash. When I jump the power to ground it blows the #18 15amp fuse. If I jump ground terminal to the terminal that dimmly lights the test light the brake warning light on dash becomes bright. If I jump the power terminal to the terminal that dimmly lights the test light the brake warning light on the dash goes out. I'm trying to determine if the switch itself is bad or if I have a wiring issue. With these results I can't make sense of how the switch operates. Any help would be greatly appreciated. Thanks

 

[Cubey]

It hasn't worked in any of the three IDIs I've had, 85, 87, 89.

Get a cheap Chinese vacuum gauge. A couple of examples.

Analog: https://a.co/d/cwGMiUZ

Digital: https://a.co/d/9P8Sqjd

I have the digital one I have yet to install.

 

[IDIBRONCO]

It hasn't worked in any of the three IDIs I've had, 85, 87, 89.

Get a cheap Chinese vacuum gauge.

Mine doesn't work either. I agree with the vacuum gauge. It's nice to know if your vacuum is low, but still high enough to not light up the light.

 

[WrenchWhore]

Where is this warning light anyways?? All of my IDI's never worked either. Didn't even know these were a thing until my 87 IDI. It would be nice to have it working since I had the OH SHI*! moment awhile ago while getting off the freeway in Mohave desert. It's like one moment everything is fine and then those vacuum pumps just SH** the bed. If I was towing something it could have been very VERY bad.

 

[Cubey]

Where is this warning light anyways?? All of my IDI's never worked either. Didn't even know these were a thing until my 87 IDI. It would be nice to have it working since I had the OH SHI*! moment awhile ago while getting off the freeway in Mohave desert. It's like one moment everything is fine and then those vacuum pumps just SH** the bed. If I was towing something it could have been very VERY bad.

BRAKE light

I lost power brakes three times on the 27ft RV. First time in 2019, the alternative belt was lost and I didn't know.

Second time, in late 2022, a nearly new vacuum pump stopped working entirely.

Third time, in early 2023, a nearly new vacuum (that replaced the one from late '22) literally fell apart. I found most of it on the cross member and bagged it and gave it to O'Reilly for the warranty so they could see I wasn't joking.

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Second and third times I had a vac gauge but it was this one, and I found it very hard to see see needle, so I was still caught off guard. So, I don't recommend it: https://a.co/d/6pblAnX

 

[WrenchWhore]

@Cubey Man! And I thought losing brakes in my single cab was bad. I probably would have pooped my pants in a motorhome. Good thing the toilets close by...

 

[KansasIDI]

Funny, the brake warning light came on when my vacuum pump went bad in my 86… wondered what it was all about, then stepped on the brakes and realized that I no longer had vacuum…

 

[Cubey]

@Cubey Man! And I thought losing brakes in my single cab was bad. I probably would have pooped my pants in a motorhome. Good thing the toilets close by...

First time, i wasn't towing. I was braking on a probably 1/3 mile long right turn lane, coming into the NE corner of Amarillo TX. The divided highway I was turning onto, I knew lead straight to a big TA truck stop, but with a few red lights by i40. flashers on, going 40mph in a 65, stomping the pedal with both feet and butt up off the seat. waited until the next morning to replace the belt.

Second time, exiting at railroad pass SE of Vegas. It was an upwards exit ramp so that helped. A truck stop I was going to nearby gave me a place to put on the spare pump. I was pulling the bug so I had extra weight behind.

third time, was exiting i40 on the east side of Kingman AZ, kind of a level exit, as I recall. again pulling the bug.

that's why I opted for the digital gauge for the van. I'm just being too darn lazy about installing it

 

[Cubey]

Funny, the brake warning light came on when my vacuum pump went bad in my 86… wondered what it was all about, then stepped on the brakes and realized that I no longer had vacuum…

The brake light would randomly go on and off in the 87 F250, but that was likely the speed sensor in the diff or whatever.

 

[XOLATEM]

likely the speed sensor in the diff or whatever.

I could be wrong....but I faintly remember a bulletin that told us (mechanics) that the speed sensor was moved from the speedo cable at the trans or transfer case to the diff beginning in '92...

So...my bet is on the 'whatever'...

In Virginia...during a 'state inspection'...if you are unlucky enough to have that warning light stay on while the inspector is checking out your truck...it won't pass...

Every now and again I see that light...but my brakes are working...

at some point I am going to brain out a substitute for the vacuum switch...just have not gotten to it...

Mine doesn't work either. I agree with the vacuum gauge. It's nice to know if your vacuum is low, but still high enough to not light up the light.

This is sage advice...you got me thinking in this direction, also...

Thanks, Guys...

 

[Bo-Jangles]

Just find it weird that the light is still dimmly lit on the dash whether the electrical connector for the switch is plugged in or not. Also find it weird that jumping the two terminals on the plug that have voltage - one higher voltage, one lower (based on the brightness of my test light) is what turns the warning light off. Don't want to cause more damage if there is a wiring issue. Have to get it inspected soon so just trying to have the light off for that. Thanks for the replies.

 

[Rocknit4x4]

I have no familiarity with the circuit but it might be that the wire with the lower voltage is getting it from somewhere it shouldn’t - nicked wire or partially melted wire or something - somewhere else on the vehicle. The dim light / low voltage could indicate a partial ground that’s enough to complete the circuit when connecting the 12v source from the switch to it. But don’t take that as any more than a wild guess.

 

[Rdnck84_03]

If the light is still lit with it unhooked from the alternator, I would say that you have an issue somewhere. I unfortunately don't have any recommendations of where to look though.

James

 

[WrenchWhore]

This is hilarious. I had to ask about the low vacuum light and how to know it's working and guess what? NOW MY dash brake light is staying on! I guess my spare used backup vacuum pump I swapped out in the desert is finally craping out. UHH! I definitely need to get another ASAP with a warranty. This is the second one is 2 years!

 

[Cubey]

This is hilarious. I had to ask about the low vacuum light and how to know it's working and guess what? NOW MY dash brake light is staying on! I guess my spare used backup vacuum pump I swapped out in the desert is finally craping out. UHH! I definitely need to get another ASAP with a warranty. This is the second one is 2 years!

O'Reilly's is lifetime warranty. I'm on my third since I bought one in 2017.