Brake Booster light on

Andrew Technician Milwaukee, Wisconsin Posted   Latest   Edited  
Question
Heavy Duty
2012 Isuzu NPR 6.0L (G L96)

Hopefully someone can shed some light on this for me. Against my advise, out service writer decided to take in a 2012 NPR for a steering concern. Not too complicated, needed kingpins as they were seized and causing lack of return. I upsold front brakes because they only had 20% left and I had a lot of it apart anyways. After a lot of smoke and swearing I got the job done and went to pull it out only to find the “Brake Booster“ light on the IPC when I depressed the pedal. This one uses a mechanical vacuum pump and booster. From what limited information I have (alldata only shows an electric pump schematic) I’ve found that the only way that light can come on is from low vacuum pressure. I’ve used a DVOM to confirm that when the switch grounds the light comes on and that seems to occur at 10in hg . If the vehicle idles it stays around 20-25 in Hg until the brake pedal is pressed 2-3 times then goes to 10-5 in Hg And the light comes on. I’ve tried a pump as it seems to be a volume issue. I smoke tested the system and vacuum resivoir to look for leaks and found none. I’m thinking maybe a switch or booster, but I’m not sure what the spec for the switch is as far as when it is supposed to trigger the light. Thanks in advance for any input. 

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Michael Mechanic
Montgomery, Texas
Michael Default
 

If there was no booster light before the brake work was done... you wouldn't think installing new brake pads is then going to cause a booster light to begin showing a fault. You are definite about the lite is booster light. When you pump brakes and lite comes on, if you hold brake pedal will lite go out? Does vacuum increase while holding? Put high idle on and try... does lite still come on?

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Andrew Technician
Milwaukee, Wisconsin
Andrew Default
 

As far as I know it wasn’t on before, but it’s my problem now. Im always skeptical and critical of my own work, but I really don’t think it’s something I did. Im positive it’s the brake booster light because it actually says “brake booster” and the brake and ABS lights are different and functional during key on bulb check. If I hold the pedal down at idle it stays on with low vacuum. If I bring…

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Michael Mechanic
Montgomery, Texas
Michael Default
 

Does the pedal stroke feel proper or too close to floor?

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Christopher Technician
Saskatoon, Saskatchewan
Christopher Default
 

Don't have a lot of experience with the gasoline version of the NPR chassis so this may not apply but I have come across similiar concerns on the diesel version of this chassis where moisture buildup in the reservoir has caused a lack of volume availability, which I have found is minimal to begin with even when all working correctly so it can seem quite dramatic as far as brake applications…

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David Mechanic
Stockport, United Kingdom
David Default
 

Hi Andrew, do the brakes function ok on the road ? Or does the pedal go hard after a couple of applications ? How many miles has the vehicle done ? Did you full stroke the pedal at any time ? I have just had bad experience after changing rear brake cylinders and manually bleeding the brakes . Simple job that went ok till I drove the vehicle, one press of the brakes and no boost on the second…

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Andrew Technician
Milwaukee, Wisconsin
Andrew Default
 

Thank you guys for the input thus far. I did pump the pedal after I did the brakes, maybe a bit further than it’s normal travel but not to the floor. I suppose its possible that damaged it. Ive always made a habit of not putting the pedal to the floor as to avoid that. The pedal has normal travel now. I have not driven it on the road but the brakes hold and the pedal is normal. Christopher, is…

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Christopher Technician
Saskatoon, Saskatchewan
Christopher Default
 

Personally I try not to condemn anything specific without proof of failure, especially in circumstances where you cant be certain how the vehicle operated like prior to the repairs your shop performed. Example does the vehicle operate normal in normal driving conditions , and the warning come up only during what would be considered panic stop pumping the brake actions? I have found in the diesel…

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David Mechanic
Stockport, United Kingdom
David Default
 

Andrew , does the booster hold vacuum with the engine off ? Not being familiar with this vehicle is there a one way valve at the vacuum inlet at the booster or is it in the vacuum tank ? If you can isolate the booster and still retain vacuum in the booster then with engine off press and hold the pedal. If the reserve of vacuum in the booster it’s self is all used up with one press the that’s the…

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Jim Technician
Dover, New Jersey
Jim Default
   

The vacuum pump is on the back of the alternator. Rubber line to steel on the frame. It goes around the front of the frame. Im certain yours is like all of ours, and is a super rust bucket. Well the vacuum line on the frame likely rusted through. I'd look closely for something like that Edit: nevermind. I see you wrote 6.0. no vacuum pump on that. Id still look for a rusted through vacuum line…

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Rex Technical Support Specialist
Alliance, Ohio
Rex Default
 

Worked on a lot of vacuum assisted systems many years ago. The ones with a reservoir always had a check valve lurking somewhere as others mentioned. Gas or diesel. Try this link to ISUZU SI. isuzutruckservice​.​com/terms​.​php $15.00 for 72 hours looks pretty reasonable. The link is off the NASTF website. nastf​.​org/i4a/pages/inde… Good SI links there…

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Tommy Owner/Technician
Holly Ridge, North Carolina
Tommy Default
 

Andrew, Did you have someone push the brake pedal during the smoke testing?

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Andrew Technician
Milwaukee, Wisconsin
Andrew Default
 

Well, I think it’s all good now. I think the pump did help, it just didn’t change it as much as I wanted it to. Christopher has mentioned that on the diesel trucks it’s not that great to begin with, and I thing the gas version is similar. With the old pump the light would come on after 2 consecutive pedal pumps at idle. With the new vacuum pump it takes 6. I smoke tested again, bypassed the…

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Andrew Technician
Milwaukee, Wisconsin
Andrew Resolution
 

Well, I think it’s all good now. I think the pump did help, it just didn’t change it as much as I wanted it to. Christopher has mentioned that on the diesel trucks it’s not that great to begin with, and I thing the gas version is similar. With the old pump the light would come on after 2 consecutive pedal pumps at idle. With the new vacuum pump it takes 6. I smoke tested again, bypassed the…

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Michael Mechanic
Montgomery, Texas
Michael Default
 

Glad to hear.👍 Thought earlier you had said you already replaced the pump. Stay safe.

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Andrew Technician
Milwaukee, Wisconsin
Andrew Default
 

Yes, I retested it with the old pump then put the new pump BACK in. Repeated tests to compare and be sure.

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