Stoplight Switch adjustment??

[tommac95]

There is discussion , in forums and the manual , of adjustment of the pedal/shaft-mounted brake light switch (which also signals the PCM when the brake has been applied). From Helm Drive/Emiss(Dies, C/K '95) pg3-7: "The TCC normally closed brake switch supplies a B+ ... PCM. ~ The cruise control normally open brake switch supplies a B+ signal to the PCM when the brake is applied. These signals are used by the PCM to control transmission and cruise control functions. The brake switches are part of the stoplamp switch."
And (page 10A-6, D&E,Dies): "The PCM uses this signal {brake pedal applied--OPEN} to de-energize the TC solenoid when brake pedal is applied".
Also, on pg 3-18 , same book: "A scan tool will display the status of the brake switches . ~ A malfunction in the brake switches circuit should set a DTC 37, 38, or 41." [I couldn't find listing for 37/38 in diesel].

On my '95 i see no provision for adjustment ... there is a clip (on the switch housing) that grabs the MasterCylinder pushrod , and the switch has a hole which fits/slides over the same pin that the MasterCylinder pushrod rides on.

I've replaced the switch to address a "No Brake Lights, while Signals/Flashers OK" problem, to no effect.(???) Visibility is extremely limited!! Any hints on the adjustment procedure?
* * *
In the Helm manualset , there are troubleshooting charts on 3-76/3-77 for DTC 41 , BrakeSwitchCircuitFault , and also on 5E1-50/5E1-51 for DTC 81, BrakeSwitchCircuitShorted(or)Open. There's a figure illustrating the brake switch circuit on page 3-76, '95 CK Drive/Emissions.

The Brake Switch circuit fault diagram on pg3-76 Helm references "CHMSL". What is "CHMSL", or "CHMSL Relay"?

Thanks, T m.

[DmaxMaverick]

CHMSL = Center High Mounted Stop Lamp

One possibility is the Hazard switch. The brake circuit goes through it, and your problem is common when they fail. Sometimes, the switch will fail, but the flashers still work.

[Hubert]

Not much help on brake switch but

CHMSL is center high mounted stop lamp. The 3rd brake light at top of cab w/ cargo light if equipped.

[JohnC]

There is no provision for adjustment.

[tommac95]

Thanks for all the advice.

John-Having just removed, examined, re-installed, i agree. The mechanical (switch-activating) pushrod is moving nicely.

DMax--
I was directed toward the stoplight switch by the 'electro/mechanical' liklihood of failure at 196KMi , and the fact that the cruise control had become intermittent ... when i read that the brake switch also had contacts
signalling to PCM for cruise and TCC status.
I've started by replacement of the stoplight switch (cheap too!). I'll check into the hazard switch....

[tommac95]

I note that at ['95 C/K Truck, Driveability and Emissions...]
8A-44-0 , "AntilockBrakes" , wiring diagram references "TCC/Stoplight andCruiseSwitch". At 8A-34-2 , "CruiseControl(DieselEngines)" ref is to "TCC/StoplampSwitch".

In view of DMax's advice , the best wiring diagram is at 8A-110-4 , "ExteriorLamps:Stoplamps-Suburban/Utility" .

A dedicated section for StoplampTroubleshooting is at 8A-110-12.

A nice component location view of the 4L80E + TransferCase (diesel) , with OutputSpeedSensor and TransferCaseHarnessConnector , is found at
8a-201-21 .

I need some tuner cleaner.

[Warren96]

Try WD-40 its cheaper,and more available.

[tommac95]

Thanks, Warren
I use WD-40 for non-critical mechanical lube/rust-prevention for short term.

I prefer silicone spraylube , silicone-dielectric-grease , TV-Tuner-Cleaner[RadioShack's is comprised of: a)mineral-oil-lube, + b)chemical-cleaner-degreaser, + c)silicone-spray-lube] for most electrical contact work , with proprietary electrical cleaners for some electronics , and DW Electrochemicals Stabilant22a for critical applications (my PC , and the 6.5L's engine control wiring harness !). The issue is time to apply , and time between intervals.

I gained the impression, using WD-40 , that it was hydroscopic , and susceptible to saturation , like standard brake fluid. On Long Island , some areas have enough moisture to saturate exposed surfaces before long. Our atmospheric salt spray (and rain when the clouds come off the ocean) coat exposed surfaces with salt that continually absorbs atmospheric moiusture.