Center High Mounted Stop Lamp Problem

**Rob** · 10-09-11, 09:13 AM

So I brought my ZR-1 in for a state inspection yesterday and they noticed that the CHMSL was not working.

They ran some electrical tests and determined that the CHMSL switch is working, but the entire CHMSL assembly was not and needs to be replaced. They didn't have any in stock, and ran a search finding that there were only two down in the State of Mass. (I assume they did a local search).

I have my doubts that it is not the switch, however, how often does the CHMSL assembly go south? Is this a common issue that other ZR-1 owners have encountered over the years?

**Hib Halverson** · 10-13-11, 01:41 PM

"CHMSL switch"
WTF?!

There is no "CHMSL switch". On a 90 (all, not just ZR1) the center high mount stop light is turned on by the brake light switch. But...the brake switch has separate outputs for the brake lights and the CHMSL so it's possible for a faulty switch to turn on the brake lights but not the CHMSL. That would be easy to check with a test light. Connect the light to ground, touch the probe to pin C of the switch then push the brake pedal. If the test light comes on, the switch will turn on the CHMSL.

First, I'd check the bulbs in the CHMSL.

If they're all good, then I'd be looking at a wiring and connection problem in circuit 20 between pin C on the switch and the CHMSL or circuit 150 between the CHMSL and ground.

The FSM will have instructions on how to check the bulbs or remove the unit. Section 8A of the 90 FSM (that year it was a separate volume) has a wiring diagram and a component locator.

**jrd1990zr1** · 10-15-11, 09:18 AM

First check the fuse panel for a blown fuse.

Common problem on a C-4 is the brake lite switch has moved back in the mount. Pull the driver hush panel and check to see if it has moved. I've got 3 C-4's had to fix the switch on two.

Fixed mine with 1 wrap of electrical tape to make it seat in the bracket more securely. Looks like when you release the brake petal it pushes the switch back in the bracket, eventially the petal does not make contact.

It could also be the brake light switch itself which controls both the CMHL stop light and the brake lights (2 circuits)
BTW this part is widely available and not ZR-1 specific. GM part 25529860. Also ACDelco #D852A. Last one I bought was about $10.00. Corvette Central and Ecklers also carry them if memory serves.