ZR WON

Hey all!

I realize this is also a 'general' C4 / 1990 issue, but figured one or more of you have encountered this recurring issue over the years. Honestly, it's happened to me a handful of times over the last 18 years of ownership, but I've always been able to simply clean the frame rail, coat the bolts with dielectric grease and it would go away for years at a time. Over the last 12 month, it has come on pretty much every single time I start / move the car. Sadly, that's only been about four times, but it is getting ponderous to take things apart and then go through that terrible procedure to clear the code (although I've gotten relatively good at it ). Is it always the same code (26), for the passenger side sensor, would it just make sense to replace the entire sensor?

At this point I have to think something is wrong besides corrosion / bad ground. I simply don't drive it enough for rust to build up in that area (due to AC usage). I shudder at the thought of it actually being the DERM. My understanding is the seatbelt light will not go off (ever) if the DERM is bad, but who knows, these cars are getting pretty old. I can get it to reset no issue (did it again a few minutes ago, stayed off when cycling the key off and on a few times), only to come on the next time I start it after it cools off(can be many months, battery is unhooked during that time), I don't think that would be a DERM issue? I guess the next big question would be, where is the best source for a pass side sensor? These days I typically order things through Rock Auto, but either they don't have it or I'm not searching with the right keywords. Since this is a 'generic' 1990 Corvette part, would think they are relatively plentiful, but even eBay searches seem to only come up with 91-96s. Honestly, I thought they were all the same.

Brian A.
90 ZR-1
ZR WON
12.09@118
01 Z06
not quite stock

Goldcylon

In a lot of ways the 90 ZR-1 is a one year only due to changes in the car starting with the 91 model year. The DERM and the sensors are a one year only as well. However the good news is the part or parts you need are the same as a base C4 MY 90. So keep looking its time to replace that sensor. GC

ZR WON

GC:

Thanks for the reply. It looks like the GM part number is:

10094613

Big question, where might a person find one of these? It's of course been long discontinued from GM, nothing on Rock Auto, a few Google hits but every site that has one listed says 'no longer available.' I wonder if the moderator could move this to the General C4 section? If not, I can repost there.

Brian A.
90 ZR-1
12.09@118
ZR WON

01 Z06
not quite stock

Goldcylon

Hi Brian NLA as you have figured out. Unless EBay pans out you will need to keep your eyes peeled for a part out of a base 1990. While you are there I would grab the DERM and the Airbag as well if it was not involved in a Airbag deployment event. GC

DRM500RUBYZR-1

Code 26 is as you very well know from past experience, a sensor issue.

Double check to make sure your current code is not a different one.

If the code is a 26, replacing the DERM, when you find one, may not make a bit of difference.

Did you perform the resistance checks ( 4 of them ) and did the values fall within the required range?

Did you "wiggle" all of the wires during the checks?

Codes above 26 involve the DERM or wiring in and out of it.

While 26, if all else fails, could be a DERM issue, that would be a low probability, compared to the sensor and or it's wiring.

Good Luck, don't give up, you WILL get it squared away.

Marty

ZR WON

Marty, GC:

Thanks for the comments. GC, no airbag event, this car has never been wrecked. Marty, yes, it is, and always has been code 26, which I've always associated with a sensor issue, almost certainly not the DERM. The next time it comes on (likely when I start it the next time ), I will unhook the battery for an hour (just to be safe), disconnect the sensor wires in the passenger footwell and do the resistance check. My gut tells me this is a sensor problem regardless, but the resistance check makes sense to be certain.

Brian A.
90 ZR-1
ZR WON
12.09@118
01 Z06
not quite stock

dmaxx3500

do you have pics of the sensor,it could fit other gm vehicles and have different part numbers,,

billschroeder5842

Sorry this one is the bane of all ZR1 owners. PM Sent.

Goldcylon

Brian you misunderstood what I telling you. Keep your eyes open for a base C4 1990 part out. If that part out car was not involved in a air bag deployment wreck the suggestion was to buy the sensors, the airbag and the DERM while you are there as well. Why because they are ALL MY90 parts only. GC

Goldcylon

Bill not really this is a MY 90 specific problem only. 9/10 times it's a coroded frame under the sensor where the sensor mates that causes the infl error. Normally a good cleaning and a healthy dose of dielectric grease resolves the prob. However the OP is familiar with that procedure and now has a suspect sensor. GC

lrj975

after the ohms check on the sensor passes then back to addressing the ground. Forget the derm. Follow the guidance in the TSB 100% not 90 or 95 especially

-apply a thin layer of special grease P/N 12345579 to frame rail mounting surface and mounting flange of the sensor. Do not apply the grease to the mounting bolts or holes.

-external lock washers, P/N 11501869 (8mm) and 121753 (6mm) to mounting bolts. Ensure that washers are between bolt head and sensor mounting flange, not between mounting flange and frame rail.

Clean surfaces, clean the bolts, grease it as directed, this isn't the place to slop it all over, use the external tooth washers, even use the GM part numbers as much as pains me to even say that, is the special GM grease 3x price same as permatex, and is a GM washer at $3.95 same as generics 1000 for 99cents? who knows but that's the hobby and 100% best effort.

Scrrem

I'm fighting this battle myself with an error on my 90. I have tested the driver sensor and tested good but after cleaning the frame area decided to replace the sensor anyway, not joy! The DERM was the last component and I found a used one on Ebay a while back and have yet to install it. The job is not for the faint of heart because of where it's located in the dash. I may tackle the job in the fall.
Rich

Goldcylon

Yep the DERM is buried low below the radio. No joy

WVZR-1

I don't have a '90 DERM here any longer but I believe all of the connections to a '90 DERM were on "pig-tails". I believe with an "unknown" you could actually accomplish a "TEST" before install. '91 and later I'm quite sure that's NOT possible but for a '90 I'm quite sure you could.

You should be able to confirm that (pig-tails) from your recent purchase.

Scrrem

Dave, Yes you are correct the 90 DERM has pigtails. Although to get a full functional test, the DERM would have to be tied into the factor wiring harness...right? I need to pull out the spare and check the connectors. Is there a procedure for testing the DERM in the FSM?
Rich

WVZR-1

All of the "pig-tails" should be very accessible without removing the DERM that's in the car. You plug in the "recent purchase" and it either works or NOT. Check your codes 1st that are stored in the original, after connecting the "more recent" check codes for it. If the "more recent" DERM displayed NO CODES when connected and the DIC display did as it should @ "KEY ON" you would think job done and now you disassemble the dash/console.

The FSM has diagnostics for specific codes and "YES" it could be assumed that the first section "SIR DIAGNOSTICS SYSTEM CHECK" would indicate a potential DERM" issue.

My thoughts on a nearly 30 year old SIR system is - what damage is that Inflator going to do if it does actually still work? I've spoken to people who had deployments with earlier GM SIR and most weren't pleasant ones. Never with a C4 owner though. An NCRS car I believe you go through the effort. A non NCRS car I believe you think HARD about a repair. A resistor will turn off the DIC and belt lights. Actually I believe a resistor placed well would be undetectable visually and the car would do NCRS easily!