Car is a stock 1988 L98 GTA, with a 9-bolt Posi.


For about a month now when car is thoroughly warmed up (like entire car is heat soaked), the driver side rear-wheel will catch, and make banging noises, on low speed (30MPH and under) left hand turns. This does not happen at higher speeds. This does not happen when car is cold, or even a little warm. I have to drive the car long enough for both fans to be on, for oil press to drop into the 25-30 range at idle (which takes usually 10-20 min of driving with OAT of 90 F).


First I thought it was the back brakes so I removed the wheels, and checked them. No leaks, brakes where good PLUS there is no difference when I pump and release the brakes during the banging/catching.


Second I thought it could be a tranny issue since it only occurs warm. Did a lot of slow speed driving in 1 and 2 and noticed the issue would occur in either gear. I can't really get the car to go into 3rd below 30 MPH so I ruled the tranny as a possible but unlikely.


Third I'm thinking it's some problem with the diff since it only occurs when the car is as hot as it's going to get, and only occurs on left hand turns under 30mph. It can be a sweeping left hand turn, or a sharp left hand turn. Does not happen at highway speeds. I've tried sharp lane changes, sweeping left hand turns at 65+ where body is leaning to right with the popping/banging not occurring.


Ruled out bearings cause this does not occur at all speeds but I could be missing something here.


Last time I worked on a rear-end was my 85 Z28 where I drained the fluid and changed some dollar sized washers (forgot the nomenclature for the actual part) on the smaller sized gears in the rear. So I'm not really familiar with the 9-bolt Posi...yet.


Do these symptoms sound like a diff problem?

 

Ah yes the Auburn cone type posi failure mode. JUNK!

For whatever reason, it's always the drivers side cone that's trashed. Anyway they aren't fixable in any meaningful or lasting way.

You need one of these:

https://www.thirdgen.org/forums/tran...installed.html

Here's my follow on post:

https://www.thirdgen.org/forums/tran...rg-warner.html

GD

 

Hmm... excellent write up. That's funny because I planned to move to a shorter gear once I put the 6-speed in. I was thinking 4.10's but it looks like the only thing available for the rear is 3.70 or 3.90... and yes I will be staying with the TPI longterm. Plan is 450HP, once all the gremlins are gone in this car and she has a nice coat of paint on her.


Thanks GD. I need to do some reading on how the system works to determine if this is a "must fix now" or whether I can wait 500-1000 miles before I fix it.

 

This isn't an Auburn, it's a Borg Warner. Same cones, but the Borg Warner can be reconditioned. I would recommend an oil change and a bottle of GM or Ford limited slip oil additive. This usually fixes low speed turning popping problems with the differential. Sometimes a second bottle of the additive is needed, especially with the cone type differential. If you want to upgrade to a better differential then do it when you change gears. For now the oil change should fix it.

 

You *can* get 4.10's or similar. As noted that stuff has to come from Australia. I'm sure if you contact Dave at Supa-F he could set you up with some gears of your choice. Really they are a good rear end. Anything that fits a Holden Commodore VL will work in our diff.

That said, if you are going to 450 HP, you won't need 410's. It seems like a good idea now, but with that much power and the torque that goes with it, you won't need that low of gearing.

GD

 

Copy that. That takes care of my concern. I can do the short term fix now until I decide to do a gear change. Ideally I wanted to do it AFTER the T-56 swap, but that keeps on sliding right because of the new issues I keep finding. For example I'm currently battling a code 36 after replacing a MAF sensor, and I just got my headlight motor bearing kit in from TopDown that I'll be doing this weekend.


Seems like I'll never get around to painting this bad boy with all these problems.

 

Sorry, I mean "Auburn cone type". Yes the diff was made by Borg Warner, who bought the BTR facility in Australia. Borg Warner, or at least that facility, is now owned by Dana.

When you change the oil - if it looks like silver paint, then the diff is shot. I tried all manner of additives and different gear oils. Didn't help for me.

GD

 

For the code 36 - when and how did that come into play? After *only* swapping the sensor? Why was the sensor swapped?

GD

 

Well 450 is the goal. Whether I reach it...
Mind you I plan on doing this N/A, stock displacement, with a high duration/lift cam that is properly matched to a good set of aftermarket heads, and all the supporting intake/exhaust modifications. Basically I want to build a high revving small block similar to how my old cam/bolt-on 99 Trans Am. Just for the "see if I can do it with a TPI" factor.

 

https://www.thirdgen.org/forums/tpi/...5-33-36-a.html


Here is a link to another thread explaining my quest so far but to summarize old MAF died (stock original). I purchased a new Duralast replacement MAF and bolted in. Car runs fine with the exception of the Code 36. Found that the default circuit on the burn-off relay had high varying OHM reading where so I replaced it, and the MAF power relay. Still throwing A code (haven't jumpered it yet) but I suspect that didn't fix the code.


Read the thread if you're curious because I got some insight from Vader on the differences between the hot wire, and thin film type MAFs which is probably the root cause of my problem (the one I have is the thin film type).

 

This is just pushing up my timeline to buy a data logging cable and program for my computer to help me fix this issue. Was going to wait post paint/ t-56 so I can start learning to program in preparation for engine modifications but I really need to see what the computer is seeing to properly fix this problem.


Car drives great it, I just have a big orange light staring me right in the eye every time I drive the thing and that just wont cut it.

 

Curious because I had that code and turned out to be someone (maybe me) swapped the burn off and power relay plugs. The relays aren't interchangeable and there is quite a trail of service bulletins, frustration, scalp hair, tears, and sadness associated with those two relays. One thing I can tell you that took weeks to figure out and isn't printed in the manuals - the burn off sequence is only run every 6th time your start the engine. So it is only on that 6th start that the ECM will find fault with that circuit and throw this code. I had a month long nightmare before figuring out that foolishness.

GD

 

Every 6th time eh? How did you discover that and how did you fix the issue? this finally gives me an excuse to purchase a data logging and programing setup. Leaning towards figuring out to program it out. Before I do that I'm going to exchange my MAF for another to rule out the possibility of getting a MAF that wasn't wired properly to send out the burn off signal to the ECM.

 

So it does a burn off cycle every time you shut off the engine, but it takes 6 failures of the burn off cycle to set the code 36.

The ECM code checks to see that the MAF voltage is greater than 1.9v or less than 0.4v (engine off so zero airflow) and if it is either of those it considers the burn off cycle a FAILURE and increments a counter. When that counter reaches 6 it sets the code 36. If a burn off cycle is completed successfully the counter is reset. If you disconnect the battery, the counter is reset, and if you clear the code memory, the counter is reset.

Here's the microcode posted in post #7 of this thread:

https://www.thirdgen.org/forums/dfi-...ml#post4383052

In my case the problem was my power and burn off relays were swapped (or rather, their connectors were). The burn off relay has a specific resistor inside it that I believe alters the voltage being sent to the burn off circuit of the MAF. Thus using the wrong relay results in a MAF voltage output that is outside of the 0.4 to 1.9v window and causing the ECM to consider the burn of cycle a failure.

GD

 

Hmm... The 6th failures to trip a code 36 may be specific to the 86 year then because mine tripped to first time I shut the car down when I first replaced the MAF sensor. After I replaced both the MAF Burn-Off and MAF Power relays, reset the computer by disconnecting battery the code tripped as soon as I started the car back up.


Either way the link you posted is good info and it gives me a new path forward to trouble shoot. I need to see what the voltage reading the ECM is getting IOT find out if the new MAF sensor is sending the proper voltage the ECM is expecting to see. I think from there I can determine if this MAF sensor is sending the proper voltage to the ECM and determine a way forward to kill this light.


Probably going to have to data log and program because I really don't know if the MAF sensor I have now is sending the correct voltage readings for airflow to the ECM. The car runs but it may be "close enough" for it function, but not spot on with the old factory unit's parameters.


Thanks for the pointers.

 

Well I don't think it's specific to 86 since I'm running $6E (89) microcode on my 86 for cold start injector delete, 24 lb injectors, etc. Perhaps it's specific to 89 and that's why it's not mentioned in my 86 service manual.

But yeah I would *imagine* the 86 to 88 and 89 are looking for the same burn off voltage values from the MAF since they both use the same part number MAF. So at least you can scope that and see if it's in the right range. You may be able to tweak the burn off input voltage to compensate by either adding a resistor or routing it a new voltage source with appropriate resistor triggered with another relay using the existing burn off relay as a trigger only.

GD

 

So here is an update on my rear-end (giggity).


When I pulled the diff cover off a little less then 1 quart drained out in my drain pan. So I put a new gasket and refilled with synthetic diff. oil and put in 7 oz of oil additive. Put in 1.8 quarts of total fluid (per the Firebird manual).


After the car was warm the popping returned at first. Any speed under 30 mph with a slight turn left or right (I could slightly turn in my lane to elicit the pops when I was coming to a stop at a red light).


HOWEVER after turning into my home street (about 5 more miles of light driving) the popping/catching disappeared. So I proceeded to drive around my neighborhood at 15-25 MPH zigzagging down the street in my car with a bunch of u-turns thrown in for good measure. Problem gone. After that I did 2 full throttle launches from a dead stop up to 80-90 mph. I feathered the throttle a little on the launches to break the tires loose a little but not do a full on burnout. Then drove it around again slow around the neighborhood zig-sagging and doing a bunch of u-turns.


No popping, no catching. After a little bit of a drive around town today between some errands the rear-end didn't act up again either. I'm calling it fixed for now. Thanks for the help fellow thirdgen-ers!!

 

On a side note my car started to click and turn over slowly today on startup. So either the battery is bad (Year old Duralast battery) or my starter is bad. Battery read 12.76 volts with a digital voltmeter so I'm thinking starter. We'll see I suppose.

 

I'm glad it was an easy fix for the rear end. Don't overlook bad battery cables. I've seen that a few times.

 

I did kind of jump the gun and bought a new starter over lunch. I'm going to play around with it first before I install the starter though just to make sure I didn't waste some money.


I replaced the alternator about 2 months ago because it crapped out on the highway. Probably should of replaced the battery cables too.

 

Yep it was the starter. No more clicking and much easier starting now too. Man that starter was a PITA to get back in though.

 

The mini starters (gear reduction) are MUCH MUCH easier to fit. I got mine from Summit. I got a Powermaster one. Think it was about $145 or so. They are tiny and don't heat soak like the old stuff.

Rick

 

I got the duralast unit from AZ for $48 with the core charge. In hindsight I honestly don't know if the $100 extra could justify the trouble I could have saved myself with a min-starter. Now if this re-man starter fails in less then a year that would change the my whole cost/benefit equation.

 

I do a lot of starters and alternators at my shop on all makes and models - Duralast is the last brand I would consider selling a customer. I've had alternators, starters, and distributors fail right out of the box from them. I usually get my stuff from Mitsubishi, Denso, Bosch, etc. We buy mostly from Worldpac and IMC. Funny - IMC was bought by Autozone about a year or two ago so they *can* get good stuff if they want to look it up. I also buy from ATech motorsports which is Summit Racing's wholesale division. If you need anything feel free to PM me and I'll hook you up. I can have stuff drop shipped directly to you from my various suppliers, etc.

Rick

 

Ok will do!


Yeah the temp sensor I got from AZ is becoming suspect now with an intermittent code 15, however I put a 190 (maybe 195?) a couple of years ago because I live in a hot climate, and never told the ECM so my guess is that is a possible cause.


Since this one is intermittent (comes on for 4-5 minutes and goes away) I'm not inclined to mess with it yet.

 

That's a super easy change to make in the ROM. I put a 180 thermostat in and have my fans set for 190 on and 170 off.

GD

 

Yeah I really need to paint this thing before I dive into the DIY Prom world. Hopefully I can make a strong push and have the prep work done by mid June and painted by July so I can get back into the fun stuff.

 

Longshot08..
I just now read your entire post and had to chuckle...not at your problem, but what I went through about 25 years ago with my 88 GTA. I had the same issue. After driving a long distance or after it was warm. I would come up to a stop sign and would get this horrendous sound from my diff. Also while turning left/right.
I got to thinking about the donut I had done in the cul-de-sac the day before.( mail man pi**ed me off, another story)
So after a few days, I took it to a diff shop and told the guy my issue. He was a big burly of a man and he got in and drove it without saying a word. He went forward,backwards, figure 8s...you name it.
We made our way back to the shop, he got out and went inside and disappeared behind the counter. He came back and threw a tube of stuff at me.
He said "Add this and you'll be fine....that'll be 6 bucks."

Long story....I added it and it went away. Still runs fine, Just add every time you do a diff fluid change.