One of my all time favorite motherboards.
Very fast, if not the fastest 486 VLB graphics.

Check if level 2 cache is not disabled from the BIOS.
Lower base frequency to 33MHz to minimize problems from flaky components and/or SRAM chips. Somewhat unlikely this is the case, but still.
Inspect PCB for damaged traces or components - happens often with old hardware.
High-chance some of the cache chips are broken - happens very often. If none of the above turns positive, i would buy another 10 cache chips and hope for the best.

IS61C256AH, 10/12/15 ns
W24257AK, 15ns
UM61256FK, 15ns

Thanks! I do like the features of the board and I'd like to realize it's full potential. I double-checked the BIOS and the L2 cache is enabled. The startup information screen even shows 256KB of L2 cache. I don't overclock so the base clock is at 33MHz for the DX4/100 I have installed. I checked the board for broken traces when I built it a few months ago, but its possible I missed some. I can check again.

Thanks again for your input!

I double-checked the traces on the motherboard. The motherboard is clean with no damage as best I can tell. It almost looks mint. The cache chips I bought off eBay are labeled W24M512AK-15. Not sure why this cache chip has an M in the model but the ones pshipkov suggested do not have an M. Are these lower power or otherwise not intended for this system?

many 486 boards have those timebomb barrel batteries that will leak and damage traces. and also, there are tantalum or electrolytic capacitors that can also have issues, so recap the board if possible, and remove the barrel battery if it hasn't leaked, if your 486 board has one.

Did you also make the jumper adjustments for the new L2 cache size as written in the manual (page 4-5)?

Thanks for the suggestions, Winodws9566 and bloodem. I made sure this board is using a coin battery, so no leakage from there. And yeah, I made the proper adjustments to the jumpers. The startup info screen even displays the 512KB of L2 cache, but SpeedSys and Cachechk are still showing no L2 cache in their tests.

OzzFan wrote on 2020-12-11, 21:02:

Thanks for the suggestions, Winodws9566 and bloodem. I made sure this board is using a coin battery, so no leakage from there. And yeah, I made the proper adjustments to the jumpers. The startup info screen even displays the 512KB of L2 cache, but SpeedSys and Cachechk are still showing no L2 cache in their tests.

Consider that maybe your RAM so so fast that the cache is unable to be so much faster than the RAM to be detected by those tools. Try to slow down the RAM temporarily (by adding RAM wait states in the BIOS setup). If that makes the cache detected, it is working fine, even with RAM at fast settings. Also check that the cache timing is not way too slow. At 33MHz with 2 banks of cache, any respectable motherboard should be able to run the cache at maximum speed, that is 2-1-1-1 timing.

Thanks for the suggestion! I went ahead and set the RAM to the slowest speed, inserted wait states and set the command rate to 2T. SpeedSys shows my memory bandwidth reduced from 81MB/s to 40MB/s. I believe these are 60ns SIMMs. Unfortunately, SpeedSys and CacheChk are still showing no L2 cache. I noticed that SpeedSys indicated my L1 cache was write-through even though I have it set to Write-Back in the BIOS (and the Am486 chip is capable of wb).

Have you tried running some benchmarks like 3DBench 1.0c (with and without L2 'external' cache enabled in the BIOS) and see if there's any difference?
Also, did you run any benchmarks prior to adding the extra 256 KB of cache? Technically, you should have seen a small difference after increasing the cache to 512 KB.

I should have benchmarked the system before I installed the cache upgrade. That would have been smart. I will have to try the benchmarking when I get a chance later this week. Unfortunately there is no option in the BIOS to disable the L2 cache so I have to do it through the jumpers. Thanks once again!

You're welcome, keep us posted 😀