M52 swap coding plug for tachometer

[DiamondschwarzE30]

Hello everyone,

I recently swapped the coding plug on my '91 318is (M52 swap) with a 6 cyl one from a 325i thinking that would fix my tach reading too high. It seems more accurate now, but still reads about 1300 rpms too high cruising at 60 mph in 5th. Should be about 2800 rpm, tachometer about 4100 rpm instead.

Idle seems about 900 rpm, still too high.

Was reading somewhere that an e28 535i coding plug might work. Can anyone confirm, or is there something else wrong?

Thanks!

[nando]

Sounds like the needle is just not in the right position. a 6 cylinder coding plug is a 6 cylinder coding plug. Doesn't matter which 6 cylinder engine it is. Mostly it just needs to match the sweep of the tach itself (ie, eta vs 325i).

[DiamondschwarzE30]

Sounds like the needle is just not in the right position. a 6 cylinder coding plug is a 6 cylinder coding plug. Doesn't matter which 6 cylinder engine it is. Mostly it just needs to match the sweep of the tach itself (ie, eta vs 325i).

Any suggestions on where/what to start checking?

[Gregs///M]

If your tach is a 7k redline one, and you have the 325i coding plug, it should work perfect. If you have an 8k tach, you would need the E28 M5 one.

[DiamondschwarzE30]

If your tach is a 7k redline one, and you have the 325i coding plug, it should work perfect. If you have an 8k tach, you would need the E28 M5 one.

It should, since I still have the 7k tach. I’m confused to why it’s still off.

[DiamondschwarzE30]

I did notice that the needle will jump quick a few seconds after turning the engine off and removing the key. Only does it once, never has jumped with the engine on or the key in. Possibly something internally wrong with the tach? How can I check to be sure?

[mjweimer]

If your tach is a 7k redline one, and you have the 325i coding plug, it should work perfect. If you have an 8k tach, you would need the E28 M5 one.

This is interesting, I had not yet come across this information.


So, for example, if I had an E30 M3 gauge cluster with the 8K RPM tach, I would need to source an E28 M5 coding plug to make the tach accurate through the range?


My plan is to use an E30 M3 cluster with my M52 swap...




Thanks!




MJ

[DiamondschwarzE30]

Alright, so I think I figured it out. I think the 6 cyl 325i coding plug I received had its chip swapped with a 4 cyl one at some point prior. The plastic case had been opened before. Tachometer reads the same regardless of which chip is in it. Part number on the case is correct for a 6cyl one, but the chip in it appears to be a 4 cyl one.

Contacted the eBay seller and hes shipping me a new one. Hopefully that one is correct.

[Gregs///M]

This is interesting, I had not yet come across this information.


So, for example, if I had an E30 M3 gauge cluster with the 8K RPM tach, I would need to source an E28 M5 coding plug to make the tach accurate through the range?


My plan is to use an E30 M3 cluster with my M52 swap...




Thanks!




MJ

If you are using a 6 cyl engine, yes. Will the E28 chip fit into your E30 cluster? No, you will need to break open the housing of the plug and cut the circuit board to make it fit. We can perform this mod or sell you an aftermarket chip that will fit good.

[Gregs///M]

Alright, so I think I figured it out. I think the 6 cyl 325i coding plug I received had its chip swapped with a 4 cyl one at some point prior. The plastic case had been opened before. Tachometer reads the same regardless of which chip is in it. Part number on the case is correct for a 6cyl one, but the chip in it appears to be a 4 cyl one.

Contacted the eBay seller and hes shipping me a new one. Hopefully that one is correct.

That happens, best to use one with a case thats never opened. It should fix your problem. If not, it may be the placement of your needle that is affecting the accuracy.

[nrubenstein]

This is interesting, I had not yet come across this information.


So, for example, if I had an E30 M3 gauge cluster with the 8K RPM tach, I would need to source an E28 M5 coding plug to make the tach accurate through the range?


My plan is to use an E30 M3 cluster with my M52 swap...




Thanks!




MJ

You might want to just sell the E30 M3 cluster to pay for your M52 swap. :p

[mjweimer]

If you are using a 6 cyl engine, yes. Will the E28 chip fit into your E30 cluster? No, you will need to break open the housing of the plug and cut the circuit board to make it fit. We can perform this mod or sell you an aftermarket chip that will fit good.

Thank you. I will contact you when the time comes.

You might want to just sell the E30 M3 cluster to pay for your M52 swap. :p

Ha Ha. E30 M3 parts are valuable these days for sure, however I've had this cluster for years and it will be one of many cool details of my build.




MJ