5500 rpm Redline on M52 Swap

[DiamondschwarzE30]

Finally got the tachometer working on my M52B28 (OBD2) swapped 1991 318is. Noticed that my redline seems to be set at 5500 rpm. It feels like it cuts fuel at that speed consistently, both at WOT and partial throttle. Not throwing any relevant codes outside the deleted SAP, rear O2 sensors, and evap valve (all are unplugged). ASC has also been deleted.

I've heard this could possibly be the ECU running in a limp mode, would any of these systems unplugged cause this? Seems to run great otherwise, no misfires, idles smooth, and pulls hard until it reaches 5500 rpm.

[Northern]

No VSS limiter is 5248rpm, so I'd say check that your VSS is connected.

[DiamondschwarzE30]

No VSS limiter is 5248rpm, so I'd say check that your VSS is connected.

How can I check that? Speedometer works correctly, not sure how that signal integrates with the M52 DME.

[AndrewBird]

Have you double checked the tach is ready accurately? Did you swap the coding plug out in the cluster for a 6-cylinder one?

[moatilliatta]

You can get a tune to delete the need for VSS input also..

[Northern]

I don't know what the late 318is pinout is, but here's your OBD2 pinout showing it needs to go to the DME via pin 9 on X20. The signal comes from the cluster, and should be in C101, maybe pin 14 with a BLK/WHT wire according to a few threads?





^ Pic by Kadunza. More of these on their Facebook page, almost at the very beginning of their pictures.

[DiamondschwarzE30]

Have you double checked the tach is ready accurately? Did you swap the coding plug out in the cluster for a 6-cylinder one?

Tach should be reading out correctly. I swapped in a 6 cyl plug out of a 325i and it appears to be accurate.

[DiamondschwarzE30]

I don't know what the late 318is pinout is, but here's your OBD2 pinout showing it needs to go to the DME via pin 9 on X20. The signal comes from the cluster, and should be in C101, maybe pin 14 with a BLK/WHT wire according to a few threads?





^ Pic by Kadunza. More of these on their Facebook page, almost at the very beginning of their pictures.

I'll check out the wiring tonight. I did not do the wiring on this car (bought it already swapped, but not complete). I do remember seeing a disconnected brown wire on the E36 harness, that could possibly be VSS input just not wired in.

[DiamondschwarzE30]

So I found something. The brown/red signal input wire on the E36 harness was not wired to the C101 black/white speed signal by the previous owner. Unfortunately, he cut the wires on the C101 plug super short AND cut the unused pins off inside the plug, so I can't wire it in until I fix the pins in that plug or find a new one. Overall, I am not super pleased with the wiring that was done on this swap. For a new plug, Im considering just wiring my own 30 pin Deutsch cannon plug instead of trying to source the engine side of a C101.

Seriously considering finding another M52 harness and removing the extra plugs I don't need (SAP, rear O2, etc.) and just starting from scratch. Keep my current hacked up one as a backup. I'm a little picky about wiring...

[pazi88]

You could just flash the vss error away from your ecu. It's easy to do by yourself with 5usd obd2 cable.

[DiamondschwarzE30]

You could just flash the vss error away from your ecu. It's easy to do by yourself with 5usd obd2 cable.

Where do I get started with that? Any recommendations for software/cables?

[Northern]

Assuming you don't have an aftermarket tune on your ECU, you should need:

• INPA K+DCAN cable (OBD2 to USB)
• Wired up OBD2 port (only 4 wires: 12v power, 2 ground, one Transmit/Receive. Relatively easy to wire if you have the port from another car) OR you can use a BMW 20 pin(ADS) to OBD2 adapter.
• Romraider software (free) + current definitions + a week of reading that forum to understand what to do.
• Flashing software ( https://sites.google.com/site/openms...s41-flash-tool is most popular, and $25)

Expect a day of messing around with COM port settings, installing drivers, and getting things set up. Once you do it all once, it's pretty easy.

If all you want is to get rid of the VSS error, it's less work to just wire it up somehow, even if you just run a new wire around C101/X20 for now.

[bimr84]

Pm’d sir

[ForcedFirebird]

You can always temporarily jump the tach signal to the VSS signal and "fake" the input. As long as the ECU "thinks" the vehicle is moving, it won't limit it to 5500. Seen this out at the track with endurance car swaps - also eliminates the speed limiter.

[DiamondschwarzE30]

You can always temporarily jump the tach signal to the VSS signal and "fake" the input. As long as the ECU "thinks" the vehicle is moving, it won't limit it to 5500. Seen this out at the track with endurance car swaps - also eliminates the speed limiter.

Temporarily wired up the VSS signal to the ECU and it worked like a charm! Got my 6500 rpm redline back. Going to figure out a permanent solution soon.

Thank you everyone for the help!