M60B40 5k Rev Limiter?

[GB / John]

Was testing the car, and it suddenly starting hitting the rev limiter at around 5k revs. Couldn't find any problems, so disconnected the negative battery lead for 5 mins, reconnected and car ran faultlessly, rev limit back upto 7k revs.

Anyone know what could have caused it, don't want it happening during a competition in a few weeks. I'm thinking just a freak ecu/dme freak out and a reset sorted it? I have a spare ecu from a 530 if it ever happens again.

[JGood]

Was testing the car, and it suddenly starting hitting the rev limiter at around 5k revs. Couldn't find any problems, so disconnected the negative battery lead for 5 mins, reconnected and car ran faultlessly, rev limit back upto 7k revs.

Anyone know what could have caused it, don't want it happening during a competition in a few weeks. I'm thinking just a freak ecu/dme freak out and a reset sorted it? I have a spare ecu from a 530 if it ever happens again.

I think BruceBe was having that issue on his m60 e30 race car, it was an issue with the DME not getting a speed signal, since he wasn't using the stock cluster that provides the VSS to the DME. It just cuts out at the RPM that would equal the top speed limiter in 5th gear in an e34 (5k RPM's). Since it doesn't know what gear you're in, it does it in every gear.

Was it definitely a hard limiter, or did it just lose power? Do you have any fault codes?

[GB / John]

Def a limiter, rev counter was bouncing around 5k. Its the 1st time its done it, done 2 full drift days, and 5 couple hour test sessions all being ok. Was doing it when stationary aswell, when just revved to 5k. Seems strange that a reset fixed it, if it was speed limiter it would of done it during all other days. My ECU is chipped, and pretty sure speed limiter was lifted to 7k aswell.

[JGood]

Def a limiter, rev counter was bouncing around 5k. Its the 1st time its done it, done 2 full drift days, and 5 couple hour test sessions all being ok. Was doing it when stationary aswell, when just revved to 5k. Seems strange that a reset fixed it, if it was speed limiter it would of done it during all other days. My ECU is chipped, and pretty sure speed limiter was lifted to 7k aswell.

Maybe you have an intermittently bad connection on the VSS. I'd check continuity on that across the x20, and wiggle the wires as you're checking. You should have a code thrown for VSS as well if that's an issue, so check the codes. Actually, anything that would cause the engine to rev limit at 5k should throw some sort of code.

Maybe your cluster is doing something flaky with the VSS output as well, or even the input to it from the diff. Is the speedo doing anything funny when this is happening?

[GB / John]

Thanks for the replies.

Speedo acts fine, but on idle rev counter should be 700 ish, it often drops to zero and only moves when revved. Battery out of the 540 goes flat quite quickly (assuming its a shit battery and needs replacing), I replaced that on the morning with a smaller battery which worked, but rev counter was same on idle. I'll check the wiring, incase anything has been pulled loose.

[JGood]

Thanks for the replies.

Speedo acts fine, but on idle rev counter should be 700 ish, it often drops to zero and only moves when revved. Battery out of the 540 goes flat quite quickly (assuming its a shit battery and needs replacing), I replaced that on the morning with a smaller battery which worked, but rev counter was same on idle. I'll check the wiring, incase anything has been pulled loose.

Ahh, I bet it's the crank position sensor going bad. You could check resistance to start, IIRC it's supposed to be 1080 ohms, but even if it is, I'd still consider it suspect.

[GB / John]

Would that limit the revs, thought it would stall if that was on its way out?

[JGood]

Would that limit the revs, thought it would stall if that was on its way out?

Could cause all sorts of weird issues. When I first did my m60 swap a few years ago, it started the first 2-3 times just fine, but after that, would require 20-30 seconds of cranking, sometimes more, sometimes it would fire right up. That was the only symptom, once it was started and running, there were no issues. Replaced the CPS, all has been well since.

I've heard of CPS's causing issues at higher RPM's, causing cutting out, which would feel like a rev limiter. Can't say I've ever heard of one causing the tach to go dead and not cause a drivability issue at the same time though, that does seem strange. Maybe a wiring issue.

Shitty batteries can also cause some very strange electrical issues, but if it's doing the same exact thing with both of your batteries, I doubt that's the problem.

[GB / John]

Thanks, I'll look into it, might just change it for peace of mind and see what happens.

[GB / John]

Changed the crank sensor, and put in a new battery. All seems ok.