How far out can one of these be before it starts causing problems? I checked these on our M30 and they were both about 1050 ohms (I think 960 +/- 10% is target) but once hot the speed reference was up to about 1200. How much is too much change? We have had random stumbling on this car that seems to occur when it is the hottest but then it usually comes right back to life, maybe like it is restarting on it's own? Not sure it is the speed sensor but think maybe it is possible especially if the resistance keeps going up with heat.
As a point of reference the sensor on our 91 M20 (single front like a SETA) has a spec of 540 but once the car was failing to start I got about 100k ohm. Believe it or not it still produced a very faint pulse (not enough for the ECM) and had still been randomly starting then running fine a week before I measured that and determined it was bad. The new one I purchased was 542 ohms btw.
Crank Sensor Resistance
Sorry for the dumb question, but is there any way to test voltage while it is installed? Crank the engine with a friend looking at the meter or something?
I ask because my ABS sensors were all reading roughly the same ohm reading(1k) but vastly different voltage(1x 1vAC 2x 0.5vAC and the bad one at 15m vAC), and they work along the same principal as the crank sensor IIRC(pulse generator/hall effect sensor)
I ask because my ABS sensors were all reading roughly the same ohm reading(1k) but vastly different voltage(1x 1vAC 2x 0.5vAC and the bad one at 15m vAC), and they work along the same principal as the crank sensor IIRC(pulse generator/hall effect sensor)
Yes you can see a/c voltage from them. You can either backprobe the terminals or unplug them and test directly at the terminals. The speed sensor will be about 10x that of the ref sensor.KegCoyote wrote:Sorry for the dumb question, but is there any way to test voltage while it is installed? Crank the engine with a friend looking at the meter or something?
I honestly do not know if there is any power supplied by the ECM (never probed it for voltage) or if all the signal is self generated by the sensor? Even a really bad sensor like the one we had in the e30 will generate some weak voltage but I don't know when it is so weak as to not be 'readable' be the ECM?
So that is the basic question I posed if anyone knows the limits so-to-speak of a good sensor and signal level and if that can be determined from just a resistance check. Assume that higher resistance = lower output. I think I will put a scope meter on the sensors I have i question and see if the output is dropping with heat. I have scoped sensors before trying to troubleshoot problems but have always got some sort of reading even when the car was falling flat on power. I usually found the problem was elsewhere anyway.
Also, I'm not sure how most meters respond to an AC signal that is not true 50-60 hz as it expects. It may be the voltage reading is no longer accurate. I imagine voltage increases somewhat with the freq of the pulse in this case even if the real pulse remains at the same level. So you probably need a true scopemeter to diagnose condition.
I plan to do a little troubleshooting this evening on the M30 and compare it to the M20 and S38 which use the same sensors.
So that is the basic question I posed if anyone knows the limits so-to-speak of a good sensor and signal level and if that can be determined from just a resistance check. Assume that higher resistance = lower output. I think I will put a scope meter on the sensors I have i question and see if the output is dropping with heat. I have scoped sensors before trying to troubleshoot problems but have always got some sort of reading even when the car was falling flat on power. I usually found the problem was elsewhere anyway.
Also, I'm not sure how most meters respond to an AC signal that is not true 50-60 hz as it expects. It may be the voltage reading is no longer accurate. I imagine voltage increases somewhat with the freq of the pulse in this case even if the real pulse remains at the same level. So you probably need a true scopemeter to diagnose condition.
I plan to do a little troubleshooting this evening on the M30 and compare it to the M20 and S38 which use the same sensors.
From my experience, an unconnected ref sensor will read approx .4 volts a/c at crank, and an unconnected speed sensor will read approx 4 volts a/c at crank. This is with a fully charged battery. Anyone know what engine rpm a starter will give with a charged battery?
Not getting any a/c voltage on the ref sensor? Then it's time to check the flywheel pin to see if it's still there.
Not getting any a/c voltage on the ref sensor? Then it's time to check the flywheel pin to see if it's still there.
Just for future reference. The remark above is about right for cranking voltage present. There is a slight voltage from the ECM that is a little different on the speed and reference. Somewhere between about 0.2 and 0.3 to ground on either of the first two pins but nearly nothing between those two. So there is no real voltage supplied from the ECM I think, just some very slight floating potential.
At idle I saw about 12vAC from the speed sensor and 2+ vAC from the ref sensor. That was true RMS voltage on a scope meter and I'm not sure how some DMM's might measure this since it is not the usually expected 50/60 Hz. The actual voltage spike is something like +20 volts to -20 with the rise and fall of the signal from the speed sensor and nearly +/-30 from the ref sensor. This was the same on either my e28 or e24 and I was trying to determine if the slightly higher resistance on the e28 was becoming a problem. Apparently not.
Bentley say 960 +/- 96 or 10% and it should be replaced. I don't agree. My e28 was 1050-1060 at each sensor going up to almost 1200 when hot. My e24 was about 1120 for both sensors cool but I did not let them get fully hot. I still don't know an acceptable range but these sensors clearly work just fine well above the 960+96 resistance level Bently tells us.
At idle I saw about 12vAC from the speed sensor and 2+ vAC from the ref sensor. That was true RMS voltage on a scope meter and I'm not sure how some DMM's might measure this since it is not the usually expected 50/60 Hz. The actual voltage spike is something like +20 volts to -20 with the rise and fall of the signal from the speed sensor and nearly +/-30 from the ref sensor. This was the same on either my e28 or e24 and I was trying to determine if the slightly higher resistance on the e28 was becoming a problem. Apparently not.
Bentley say 960 +/- 96 or 10% and it should be replaced. I don't agree. My e28 was 1050-1060 at each sensor going up to almost 1200 when hot. My e24 was about 1120 for both sensors cool but I did not let them get fully hot. I still don't know an acceptable range but these sensors clearly work just fine well above the 960+96 resistance level Bently tells us.
Re:
KegCoyote wrote:Sorry for the dumb question, but is there any way to test voltage while it is installed? Crank the engine with a friend looking at the meter or something?
I ask because my ABS sensors were all reading roughly the same ohm reading(1k) but vastly different voltage(1x 1vAC 2x 0.5vAC and the bad one at 15m vAC), and they work along the same principal as the crank sensor IIRC(pulse generator/hall effect sensor)
yes you can check for voltage at the plug where the sensor connects these plugs on the m30 are located on top of the intake in a metal valley sort of deal you can use needle nose pliers or a flat head screw driver to remove the metal retaining clip for the plug be sure to remove it completely be careful your cps sensor plug may be cracked as mine both were remove the plugs set your multimeter to dc volts turn the ignition on and start probing the 3 metal prongs you should see 2-3 volts with the ignition on. if you know the sensor is getting voltage set your multi meter to ohms and ohm each one out they should be inbetween 850-950 usually you will know when these sensors start to go bad a sign could be if your tach gauge is totally off or rough/no start conditions.