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940: No speedo sensor = idle issues?

P

PNW940Pwnage

New member
Joined
Oct 1, 2015
Location
Olympia, Washington
Hey gang,

I had swapped out the G80 in favor of a welded open diff from a 240. Now I have no speedo sensor pickup.

I've been having idle issues (sometimes won't recover the idle when slowing to a stop and dies) and it was suggested it's because I have no speedo sensor wired up. It's also triggered the CEL.

Is there a fix for this? I don't need a speedometer, so even if it's not possible to get a working speedo sensor in the car, I'd settle for tricking the car into thinking it's there. Car is a 95 940T. Thank you!
 
Yes that's the issue. Lh2.4 has to have a speed sensor or it high idles and possibly other things. I drove my old 740 without one for a while but it was annoying. Only fix is give the ecu a speed signal. Why didn't you swap the tone ring and diff cover?
 
Canadan said:
Yes that's the issue. Lh2.4 has to have a speed sensor or it high idles and possibly other things. I drove my old 740 without one for a while but it was annoying. Only fix is give the ecu a speed signal. Why didn't you swap the tone ring and diff cover?
Click to expand...

I guess it was just in the interest of time. I haven't separated the tone ring before - does it just pull off? I can get the tone ring and diff cover from a 240 I guess.
 
It's been a while since I looked but I believe it's held on by the bolts which hold the ring gear. Could be wrong but if you want it to run/idle right, it's what you have to do. Why'd you get rid of the g80 for a welded diff btw? I'll give you about tree fiddy for it
 
Canadan said:
It's been a while since I looked but I believe it's held on by the bolts which hold the ring gear. Could be wrong but if you want it to run/idle right, it's what you have to do. Why'd you get rid of the g80 for a welded diff btw? I'll give you about tree fiddy for it
Click to expand...

It seemed like it was starting to act up, and I've seen a couple blow up under less power than I was making...so it was a preemptive strike.

It seems to me like this is going to be a bitch of a fix...the tone rings between the 240 and 940 are totally different. At first I was hoping I could just run the 240 diff cover, but now that I think about it - I'm going to need to swap out the tone ring since they have a different number of "teeth" - is that correct?

I'm not sure if it needs to read *correct* in order for it to run right, or if it just needs to *read*. I don't care about the speedometer. Thanks!

940 tone ring:
http://s513.photobucket.com/user/simonbabbs/media/1041kinstalled.jpg.html

240 tone ring:
https://farm1.staticflickr.com/602/21628290673_e8934f4a0d.jpg
 
It seems possible that you could use a ABS sensor on front strut to provide a speedo signal. Not tried it, but it might work.....
 
crogthomas said:
It seems possible that you could use a ABS sensor on front strut to provide a speedo signal. Not tried it, but it might work.....
Click to expand...

Thank you. I don't understand how the signal is used by the ecu...so I don't know if it needs to read exactly right (48 teeth, correct diameter tone ring/tooth spacing) in order to run properly - or if it just needs to read *something* even if it's not perfect (ie, the 12 tooth 240 ring currently on the diff). I don't think I have the sensor plugged it at all right now.

I'm trying to figure out what running issues this could cause.
 
I know this isn't what you want to do but I'd swap back to g80, honestly a welded diff is more likely to break **** than a g80 (didn't they put g80s in 350/454 powered gm trucks too?) weld the paw and go. Cause either way you have to take **** back apart to get the idle to work properly. I swear I saw there was another issue caused by not having a vss hooked up on lh2.4 but I can't recall what it was and I never noticed anything else personally
 
PNW940Pwnage said:
I don't understand how the signal is used by the ecu.
Click to expand...

It uses the frequency of the signal to determine deceleration. VSS and ABS sensors are VR sensors on older cars=AC voltage with slightly incresing voltage (about 2v total) with increasing frequency with speed.

That is why, theoretically, an ABS wheel sensor "might could" be used. Its the same thing as a VR diff sensor. I really don't think the ECU cares about how many windows/teef it sees. The speedo head does, but I THINK the ECU doesn't.
 
PNW940Pwnage said:
It seemed like it was starting to act up, and I've seen a couple blow up under less power than I was making...so it was a preemptive strike.

It seems to me like this is going to be a bitch of a fix...the tone rings between the 240 and 940 are totally different. At first I was hoping I could just run the 240 diff cover, but now that I think about it - I'm going to need to swap out the tone ring since they have a different number of "teeth" - is that correct?

I'm not sure if it needs to read *correct* in order for it to run right, or if it just needs to *read*. I don't care about the speedometer. Thanks!

940 tone ring:
http://s513.photobucket.com/user/simonbabbs/media/1041kinstalled.jpg.html

240 tone ring:
https://farm1.staticflickr.com/602/21628290673_e8934f4a0d.jpg
Click to expand...

Just take the speed signal off one of the front hubs and you will be fine. It has been done many times by folks on here that switched to different locking carriers with no tone ring.
 
Canadan said:
I know this isn't what you want to do but I'd swap back to g80, honestly a welded diff is more likely to break **** than a g80 (didn't they put g80s in 350/454 powered gm trucks too?) weld the paw and go. Cause either way you have to take **** back apart to get the idle to work properly. I swear I saw there was another issue caused by not having a vss hooked up on lh2.4 but I can't recall what it was and I never noticed anything else personally
Click to expand...

I've seen them blow up too many times now, and I'm scared. lol. My buddy blew a hole in his case when the G80 exploded...it was only a 15G powered car @ 17psi. They don't seem to be able to handle much abuse once you're making 250+ lb/ft.

Might try the wheel hub route. Thanks!
 
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