Troubleshooting Flickering CTIS Ovspd Light Above 25mph in Hwy Mode

[Overdrive]

1997 S&S M1078 8300 miles. Just cleaned up after transport from GP.

Went out for the first test drive of the M1078 early yesterday morning. Warmed the truck up to 165, verified CTIS is in Hwy mode with a SOLID red light and tire pressures are at 55psi. Around 25-30 mph the CTIS Ovspd warning light started flickering on and off. When slowing down below 20mph (approx.) the flickering stops.

I know the CTIS over speed light is designed to come on when you are using one of the other modes off-road, run-flat, etc. and if you go above that settings speed threshold the warning light comes on however, shouldn't be getting this light in highway mode and the flickering indicates some connection/sensor problem.

Going to refer to the -20 to see what I can find but thought I would go ahead and start a post up here in case anyone else has the issue or fixed. My first assumption is a ground issue some where. Any assistance is appreciated.


Will keep this post updated
OD

 

[Gunzy]

Not sure on the M1078, but on the M939 series the O/S lite will come on in Highway if you excede 55 mph. I would suspect a bad speed sensor or a short in the sensor wiring.

 

[Retiredwarhorses]

1997 S&S M1078 8300 miles. Just cleaned up after transport from GP.

Went out for the first test drive of the M1078 early yesterday morning. Warmed the truck up to 165, verified CTIS is in Hwy mode with a SOLID red light and tire pressures are at 55psi. Around 25-30 mph the CTIS Ovspd warning light started flickering on and off. When slowing down below 20mph (approx.) the flickering stops.

I know the CTIS over speed light is designed to come on when you are using one of the other modes off-road, run-flat, etc. and if you go above that settings speed threshold the warning light comes on however, shouldn't be getting this light in highway mode and the flickering indicates some connection/sensor problem.

Going to refer to the -20 to see what I can find but thought I would go ahead and start a post up here in case anyone else has the issue or fixed. My first assumption is a ground issue some where. Any assistance is appreciated.


Will keep this post updated
OD

Did you ever fix your CTIS O/S issue?

 

[Overdrive]

Subscription email not working for me so sorry on the delayed reply. I have not had a chance to start troubleshooting this gremlin. Also have a flakey fuel gauge. There is a little wire along side my CTIS harness to the cab controller attached to one of the mount bolts that doesn't look stock so I am assuming someone had previously tried to work the problem. My CTIS controller looks fairly new.

 

[Suprman]

I have learned that the trans control also uses a throttle position sensor on top of the engine its a small box over the fuel filter with primer button and has a cable running up to the IP. This with the sensor on the trans and it works together with the ctis controller. If you are in a lower tire pressure setting you will get an overspeed warning at lower speeds accordingly. There are a bunch of 12v ctis controllers around that are compatible with the LMTV but are really MRAP and the program on them is slightly different.
Will

 

[Overdrive]

Thanks for the info Suprman. The overspeed light flickers in HWY mode above 30 which it should not do. I haven't started down the TM troubleshooting path yet but when I did glance at the -20 I recall that I didn't see this particular condition in the troubleshooting tree...

It is on my list of things but is down there a ways
OD

 

[Suprman]

If the light flashes and your truck shifts funny then replace the speed sensor on the underside of the transmission. Its an over the counter Allison part. 29543432 there are a dozen or so alt part numbers listed in parttarget.

 

[Retiredwarhorses]

Thanks for the info Suprman. The overspeed light flickers in HWY mode above 30 which it should not do. I haven't started down the TM troubleshooting path yet but when I did glance at the -20 I recall that I didn't see this particular condition in the troubleshooting tree...

It is on my list of things but is down there a ways
OD

My 1078 still flashes above 30mph as well....I have replaced the speed sensor, that did make my speedo accurate within 1-3mph....before that, it was way off and jumped all over the place. But if I ever get time, it would be nice to get that lamp to stop flashing.

 

[Suprman]

I am still learning about these trucks. If there was a theory of operation TM with a schematic of the whole truck it would make the process much easier. I didnt know the tps was only used by the the trans computer until recently. Since the IP is mechanical I am wondering if the ecu expects a certain rpm to be at a certain throttle position depending on gear of course. Or maybe it just uses it for a "kick-down".

 

[Suprman]

On the bell housing is an engine speed sensor I have been doing some reading up and this input is also used by the ecu.

 

[Lmtv772]

isn't there a sensor on the bell housing top driver side ? I saw some trucks had it unplugged , when they had ctis overspeed on

 

[wyocommo]

My '98 M1078 has that same little wire on the outside of the harness.

 

[Overdrive]

yea I just realized the wire is a standard ground.. Lmtv772 I was just looking at that connector when I had my cab up last night. Might check that out and see if there are any obvious issues with that connector.