Intermittent cruise speed sensor failure after LED lights

It's my '91 GT convertible, but AFAIK the system is the same as the alltrac (speed sensor for sure), and there are some seriously knowledgeable people here...but I didn't see anything similar when I searched, mostly people removing the system.


The cruise has always worked flawlessly on this car. Probably put 30k miles on it in the ~5 years I've had it (the salt-free days are pretty numbered up here in northern MN), never had a problem.

About a month ago, I put LED taillights and an electronic flasher in it, the cruise immediately started acting up. Thanks to the diagnostic handshake (hold "set" or "resume" while turning the system on) in the BGB, it's pretty easy to narrow down the symptoms.

1st, EVERYTIME the turn signals come on, it activates the "cancel" circuit in the cruise. This is not intermittent. This led me to believe that the flasher was causing some interference. Since I had bought the cheapest chinese flasher I could find, I ordered a genuine USA made one from RockAuto and swapped it out, no change. The chinese flasher causes no problems in my '00 Outback.

2nd, periodically the speed signal will drop out. I haven't found almost any pattern to when it works or doesn't. Seems to be a better chance that it'll work in the morning, but not always. It will frequently drop out while cruising, with zero lights on (which is what's making me question whether the LEDs/flasher are related at all). Of course, if the speed signal drops out while the system is engaged, the car has to be restarted to try again, so I'm getting good at turning off and restarting the car on the highway :twisted:


Of course, I didn't save any of the old parts, I had a couple burned out bulbs (which is what started this), and the base of the original flasher broke when removing, so I chucked it all. I suppose my next option is to either buy new incandescent for the Celica, or swap the LEDs into my Outback (which would require some fronts and rears on the Outback to cover them all, and possibly cause issues there). So I thought I'd spitball here to see if there's anything I might have missed.


When I bought the car, the transmission was damaged. I replaced it with an S51 from a Camry for the taller gears, I swapped the speed sensor over, speedometer works flawlessly, though, so I'm ASSuming that everything is connected correctly there (at least mechanically). The wiring diagrams seem to imply that there's some speed signal coming from the gauge, but I had the speedo cable break and the cruise still worked.

Anyone had intermittent random speed sensor failure? Think of anything else to check?

there are two speed signals, one that is created by the electric sending unit mounted inline to the speedo cable and the cluster also generates an electric signal to the ecu labeled SPD. the leds probably have nothing to do with your issue. the speed signal from the inline sensor is for the cruise control but if it isnt working (sometimes the ground goes bad) you can steal the signal from the cluster and send it to the cruise ecu and it should work fine. its the same for all 1990s toyotas.

Cruise is also cancelled when you touch the brakes right? At least in the vehicles I've driven it does. I'd consider the option that the LEDs could be backfeeding the brake light system enough to trigger it. TO eliminate things I'd be taking standard parts from the Outback and trying them in the Celica, not the other way round since the systems are going to be different.

black88alltrac":36aeh1r6 said:

there are two speed signals, one that is created by the electric sending unit mounted inline to the speedo cable and the cluster also generates an electric signal to the ecu labeled SPD. the leds probably have nothing to do with your issue. the speed signal from the inline sensor is for the cruise control but if it isnt working (sometimes the ground goes bad) you can steal the signal from the cluster and send it to the cruise ecu and it should work fine. its the same for all 1990s toyotas.

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Good info, once I eliminate the lights (the timing was too perfect to ignore completely, but it's certainly possible it's coincidence), I'll start narrowing to the speed sensor and that gives me another option.

underscore":36aeh1r6 said:

Cruise is also cancelled when you touch the brakes right? At least in the vehicles I've driven it does. I'd consider the option that the LEDs could be backfeeding the brake light system enough to trigger it. TO eliminate things I'd be taking standard parts from the Outback and trying them in the Celica, not the other way round since the systems are going to be different.

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Yep, Brake, clutch, ebrake, and cancel button all have the same cancel effect. The turn signal is a completely different bulb on these than the brake, so they really don't share the same circuit. They are physically near each other, and the wires run immediately adjacent, so some electrical interference from one circuit could cause problems in the other.

I'd be swapping the bulbs completely, I don't expect any symptom change in the Outback to be reliable, but I need to keep both cars driveable, so the LEDs will need to be installed there in the mean time.

Separate bulbs, but they may share a ground in the harness. If that ground isn't making a good connection I could see it causing problems.

black88alltrac":1mjvmy5j said:

the leds probably have nothing to do with your issue.

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They would if they were incorrect. . Like if he put a single filament 1156 in place where a dual 1157 should be.

underscore":1mjvmy5j said:

Cruise is also cancelled when you touch the brakes right? I'd consider the option that the LEDs could be backfeeding the brake light system enough to trigger it.

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Correct! I thought the same. . I'd be willing to bet he has defective/incorrect bulbs in place.

Check that you have the proper bulbs in place, more specifically. . 1157s for stop lights and 1156s for tails. Even if correct, they could be defective so remove them from the equation and (safely) take the car out to test the cruise. I'm with underscore, I think that you may be backfeeding power into the brake light circuit.

If I'd put an 1156 in the brake light socket, it would backfeed power to the running light circuit (ever see a car driving down the road where the front lights come on with the brakes?). I am 100% confident that the 1157s and 1156s are in the correct position.

That said, it is likely that these cheap LEDs are causing some interference which is disrupting the circuitry in the cruise unit when the turn signals come on. I wouldn't be surprised if shielding one or both of the wires would cure that problem, but that's not practical.


The part that I'm not sure about, is whether that same interference is causing the speed signal to randomly drop out.

My guess would be two possibly unrelated issues. Do you get a CEL when the speed signal drops out? I assume that's how you know it's happening. Every once in a while I'd get that in my RC, I had the sensor replaced during the rebuild and so far (knock on wood) it hasn't come back.

underscore":1e1tz5s5 said:

My guess would be two possibly unrelated issues. Do you get a CEL when the speed signal drops out? I assume that's how you know it's happening. Every once in a while I'd get that in my RC, I had the sensor replaced during the rebuild and so far (knock on wood) it hasn't come back.

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Doesn't throw a CEL. When the cruise cuts out, it stores a code in the cruise system that indicates speed sensor.

Also, when driving, I can put it in diagnostic mode and watch for the cruise light to blink while getting a speed signal. When the speed sensor works and doesn't has zero correlation with use of the rear lights.

Mild follow-up for anyone paying attention.

Nearly 2 weeks since I swapped the LEDs out for incandescents. Turn signal/cancel symptom went away immediately.

Speed sensor signal seems to work a bit more, but not 100%. I wish I'd saved the cruise unit when I parted out my '92 to try. Part of my brain is convinced that the interference from the LEDs has damaged something in there, but not enough to spend much money to buy one (if the local UPull would get a 5th gen again, I'd pull one in a heartbeat, but nothing that old these days).

I'll keep experimenting to see if I can narrow down the cause.

Another follow-up.

I bought a $13 used cruise unit on Ebay and swapped it in last night. No change. So my diagnostic continues along the speed sensor circuit (I think I'll try to track down the ground, and then jump the ECU speed signal to it as suggested by black88alltrac).

I knew I was driving my Celica to the cabin (~200 miles each way) for Memorial weekend, so I made time the week before to dig in far enough to rewire the Cruise unit to the vehicle speed signal from the dash.


Cruise worked flawlessly. It seems to be fixed.