What do I need to consider when converting a mechanical speedometer over to electrical?

Question

I am the proud owner of a 1998 Toyota Corolla E11 (bug eyed version, pre-facelift). It has suffered a problem whereby the speedometer is broken. I haven't been able to diagnose the issue, but it's one of the following items:

• The Vehicle Speed Sensor (VSS) that sits on top of the gearbox
• The speedometer cable running from the VSS to the back of the speedometer
• The speedometer itself

I'm inclined to believe the issue is down to the VSS failing, as when the car is to temperature and you do 3,500rpm for 8 seconds the check engine light comes on. The code for this points to a VSS failure, however upon further research it basically indicates the ECU has no idea how fast the car is going, and that information is provided to it by the speedometer.

I've been looking around various breakers yards etc around me in the UK and in Ireland for a mechanical VSS and they seem to be rarer than hen's teeth. I've spoken to two guys who said they had one. I visited one and he promptly broke the VSS off the gearbox, meaning it was useless. The second guy has told me it's near impossible for him to get his VSS off his gearbox to give to me without breaking it. My car is 19 years old this month, I know full well I'm going to end up breaking the VSS off my vehicle as well. Plus I've had several arguments with my Toyota dealer, as they maintain my vehicle has 4 speed sensors on each wheel and that's the problem. The car has no ABS.

I have noticed though that the market is awash with the electrical VSS that existed on higher spec Corolla's, mostly on the facelift models from 1999 onwards. Even eBay lists these as brand new parts.

To top this off, the MOT is due in August this year. We're a long way off obviously, but the car will fail outright if the speedo doesn't work. So I have two choices, which is scrap the car or turn it in to a project in where I can convert the car to use an electrical VSS instead of the mechanical one.

From what I can deduce between the two VSS's, the mechanical one has a cable plug in to it and that turns the speedo in the cabin. The electrical one has a plug and no mechanical cable, so I would do away with the mechanical cable and would need to purchase a new electrical speedometer.

But it can't be that simple? Can it? Have I missed a step or would I also need to swap out the ECU?

Answer 1

The car electronics don't care where they get the VSS signal from, they just want a correct signal.

Older cars use a spinning speedometer cable which drives a mechanical speedometer. The VSS was typically an afterthought on these cars, and is mounted inside the speedometer/instrument cluster. If the speedo cable has a problem or flutters, the VSS signal will also have a problem or flutter.

In newer cars, the VSS generator is right on the transmission and feeds this singal to the engine computer, electric speedo, anti-lock brakes, and the radio (if it supports automatic volume adjustment).

You do occasionally see your setup where the VSS is on the transmission but it "passes through" the spinning cable. I would stay with that setup if it wan't unserviceable, but since it is...

You want to convert from one type to the other.

Given the vintage of your car, you have a fair chance of success with enough care and diligence. This was around the time automakers were switching from one method to the other, so there's a fair chance that an on-the-tranny VSS generator will exist in a parts bin somewhere for that transmission (or more likely a picking yard).

You will have to be careful to get a VSS which outputs the same style of signal generated the same way - for instance GM VSS's are wired in a standard way, but generate pulses at either 1000 pulses per mile, 2000 or 4000 - you'll need to match up carefully. It will help quite a lot to consult the factory shop manual (not Chiltons or Haynes).

It also helps if there is an enthusiast community and strong parts aftermarket around this vehicle - for instance 3rd parties make VSS for GM cars, count dividers/multipliers to convert 1000-pulse signals to 2000/4000 or vice versa, and even "speedo spinners" which intake power and a VSS signal, and output a spinning cable to drive an original mechanical speedo, so you can have a VSS powertrain but keep the dash original.

Once you have a VSS signal working and wired in, you will pass smog. The next trick is to get a speedometer on the dash to indicate. For that you will be on a snipe hunt for a VSS speedo that will physically fit... And the first place to look is later model years of that same chassis. (Same or next generation, that is.) Keep in mind that chassis may appear as other makes, such as Lexus or even General Motors, and those models may be specced with different powertrains (and dashboards).