VDO Speedometer calibration how to

[emarsh]

When I checked the speedometer from my '79 SC against my GPS it seemed to be linear but about 5 mph fast. Others have reported similar results.

I had to pull the speedo apart to replace the odometer gear. While I was in there I thought I'd see if I could calibrate the speedo. I found a thread here that describes how its done but the description was somewhat sparse so I thought I'd describe what I did.

The adjustment is a small adjustible resistor near the back of the mechanism. The problem is that to get to it the housing not only has to be drilled, it is also necessary to go through one of the plugs that the instrument's light goes into. This is very soft material that does not drill so it was necessary to first drill the housing and then grind the light plug by going through the drilled hole with an 1/8" bit on a Dremel tool. Once it was ground I then squeezed it with pliars so that the light will be held snugly.

After reassembling the speedometer I purchased a set of TV set adjustment tools. These are just small screwdrivers that reach about an inch and a half into the speedo. I put the instrument back into the car but left the one light out. After checking the speedo against my GPS I gave the resistor a turn to the right (clockwise). Checking again I discovered that the speedo is now about 1 mph slow. A second small adjustment to the left was all it took for the speedo to be just about right on.

The speedo is now about one to one and a half mph fast at 30 mph or below. I checked at 40, 50, 60, 70, 80 and 90 mph and at all those speeds the speedo is within one mph of agreeing with my GPS. Unlike most instruments the VDO speedometer appears to be very linear but just set a little fast.

Once calibrated the last step is to seal the hole with a spot of RTV silicone and put the light back in.

Here is a photo of the drilled speedometer housing.

[pwd72s]

I'm impressed...really! Great job.

[350HP930]

Thanks for the info.

I am installing an 8:39 ring and pinion in the car and might need to do something simular.

The only problem is that I don't want the odometer to be out of synch so I was planning on puting a signal pulse multiplier/divider into the circuit instead.

Does anyone know whether this adjustment only calibrates the speedo or if it also adjusts the odometer circuit too?

[emarsh]

Originally posted by 350HP930
Thanks for the info.

I am installing an 8:39 ring and pinion in the car and might need to do something simular.

The only problem is that I don't want the odometer to be out of synch so I was planning on puting a signal pulse multiplier/divider into the circuit instead.

Does anyone know whether this adjustment only calibrates the speedo or if it also adjusts the odometer circuit too?

I don't think that this fix will work for a change in ratio. It is more of a zeroing adjustment.

Don't know if it will work with the Porsche but there is a product called the Yellow Box that is used to calibrate motorcycle electronic speedos. I've got one on my 'Busa and they work well. You can get one at Black Robotics (http://www.blackrobotics.com/). I'm guessing that this is the answer that you are looking for.

Eric

[RoninLB]

Originally posted by 350HP930

Does anyone know whether this adjustment only calibrates the speedo or if it also adjusts the odometer circuit too?

I did the adjustment without touching my lite.. It was linear.. glad to see someone else do this adjustment..

the odo. is not affected in my unit.............Ron

[1980SC]

I've also done this. And I did have to grind through one of those bulb holders. That was a bit of a pain since it liked to get bent out of shape.

I used a stopwatch and mile markers to get my speedo perfect.

-Rob
1980sc

[island911]

emarsh -- Great Post!

Question: Does the "drilled" hole plunge perpendicular to the case surface?

More specifics on hole location and tragectory would be very helpful.

edit; FWIW, McMasters has some caps here that might work well for a hole plug.

[RoninLB]

Originally posted by emarsh

it is also necessary to go through one of the plugs that the instrument's light goes into. [/IMG]

like I said.. I didn't have to do anything [besides backing out the lower outside plastic wire connector/posted on another thread].. then I could see inside to spot the itty bitty screw.. then I drilled in line to the screw.. I never opened the unit.

Anyway, I could still use some bigger watt bulbs..................Ron

[emarsh]

Originally posted by island911
emarsh -- Great Post!

Question: Does the "drilled" hole plunge perpendicular to the case surface?

More specifics on hole location and tragectory would be very helpful.

If you like the next time I have it out I can give you measurements of the hole. The problem is that the way I did it created metal filings so the unit needs to be disassembled. I'm very curious about those who did not have to work around the light. Could there have been more than one model with the light in different locations?

Eric

[RoninLB]

Originally posted by emarsh

. I'm very curious about those who did not have to work around the light. Could there have been more than one model with the light in different locations?

Hey Eric... check out the difference between where you drilled and where I drilled

[Doug Zielke]

Originally posted by emarsh
The problem is that the way I did it created metal filings so the unit needs to be disassembled.

Mmmm.....I was wondering about *that*.

[emarsh]

Originally posted by RoninLB
Hey Eric... check out the difference between where you drilled and where I drilled

Its way different, isn't it? When I drilled I had the unit disassembled and took measurements from it while it was apart. I don't know that I would have been able to get to the resistor from where you drilled. Next time its out I'll look and see.

Now that I think about it I had to use an European odometer gear. Could there be internal differences in the European and U.S. models?

[RoninLB]

Originally posted by emarsh
Its way different, . Could there be internal differences in the European and U.S. models?

I don't know..

there is so little written about this.. between the 2 of us, this is the most written anyware.. maybe we can contact the Pelican Research Foundation because most answers to speedo questions simply state the send them to the 2 rebuild shops.

[RoninLB]

Originally posted by Doug Zielke
Mmmm.....I was wondering about *that*.

the plastic wire connector can be opened up and cleaned up with your finger when you drill upwards into it.. so the shavings fall down against the case................Ron

[kellzey]

What's that hairy thing in the bottom of the picture?

Do you have a small balding hairy ogre living in your 911?

[RoninLB]

Originally posted by kellzey

What's that hairy thing in the bottom of the picture?
Do you have a small balding hairy ogre living in your 911?

that pic can be LOL...
maybe I shouldn't be concerned about the wildlife when I take the 911 into the open forest for a sump flushing?

[thulsadoom]

Hey all.

Hello.

The 85MPH electronic speedo in my 1980 SC is loosing teeth off the odometer gear like a centegenarian. I'm going to get another gear and fix (see pict).

However, I purchased a 150MPH replacement. It "works," but the needle is going as far in the 150 as it was in the 85, and thus reading WAY high. So, question is - can I calibrate the 150 (there's a potentiometer in there - see pict, what is it?) or do I need a new sender (ugh!)?? How does one simulate driving... what voltages applied to which leads.. pulses?



Thanks for any advice.
-John.

[fanaudical]

Bringing back a really old thread; this one got me going in the right direction so I'm going to add to it.

I needed to recalibrate my speedo, so I used my GPS to determine the error at 60 mph (it read 65 when I'm really doing 60). I removed the speedo from the case to find the potentiometer. Next I made a simulated speedo sensor with a set of old breaker points, a piece of old distributor shaft, and my drill press. (Drill press spins the distro shaft, trips the points, which provide a signal to the speedo.) I wired up the speedo innards to a bench-top 12V supply and my makeshift signal generator, adjust drill press to a simulated speed of 65mph as read on the speedo, and then dialed the indicated speed down to 60mph.

This may not be as accurate as calibration directly against a GPS, but probably will be "good enough". (I'm doing some rust repair so won't have the car on the road for a while to test.)