Temp Gauge and Speedo

[Ian B Walker]

Probably not the place to post, but.......... My Speedo is reading 15mph high and also my Temp gauge reads low (under 80) also the outside temp gauge reads 10 degrees low as well. Could this be an electrical problem and what should I look for if it is? Can the Speedo be re calibrated at home? Hope someone can help. Oh car BTW is E220 Coupe as in siggy pic. Thanks in advance.

Ian

 

[andy_k]

Perhaps if you were to add the outside temperature to the water temp, then divide by two and subtract that figure from the speedo you may get a more accurate readout

My guess would be you have a bad earth somewhere but things to check

1, is the speedo drive mechanical or electrical? if it's cable driven you can kind of rule that out the equation but if it's not then maybe there is one earth for all the senders?

2, is there a control module on the back of the instrument cluster whhich could have failed?

3, Perhaps a dry joint on the circuit board?

All of these involve taking the unit out to investigate so my first check would be that all the senders have clean and good contacts at the engine.

Does the temp guage stay belkow 80 even when the car is stationary with the engine running, in the winter months mine seldom got above the very low eighties unless i was stuck in traffic.

Andy

 

[prprandall51]

Can't speak specifically about MB but:
Dash instruments need a constant voltage to display correct values. Battery voltage varies significantly from a low of around 11.5 up to 14.4 when the alternator is charged at full rate.

Therefore, there is a voltage regulator in the back of the dash (probably now combined in a single "control unit") that may no longer be providing the correct, constant voltage (often around 10.5 Volts). This could explain why several instruments all appear faulty since it is hard to imagine all of the seperate sender units playing up at the same time.

I agree with the "check connections" advice - especially with regard to the back-of-the-dash connectors.

However, could your engine just be running cold due to a stuck-open thermostat? Also, in what way is your speedo over-reading? You say 15mph too high - but at what speed? How innacurate is it at 30mph, at 50mph, at 70mph? I assume that the tyres are the correct / factory-fit size for the vehicle. And they aren't under-inflated, are they (though it's hard to imagine a 15mph error from all but a flat tyre!)
Philip

 

[Ian B Walker]

Thanks for all the replies. First I will acertain whether or not its an electrical speedo, then check as Andy suggests all the earths etc. Now this could be fun, mechanics I am ok with ...........electrics? Ehm, cough. I am to electricity what Hitler was to World Peace, but I am going to try.

Note to self, check insurance of house, car, other car and thrid party liability to neighbours.

prprandall51, the speedo reads 15 above whatever speed I am doing. At stand still it reads zero. The battery is ace and has a full charge (its quite new) With regard to the engine thermostat being stuck, nope as it throws out mountains of heat when the engine has run for about 5 miles. During that time it only pushes out cold air so I asume that the thermostat is functioning correctly.

 

[PJH]

Ian,
If the thermostat has 'failed open' the top water hose, thermo to radiator,
will feel warm within 1 minute of a cold start.
Patrick

 

[prprandall51]

The speedo: has this problem just appeared or has it been like that since you bought the car?

If the tyres are factory-fit size and correctly inflated, and if there have been no modifications to the gearing of the car (through an incorrectly swapped gearbox or auto box or differential or speedo cable) then a constant 15mph over the actual speed strongly implies that at some point the speedo needle has ben removed from the spindle and placed back incorrectly. This could have been done to gain access to the inside of the dash pod.

Do you know all of the history on the car regarding these items?
Philip

 

[Ian B Walker]

Philip thanks for the reply. Yes I do know this car intamatley (spelt wrong I know but brain going to sleep) I have been privaledged to have done all the servicing on this car for some time, the ex owner is a very good friend and an excellent forum member. The wheels are 15 inch, as per the book, although now very nice alloys. I was informed of the speedo and temp problem before I purchased the car. I doubt very much that the needle has been played with as the car has a very good service history prior to its last owner.

Ian,
If the thermostat has 'failed open' the top water hose, thermo to radiator,
will feel warm within 1 minute of a cold start.
Patrick

No everything there is fine, trust me

 

[pluggers]

Ian is it possible to swap your instrument clusters over whilst your taking it out to check wether the speedo is electronic or not?Should be able to rule out instrument cluster failure then.

 

[prprandall51]

Thinking about this over lunch:

All possible speedo failures - whether mechanical or electrical - should only add a scaling factor to the speedo reading, e.g. reading 40 at an actual 30, 80 at an actual 60, 120 at an actual 90, etc. This includes all my earlier theories about flat tyres, mis-matched gears, speedo cables ,etc.

If the speedo reading is always accurate - but 15mph over (if you see what I mean) - then this constant over-reading must surely be that the needle is incorrectly positioned on the spindle. If the needle has never been removed deliberately then perhaps it has come loose and slipped on the spindle to the new position? There is a return spring on the spindle that will pull the needle round to zero in the absence of a signal (electrical) or driving force (mechanical). Thus, if the needle were loose, the spindle could wind back and past the zero position under the force of the retun spring causing the needle to move on the spindle when it hits the "bump stop". This would give rise to always read a constant amount over (15mph over in this case).
Philip

 

[Ian B Walker]

Ian is it possible to swap your instrument clusters over whilst your taking it out to check wether the speedo is electronic or not?Should be able to rule out instrument cluster failure then.

I am checking this out tomorrow.

prprandall51.....what you say makes sense, but if my speedo is electric then would the slipped spindle make my temp gauge and outside temp gauge read inacuratley? I am swaying towards an electrical speedo and a blown gubbins behind the unit. As above, I will find out tomorrow. Many thanks for all your thoughts one and all.

 

[Ian B Walker]

Ok I now know what the problem is, looks like the electrical gubbins box attached to the unit is on its way out. Gonna have to visit the Stealers to find what they cost, unless someone here knows?