Crazy Speedo

[Alison Hudson]

Our beloved Bongo has been resting up for several months but now ready to have fun with it again - Once we got it started the Speedo has gone crazy and going up to max when engine idling and vehicle not even left the driveway, also adding miles onto the clock - any ideas how we can resolve this problem would be greatfully received! Graham & Alison Mosley

[grumpo]

The speedo is driven by a low voltage square wave frequency generated
by the gearbox transmitter. As it's virtually impossible for anything else
to simulate this signal I can only assume that that the speedo head can
also respond to a DC voltage and drive it full scale. If this was the case
then you should also get a hold light error, flashing 1 second ON and 1
second OFF, although a DC signal may confuse it.

If your odometer reads in MILES then a frequency converter "Chip"
must be fitted behind the instrument panel and I would suspect that
this is faulty and sending DC on the output to the speedo. I would not
suspect anything prior to the "Chip" because the circuit only responds
to the rising edge of the frequency and it would not pass a DC voltage
if it was in good condition.

If you do have a converter "Chip" fitted I would first remove this and
re-connect the original cables, this will then convert the speedo back
to reading Kilometers and with a bit of luck solve the problem.

If you don't have a "Chip" fitted and your odometer reads Kilometers
then you will have to use an oscilliscope to test the actual input to the
speedo head to determine if the speedo head or gearbox unit is the
faulty part. A modern electronic volt meter would not reliably block
the DC component whereas an old moving iron meter may give you
some indication.

Sorry if this is becoming complicated but the symptoms are unusual
and you need to discover the actual fault before spending lots of
money on something that may not cure the problem.

Whatever you do, tell us the results.

[Allans]

Well put Grumpo! If the speedo is chipped, I would check that first. It used to be that a chip might cause problems after a few years of use. Recently, I've heard of the problem occuring more often. I prefer not to chip.

[grumpo]

I totally agree Allens.

An aquaintance had a problem with one, so I removed it and
stripped it, it was poorly made and had absolutely no spike
protection, I could have bought the components for about
90p. It seems someone is trying to make a lot of money from
unsuspecting owners. The same thing can be used with many
grey imports. Even the good quality ones seem expensive for
such a largish market and cheap manufacturing costs.

I much prefer a new speed scale as in my case it's much easier
to distinguish the 30 MPH point. I think some suppliers prefer to
fit a converter as it's quicker and doesn't involve removing the
indicator needle.

It's just a matter of getting used to kilometres instead of miles,
and it becomes useful if you often travel in Europe. The same
goes for temperature, I'm sure we've had much better weather
since we changed to degrees centipede.

[Peg leg Pete]

Our bongo has just returned from a service, autobox oil was changed etc, now the speedo is behaving very erratically, not sure if anything has been disturbed

[Alison Hudson]

Hi Grumpo
Thanks ever so much for your diagnosis - the hold light is coming on as well so will carry out your recommend treatment and let you know the result !
Thanks again
Graham and Alison

[Peg leg Pete]

Well been out today and speedo has behaved so it must have been a glitch in the system