Setting the value held in a combobox

I have a form with two unbound combo boxes, and several fields.

The idea of the form is that the user can select either of the two
combo's and the rest of the fields on the form will populate.

So, for example, if the user selects an entry from cboContractAccount,
the table will be searched and the fields populated with the values
associated with that ContractAccount. Alternatively, the user can
select an entry from cboSerialNum--if they don't know the
ContractAccount--and the fields will populate as appropriate.

However what I want to do is, if they select a ContractAccount from
cboContractAccount, I want cboSerialNum to populate with the serial
number for that record, and vice-versa. I've tried using:

.cboContractAccount.Value = ContractAcct

Where ContractAccount is the value retrieved from the database. In
debug mode, ContractAcct shows ''85002312542'
and .cboContractAccount.Value shows ''85002312542'. But, on the form,
cboContractAccount is blank.

Anyone advise?

TIA

Duncs