Summary:
When transmission malfunctions occur, it is necessary to first check the basics prior to any transmission replacement. The Hydraulic-Mechanical Checklist is intended to be a guide for evaluating the condition of the transmission.
Naturally, not all sections of the Hydraulic-Mechanical Checklist will have to be completed in every case. In certain obvious cases diagnosis can be carried out rather quickly without the need for a fully completed checklist.
The following scenario shows how all the information in this bulletin, the repair manual microfiche, and the technical data microfiche work together to assist the technician in providing an accurate and efficient diagnosis of transmission problems.
Vehicle: 1985 325iA, with the original transmission serial number 319127
Complaint: No reverse
Diagnostic Procedure:
1. Customer's complaint was verified as to no reverse under any conditions.
2. Basic mechanical checks showed external linkage correctly adjusted and a correct ATF fluid level with fluid dark, but not burnt, and evidence of fine metal particles suspended in the fluid.
Road test revealed that upshifts were earlier than specified in Technical Data Microfiche. Stall test was within specifications.
3. When using the Automatic Transmission Troubleshooting Chart, the letters next to the symptom reference the Diagnostic Hints chart and the numbers at the top of the vertical columns reference the Where To Find Information table.
Under the symptoms of No reverse on the Transmission Troubleshooting Chart, the recommended checks are fluid level and condition, adjustment of manual selector cable, and pressure check.
4. On the Diagnostic Hints chart, governor hub or support scored by sealing rings is listed as a possible cause of a no reverse condition. Looking in the Diagnostic Note column, the transmission in question was produced before the change to Viton sealing rings.
This type of failure could allow unregulated main pressure into the governor circuit simulating a higher than actual road speed. As this transmission will lock out reverse above a given road speed, this type of failure could be responsible for the complaint of "no reverse" on this vehicle. The early upshifts also support the theory of main pressure leaking into the governor circuit.
Fluid level and condition along with the adjustment of the manual selector cable were checked and found to be OK. A pressure check was not done as it was felt that the results would not have provided any additional useful information. It was decided that the next step would be to remove the transmission extension housing and inspect the governor hub and governor support.
As expected, the cast iron sealing rings had scored into the governor hub. The governor and hub assembly were replaced and the new Viton sealing rings were used. Following a methodical diagnostic procedure and using the available reference material allowed for an accurate assessment of the problem and resulted in a cost-effective on-the-car repair.