Carrier 19XR, XRV specifications Measure SCR Pairs Recorded Being Between, Checked

5.Using an ohmmeter, perform the following resistance measurements and record the results:

MEASURE	SCR PAIRS	RECORDED
	BEING
BETWEEN		VALUE
	CHECKED
T1 and T6	3 and 6
T2 and T4	2 and 5
T3 and T5	1 and 4

If all measured values are greater than 5K ohms, proceed to Step 10. If any values are less than 5K ohms, one or more of the SCRs in that pair is shorted.

6.Remove both SCRs in the pair (See SCR Removal/ Installation).

7.Using an ohmmeter, measure the resistance (anode to cathode) of each SCR to determine which device has failed.

NOTE: Both SCRs may be defective, but typically, only one is shorted. If both SCRs provide acceptable resistance measurements, proceed to Step 10.

8.Replace the defective SCR(s).

9.Retest the “pair” for resistance values indicated above.

10.On the right side of the firing card, measure the resistance between the red and white gate/cathode leads for each SCR (1 through 6). A measurement between 5 and 50 ohms is normal. Abnormally high values may indicate a failed gate for that SCR.

If any red or white SCR gate leads are removed from the firing card or an SCR, care must be taken to ensure the leads are replaced EXACTLY as they were (white wires to gates, and red wires to cathodes on both the firing card and SCR), or damage to the starter and/or motor may result.

11. Replace the SCRs and retest the pair.

SCR REMOVAL/INSTALLATION — Refer to Fig. 43.

1.Remove the SCR by loosening the clamping bolts on each side of the SCR,

2.After the SCR has been removed and the bus work is loose, apply a thin coat of either silicon based thermal joint compound or a joint compound for aluminum or copper wire connections to the contact surfaces of the re- placement SCR. This allows for improved heat dissipa- tion and electrical conductivity.

3.Place the SCR between the roll pins on the heatsink assemblies so the roll pins fit into the small holes in each side of the SCR.

NOTE: Ensure the SCR is installed so the cathode side is the side from which the red wire extends. The heatsink is labeled to show the correct orientation.

4.Hand tighten the bolts until the SCR contacts the heatsink.

5.Using quarter-turn increments, alternating between clamping bolts, apply the appropriate number of whole turns referencing the table in Fig. 43.

Care must be taken to prevent nut rotation while tightening the bolts. If the nut rotates while tightening the bolt, SCR replacement must be started over.

6.Reconnect the red (cathode) wire from the SCR and the white (anode-gate) wire to the appropriate location on the firing card (i.e., SCR1 wires to firing card terminal G1-white wire, and K1-red wire).

7.Reconnect all other wiring and bus work.

8.Return starter to normal operation.

NUT
	LOOSEN
CLAMPING	AND
	TIGHTEN
BOLT
	BOLTS
A
ALUMINUM	FROM
HEATSINK	THIS END
ROLL PIN
SCR

SCR PART	CLAMP		A	NO. OF	BOLT
NUMBER			DIMENSION		LENGTH
	SIZE			TURNS
BISCR			(in.)		(in.)
6601218	1030		2.75	11/	3.0
			(70 mm)	2	(76 mm)
6601818	1030		2.75	11/	3.0
			(70 mm)	2	(76 mm)
8801230	1035		2.75	13/	3.5
			(70 mm)	4	(89 mm)
8801830	1035		2.75	13/	3.0
			(70 mm)	4	(89 mm)
15001850	2040		4.00	23/	4.0
			(102 mm)	4	(102 mm)
15001850	2050		4.00	23/	5.0
			(102 mm)	4	(127 mm)
220012100		Consult Benshaw Representative
330018500		Consult Benshaw Representative

Fig. 43 — SCR Installation

Physical Data — Tables 13A-20 and Fig. 44-57 provide additional information on component weights, compressor fits and clearances, physical and electrical data, and wiring sche- matics for the operator’s convenience during troubleshooting.

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